Summary

Based on a comprehensive review of the available evidence and a rigorous psychometric evaluation, the clinical assessment of "rib flare" lacks reliability, scientific validity, and clinical utility.

• What Moves The Ribs: The magnitude of rib motion, and specifically rib flare, is most heavily influenced by thoracic extension/flexion, not core muscle contraction, or respiration.
• Lack of Standardization and Subjectivity: Rib flare as an assessment lacks objective quantification, definitive thresholds, and normative data, reducing it to a highly subjective visual estimate governed entirely by examiner bias.
• Failed Reliability and Validity: Without standardization, the visual estimation of rib flare is highly unreliable, and, lacking correlation with impairment, specific joint pathologies, or injury risk, the assessment lacks any construct and criterion validity.
• Physiologically Flawed Interventions: Prescribing corrective exercises to pull the ribs "down and in" relies on unassessable metrics (e.g., the "zone of apposition" and rib flare) and the physiologically absurd assumption that resting muscle tone can permanently deform cortical bone.
• Inability to Measure Outcomes: Because the rib flare assessment is both unreliable and invalid, any perceived "improvement" cannot be objectively verified. There is no means of distinguishing an actual change from measurement error, transient changes in respiration, or a change in thoracic posture. The lack of reliability and validity means rib flare cannot be used as a reassessment to determine whether the interventions were effective in reducing rib flare.
• Loss of Clinical Utility: Because of a lack of standardization and definitive thresholds, rib flare cannot reliably be used to group patients as those who would or would not benefit from a specific set of interventions. Further, this assessment cannot be used to measure progress because it lacks reliability. Lastly, there are more reliable and valid assessment options. Given these issues, this assessment does not serve a clinical purpose.
• Evidence-Based Alternatives: Clinicians should abandon the rib flare assessment in favor of rigorously researched, psychometrically sound evaluations. Our recommendations are the use of the Overhead Squat Assessment (OHSA) and the lumbar-locked rotation test (note that we often use this test as a thoracic mobilization and stability technique - Thoracic Mobilization) .

Caption: Can you assess rib flare?

What is Rib Flare?

"Rib flare" has become a widely popularized clinical target among some groups of professionals in fitness, performance, and physical rehabilitation. This assessment has likely become the most popular among Pilates instructors, certain physical therapy groups, and practitioners of systems such as those taught by the Postural Restoration Institute (PRI) . In these contexts, rib flare is generally defined as a visual sign in which the anterior and lower ribs protrude anterior and laterally, resulting in an abnormally wide distance between the infrasternal angles (ISA). Proponents of this assessment theorize that a widened appearance of the ISA indicates that the internal obliques, transversus abdominis, and additional core musculature are long and underactive. This theory is often paired with the concept of the "Zone of Apposition (ZOA) ," a cylindrical/dome-shaped area from the diaphragm's insertion on the lower ribs to the roof of the diaphragm. A loss of the ZOA is blamed for a cascade of dysfunctions, including altered respiratory mechanics, core instability, and compensatory pain in the lumbar spine or shoulder girdle.

The definition of rib flare suffers from a critical lack of standardization. There is no research or normative data on the average distance between infrasternal angles, there is no research indicating a threshold between normal and abnormal distances (in fact, there may be no hypothesized threshold published anyhwere), nor is there a standardized methodology for measuring the distance between infrasternal angles (e.g., using a tape measurer to determine the absolute distance from the most proximal edge of the 10th rib and cartilage). Note that the ZOA is also poorly defined (as by PRI). It appears to be more of a hypothesis than an actual anatomical feature. Note that research has shown that pain and dysfunction, particularly of the lumbar spine, lead to changes in EMG activity of the diaphragm, pelvic floor, and intrinsic core muscles. This research is reviewed in our course "Intrinsic Stabilization Subsystem ." We make this point to concede that the hypothesized effect of dysfunction on core musculature is likely valid, but there cannot be a correlation between core muscle dysfunction and an assessment that lacks standardization.

The Proposed Interventions

Because the clinical assumption is that rib flare is a postural fault driven by muscular imbalance, proposed interventions attempt to reverse this imbalance. Practitioners typically prescribe corrective exercises designed to "pull the ribs down and in." These protocols emphasize forced, prolonged exhalations to engage the internal obliques and transversus abdominis, along with diaphragmatic breathing drills intended to restore the Zone of Apposition (ZOA) (Boyle, Olinick, & Lewis, 2010). Additionally, these programs often incorporate "proximal hamstring" activation to facilitate a posterior pelvic tilt, theoretically anchoring the abdominal fascia to pull the anterior rib cage downward (Waldron et al., 2020). However, these proposed interventions rely on several physiological and clinical inaccuracies:

• Unassessable Metrics: The ZOA is an internal anatomical relationship that cannot be directly or objectively assessed in a clinical setting.
• Physiological Impossibility: The concept of isolated "proximal hamstring activation" violates foundational exercise physiology; per the all-or-none principle of motor unit recruitment, muscles contract uniformly from origin to insertion.
• Unreliable Assessment: Proving that these interventions successfully resolved the dysfunction would require that the assessment of rib flare was reliable, accurate, and valid—a standard that a subjective visual evaluation of ribs, without standardization, wholly fails to meet.

Independent Motion of the Ribs

Proponents of the rib flare assessment often suggest that ribs possess independent kinematics, allowing them to rest in an altered "flared" position. While ribs do rotate at the costovertebral and sternocostal joints during respiration (pump-handle and bucket-handle motions), this movement is a dynamic, transient physiological process rather than a static postural state. In-vivo a 3D kinematic study demonstrated that the actual range of motion at these joints is remarkably small. For example, sternocostal joint motion during respiration results in a mean change of only 4.4° across the entire inspiratory capacity (Beyer et al., 2017). Once inhalation ends, passive ligamentous restraints and the elastic recoil of the lungs immediately return the ribs to a neutral resting position.

Clinically, conditions exist in which ribs exhibit altered movement independent of thoracic spinal motion, such as changes in costovertebral or sternocostal joint stiffness and resting position (sometimes referred to as subluxation). However, this pathology typically presents with acute, localized thoracic pain, intercostal muscle spasm, and an unwillingness to take a full breath. The minor positional change associated with a "stiff or subluxed rib" joint does not resemble a massive structural flare. Furthermore, it is highly improbable that a patient would present with this level of chronic dysfunction and also be pain-free. Therefore, a visible bilateral flare at rest cannot be attributed to isolated, painless rib joint kinematics.

The Biomechanical Absurdity of "Bending" Bones

If rib flare is not the result of altered joint kinematics, the assessment intrinsically assumes that resting muscle tone has permanently warped or "bent" the anterior ribs. Literature from impact and forensic biomechanics completely invalidates this assumption. Human ribs are highly rigid structures with an average elastic modulus of 13.9 GPa (Kemper et al., 2005). Causing a temporary elastic deflection of just 3 to 6 centimeters requires a direct, localized load, and the bone instantly recoils to its baseline shape the moment the force is removed (Schultz, Benson, & Hirsch, 1974). Pushing a human rib past its elastic limit to the point of permanent physical deformation requires a massive localized force ranging from 137 to over 250 Newtons per rib (Yoganandan & Pintar, 1998). In short, the muscular forces required to permanently deform a rib would physically break it.

Research confirms that heavy lifting combined with a Valsalva maneuver sharply increases intra-abdominal pressure, with the highest values recorded during squats (exceeding 200 mmHg) and deadlifts (161–176 mmHg), indicating that the core cylinder generates substantial internal compressive forces (equivalent to 15-30 kPa). However, this is not direct pressure; it is not pressure applied in a direction that bends a single rib. Further, postural and therapeutic exercises would result in far less indirect pressure than squats or deadlifts, and resting muscle tone during normal daily activities would result in significantly less pressure than that. It is a ridiculous notion, contrary to basic physiology, to think that therapeutic exercises and changes in resting muscle tone could increase the activity of the abdominal musculature to generate the continuous, localized point-loading required to permanently bend cortical bone back to an "unflared position."

To further elucidate the clinical absurdity of this concept, consider applying the logic to other movement faults. No one in fitness, performance, or physical medicine asserts that flat feet are caused by permanently bent metatarsals, nor is dynamic knee valgus attributed to the femurs physically warped inward. In those instances, bones are correctly recognized as rigid levers, and visible malalignment is attributed to changes to muscle activity and joint position. The rib cage must be viewed the same way. Ribs are rigid levers articulating with the thoracic spine. What clinicians visually assess as "rib flare" is nothing more than the entire rigid rib cage pivoting upward as a direct consequence of sagittal plane thoracic spinal extension. Without an objective way to differentiate thoracic extension from actual rib position, the visual assessment of "rib flare" is biomechanically useless, as there are more reliable ways to assess thoracic mobility.

Thoracic Motion Results in the Majority of Rib Motion

If structural deformation (bending of bone) and independent rib motion via the costovertebral and sternocostal joints are not the primary contributors to rib motion, and especially of the visual presentation of "rib flare," then what is actually occurring to result in something that looks like "rib flare"? Some of the motion can be attributed to normal respiration, although it was already noted that respiration produces only a relatively small amount of motion and is transient, lasting only until exhalation. What is most responsible for observed changes in rib position is actually nothing more than sagittal plane kinematics of the thoracic spine. This one you can actually try yourself. Put your hands on your infrasternal angle, and contract your abs as hard as you can (without moving your spine); you will notice very little motion. Take a deep breath in and out; you will notice some motion as the infrasternal angles expand and retract. Last, extend your thoracic spine (chest up), and you will notice a significant expansion of the infrasternal angles that you can relatively comfortably maintain.

Foundational biomechanical research demonstrates that the ribs and thoracic vertebrae are so inextricably coupled that they have to move together. This is due to the costovertebral joints being so stiff that they allow little motion (just 4 degrees). When the vertebrae move, the ribs are forced to follow. During thoracic extension, the superior vertebra glides inferiorly and posteriorly and rotates posteriorly. This spinal extension forces the articulating rib to rotate posteriorly at the costovertebral and costotransverse joints. Because the rib acts as a rigid lever, this posterior rotation at the proximal joint biomechanically mandates that the anterior aspect of the rib travels superiorly and anteriorly (Lee, 2015). Conversely, thoracic flexion forces anterior rotation of the vertebrae and ribs, which visually depresses or "closes" the anterior ribs.

Consequently, the physical presentation of a rib cage at rest is a reflection of thoracic position. The significant, observable "flare" or elevation of the anterior costal margins in a static posture is not an isolated rib impairment, nor is it a primary failure of the abdominal musculature to hold the bones down. Rather, it is a biomechanically mandated consequence of resting thoracic spinal position. This implies that a more direct approach to assessing and correcting postural dysfunction would involve assessing the spine and correcting spinal and pelvic alignment.

More Reliable Assessments for Thoracic Mobility

If the visual presentation of "rib flare" is primarily governed by the resting position of the thoracic spine, and thoracic spine assessment is more accurate, reliable, and valid, than assessment should focus on the thoracic spine. Attempting to infer spinal posture from the anterior cartilage of the costal margins introduces unnecessary confounding variables and, as mentioned above, relies on an assessment that lacks standardization and therefore cannot be reliable. Shifting the focus to the thoracic spine allows clinicians to use established assessments that have demonstrated reliability and accuracy in peer-reviewed, published research and have shown true clinical utility. Unlike the subjective visual estimation of the infrasternal angle, the following assessments provide objective, reproducible data:

• Tape Measure Excursion (Sagittal Plane Mobility) If a clinician hypothesizes that a patient is restricted in thoracic extension (the primary driver of the "flared" rib appearance), sagittal plane mobility can be objectively quantified using standard skin-surface tape measurements. Described extensively in foundational orthopedic texts (e.g., Norkin and Levangie), this method involves anchoring a tape measure to palpable bony landmarks—typically from the spinous process of C7 or T1 down to T12 or L1—while the patient is in a neutral standing or seated posture. The patient maximally flexes the spine, and the gross distance of skin distraction is measured in centimeters. This provides an objective metric that removes examiner visual bias and can be reliably retested to measure the efficacy of the intervention.
• Dual Inclinometry (Static Sagittal Posture): To measure static resting posture and determine whether a patient presents with an extended (or "flat") thoracic spine, inclinometers or modern smartphone clinometer applications offer vastly superior reliability compared to visual estimates. Foundational research establishes that using accessible inclinometers to measure resting thoracic kyphosis yields excellent intra-rater reliability (Macintyre & Asamoah, 2012). Furthermore, using gravity-dependent inclinometers—zeroed at the cervicothoracic junction (T1/T2) and measured at the thoracolumbar junction (T12/L1)—demonstrates strong intra-rater and inter-rater reliability for assessing seated and standing thoracic curvature (Takatalo et al., 2020). This instrumentation delivers an exact degree of spinal curvature, preventing clinicians from chasing the visual illusion of a widened infrasternal angle.
• The Lumbar-Locked Rotation Test (Transverse Plane Asymmetry) Practitioners often observe a unilateral "rib flare" and attribute it to complex, systemic rotational patterns. However, if structural asymmetry is the concern, transverse plane thoracic mobility should be tested directly. The clinical gold standard for this assessment is the lumbar-locked rotation test. By placing the patient in a quadruped position and instructing them to sit back onto their heels, the lumbar spine and pelvis are mechanically locked into maximal flexion, effectively isolating thoracic motion.
  • Reliability: Using an inclinometer placed between T1 and T2 to measure pure, isolated thoracic rotation yields excellent inter-rater reliability (Johnson et al., 2012).
  • Validity: Unlike the assessment of rib flare, this test possesses criterion-related validity. Recent research comparing the clinical lumbar-locked test directly with magnetic resonance imaging (MRI) demonstrates a significant positive correlation, indicating that it accurately reflects true physiological joint motion (Odagiri et al., 2024).
  • Standardized Visual Utility: Because the lumbar-locked position effectively standardizes patient setup and eliminates compensatory movement, evidence shows that even a purely visual assessment of this test can reliably discriminate between restricted and unrestricted mobility among clinicians with varying experience levels (Whetstone et al., 2024). This serves as the ultimate contrast to rib flare: visual estimation can possess clinical utility, but only if the assessment is strictly standardized and biomechanically sound.

No Correlation with Pain, Injury, Dysfunction, or Interventions

Despite theoretical biomechanical models linking rib flare (and the subsequent loss of the Zone of Apposition) to altered respiratory mechanics and musculoskeletal pain, there is a profound lack of empirical evidence to support these claims. In the broader orthopedic assessment literature, valid clinical signs, such as restricted ankle dorsiflexion or altered scapular kinematics, have been shown to be significantly correlated with specific joint pathologies. However, there are no large-scale prospective or retrospective studies demonstrating that a widened resting infrasternal angle is associated with a statistically higher risk of lower back pain, pelvic floor dysfunction, or rotator cuff pathology than a narrower angle. Without an established correlation in peer-reviewed research, labeling rib flare as a dysfunction or an injury risk factor is a theoretical assumption rather than an evidence-based diagnosis.

Because the assessment of rib flare lacks a standardized definition and established reliability, it is methodologically impossible to prove that any intervention successfully resolves it. That is, because the results of this assessment are undefined, trying to use it as a re-assessment after an intervention is impossible... what changed? There is no definition to describe the change. Consequently, there are no randomized controlled trials (RCTs) demonstrating that specific corrective exercise protocols, such as forced exhalation drills, diaphragmatic breathing, or targeted hamstring activations to drive posterior pelvic tilt, result in a realtively permanent change in the resting structural alignment of the costal margins. While a patient may consciously contract their abdominal musculature to temporarily narrow their infrasternal angle during reassessment, there is no high-level evidence that these interventions result in a permanent, unconscious change in the skeletal resting posture of the anterior ribs.

Caption: Is rib flare addressable? What are we fixing?

The Limited Research on Rib Flare as an Assessment

To fully evaluate the clinical utility of the rib flare assessment, it is necessary to examine the sparse body of research that attempts to measure or use it. The available literature highlights severe limitations in standardization and methodology.

First, there is a substantial discrepancy between the definition used by fitness and rehabilitation professionals and that used in the broader medical literature. For example, Wright et al. (2025) validated a highly reproducible radiographic metric, the "Rib Flare Distance," that measures posterior structural deformities in patients with adolescent idiopathic scoliosis. This represents a completely different anatomical concept than the bilateral, anterior infrasternal angle visually assessed in clinical practice.

Second, demonstrating that an angle can be reliably measured does not establish clinical validity. While Kim and Weon (2015) demonstrated that the resting infrasternal angle can be reliably measured with a standard goniometer, they did not establish normative data or investigate functional validity. Specifically, the researchers demonstrated that the angle can be measured but failed to determine a normal range or to establish any correlation between altered angles and pain, dysfunction, or injury risk.

Finally, the limited evidence supporting interventions based on rib flare assessments is of the lowest methodological quality. The available literature is restricted to a single-subject clinical case report detailing symptomatic improvement in a patient with rotator cuff pathology following Postural Restoration Institute (PRI) interventions (Waldron et al., 2020). Notably, while the case report mentions assessing "rib flare," the reported outcomes are strictly changes in symptoms, not measurable changes in the infrasternal angle itself. Because case reports lack control groups and randomization, they carry a high risk of confirmation bias and cannot be used to scientifically validate that assessing rib flare improves intervention selection across a broader population. Furthermore, the failure to demonstrate, or even report, post-intervention changes in the rib flare assessment should prompt profound skepticism about its clinical utility.

The Cascade of Psychometric Failure: Why the "Rib Flare" Assessment Lacks Clinical Utility

To understand why the clinical assessment of rib flare fails the standard of utility, it is helpful to contrast it with a functional observational screen that possesses robust empirical support: the Overhead Squat Assessment (OHSA) .

Psychometric Analysis: Rib Flare vs. The Overhead Squat Assessment Evaluating the clinical utility of the rib flare assessment requires a psychometric comparison against a validated functional screen, such as the OHSA.

• Lack of a Standard Definition: Clinical utility requires a standardized definition with established thresholds for optimal and dysfunctional. Defining rib flare as a posture where the lower ribs protrude "forward and upward" lacks objective quantification. Furthermore, even if the infrasternal angle is measured directly, no established normative data defines the threshold at which an angle becomes a "flare."
  • The OHSA Comparison: Conversely, OHSA movement faults are objectively defined. For example, medial knee displacement is explicitly defined as the patella tracking medial to the first metatarsophalangeal joint.
• Subjectivity: While research demonstrates the infrasternal angle can be reliably measured using a goniometer (Kim & Weon, 2015), its use in clinical practice relies almost exclusively on visual or manual estimation. Without standardization and instrumentation, the assessment defaults to an examiner's subjective, internal model of normal and abnormal.
  • The OHSA Comparison: The OHSA utilizes clear, dynamic visual anchors (e.g., arms not maintaining alignment with the trunk, loss of the medial longitudinal arch) that are distinct to the naked eye. This minimizes subjective interpretation and standardizes the screening process without requiring goniometry.
• Reliability: Subjective visual assessments lacking standardization are not reliable, as they rely entirely on an examiner's internal frame of reference and are susceptible to measurement error.
  • The OHSA Comparison: The OHSA demonstrates substantial intra-rater and inter-rater reliability (e.g., kappa coefficients >0.60 for identifying medial knee displacement) among trained professionals, mitigating the examiner bias inherent to visual rib flare evaluations (Post et al., 2017).
• Validity: A valid test accurately measures what it claims to measure. Because the visual assessment of rib flare is unreliable and the angle itself has no established correlation with dysfunction, the assessment results cannot be accurately attributed to specific neuromuscular or structural impairments.
  • The OHSA Comparison: The OHSA possesses strong construct, criterion, and ecological validity. Visually observed signs accurately reflect underlying kinematics, such as limited ankle dorsiflexion (Rabin & Kozol, 2017) and sex-specific differences in joint mobility (Mauntel et al., 2015). Furthermore, specific faults reliably predict underlying neuromuscular deficits; for example, medial knee displacement correlates with restricted gastrocnemius flexibility, weak ankle plantarflexors, and overactive hip adductors (Bell et al., 2008; Bell et al., 2012). Additionally, OHSA compensations demonstrate strong ecological validity by accurately reflecting kinematic deviations during high-impact dynamic tasks, such as treadmill running (Sever & Kır, 2023).
• Measuring Outcomes: Without standardization, reliability, and validity, clinicians cannot objectively determine the efficacy of an intervention plan upon reassessment (e.g., did the intervention result in a measurable change in rib flare?). Any perceived visual change in a resting rib angle cannot be objectively verified and is highly susceptible to confirmation bias, alterations in the transient respiratory phase, or compensatory changes in thoracic spine posture.
  • The OHSA Comparison: If an intervention to restore ankle mobility is successful, a subsequent OHSA will objectively demonstrate measurable reductions in specific signs (e.g., less foot turnout, less pes planus, less knee valgus, and less excessive forward lean). Furthermore, OHSA performance accurately predicts broader functional outcome measures, such as full Functional Movement Screen composite scores (Clifton et al., 2015), and the absence of poor movement quality during the squat is predictive of a lower incidence of non-contact lower-extremity injuries (Eckard et al., 2018).
• Clinical Utility: The primary function of a clinical assessment is to differentiate patient subgroups that would benefit from targeted intervention plans. If a test cannot differentiate optimal from dysfunctional or reliably measure physiological changes resulting from an intervention, it lacks clinical utility.
  • The OHSA Comparison: The OHSA successfully categorizes patients into specific neuromuscular subgroups (e.g., differentiating patients with an asymmetrical weight shift originating from the lumbopelvic hip complex from those with bilateral pes planus). Consequently, targeted interventions can be prescribed, and improvements in motion can be reliably objectively verified during subsequent OHSA reassessments.

Conclusion

Based on a comprehensive review of the available evidence and a rigorous psychometric evaluation, the clinical assessment of "rib flare" lacks reliability, scientific validity, and clinical utility. The foundational assumption that resting muscle tone can permanently deform cortical bone violates established biomechanical principles. Furthermore, transient changes in rib motion due to respiration cannot account for significant changes in static resting posture. Instead, what is visually perceived as a flared rib cage is primarily a kinematic consequence of thoracic spine extension.

Psychometrically, the assessment of rib flare fails fundamental clinical standards. It lacks a standardized definition, normative data, and objective measurement criteria, rendering it a highly subjective and unreliable visual estimation. Without an established construct or criterion validity linking the infrasternal angle to pain, injury, or dysfunction, this metric cannot be used to direct targeted interventions or evaluate intervention efficacy.

Evidence-based practitioners should abandon the subjective visual estimation of rib flare. Clinicians must rely on objective assessments that demonstrate true clinical utility in peer-reviewed literature. By utilizing psychometrically sound tools, such as the Overhead Squat Assessment (OHSA) and objective measurements of thoracic mobility (e.g., the lumbar-locked rotation test), professionals can accurately identify movement impairment, identify signs correlated with an increased risk of injury, and better recommend interventions that result in verifiable clinical changes in outcomes.

Caption: Thoracic mobility will result in larger changes in rib flare angle.

Annotated Bibliography

PRI Techniques

Boyle et al. presented a clinical commentary and an intervention strategy to establish the biomechanical rationale for addressing the Zone of Apposition (ZOA) using the 90/90 bridge with ball-and-balloon exercises. The proposed clinical application focused on treating patients using specific therapeutic exercises to optimize respiratory mechanics, lumbopelvic posture, and rib alignment. The intervention protocol included forced exhalation against resistance (a balloon), combined with proximal hamstring and abdominal activation, to facilitate a posterior pelvic tilt. Outcome measures included the theoretical restoration of the ZOA and a reduction in compensatory lumbo-pelvic extension. The paper provides a detailed biomechanical model for how these specific muscular activations theoretically pull the rib cage into a state of internal rotation and depression, indicating the foundational methodology utilized by many practitioners to treat "rib flare," though it serves as an intervention framework rather than an empirical trial demonstrating permanent structural changes to resting bone alignment.

• Boyle, K. L., Olinick, J., & Lewis, C. (2010). The value of blowing up a balloon. North American Journal of Sports Physical Therapy: NAJSPT, 5(3), 179.

Waldron et al. investigated a single patient with chronic rotator cuff pathology to present a clinical case report on the use of Postural Restoration Institute (PRI) interventions. The participant underwent conservative treatment focusing heavily on repositioning the rib cage to improve respiratory mechanics and alter subsequent scapular kinematics. The measurement protocol included the assessment of rib position and the infrasternal angle as dictated by the proprietary PRI system. Outcome measures included patient-reported pain reduction and functional improvements in shoulder mobility following the intervention. The findings demonstrated symptomatic improvement in this isolated subject, indicating a high risk of confirmation bias.

• Waldron, J. L., McKenney, M. A., Samuel, M. N., Girouard, T. J., Turner, C. L., & Radzak, K. N. (2020). The use of postural restoration for treatment of chronic rotator cuff pathology: A case report. International Journal of Sports Physical Therapy, 15(5), 832.

Rib Motion

Beyer et al. investigated asymptomatic subjects to quantify the precise in-vivo kinematics of the sternum, sternal angle, and anterior sternocostal joints during respiration. Participants underwent codified spiral computed tomography (CT) scans in a supine position to capture dynamic joint physiology across different lung volumes. The measurement protocol included processing 3D models of the ribs, manubrium, and sternum at functional residual capacity (FRC), middle of inspiratory capacity (MIC), and total lung capacity (TLC) to compute the exact spatial transformations and rotational axes of the anterior joints. Outcome measures included sternum cephalic displacement, sternal angle variations, and the exact degrees of rib rotation relative to the sternum. The findings demonstrated that while the anterior ribs and sternum possess independent kinematics for breathing, the actual joint variations are remarkably small (e.g., the sternal angle varies by a mean of only 4.4° across the entire inspiratory capacity). Note, this study likely indicates that the independent movement of the anterior ribs is relatively small, dependent on transient respiratory expansion during inspiration, and does not result in changes from static posture.

• Beyer, B., Feipel, V., Sholukha, V., Cheze, L., & Jan, S. V. S. (2017). In-vivo analysis of sternal angle, sternal and sternocostal kinematics in supine humans during breathing. Journal of Biomechanics, 64, 32-40.

Bending Ribs:

Kemper et al. investigated human cadaveric subjects to determine the baseline material properties and elastic modulus of human rib cortical bone. Specimens underwent dynamic tension testing to assess the absolute stiffness of the skeletal structures. The measurement protocol included extracting cortical bone from multiple ribs and applying controlled mechanical loads to failure. Outcome measures included Young’s Modulus (measured in Gigapascals), yield stress, and ultimate strain energy. The findings demonstrated that the human rib has an extremely high elastic modulus (averaging 13.9 GPa), indicating that while the bone possesses enough microscopic elasticity for natural respiration, it is a highly rigid structure.

• Kemper, A. R., McNally, C., Pullins, C. A., Freeman, L. J., & Duma, S. M. (2005). Material properties of human rib cortical bone from dynamic tension coupon testing. Stapp Car Crash Journal, 49, 199-230.

Schultz, Benson, and Hirsch investigated human cadaveric rib specimens to measure the exact force-deformation properties and mechanical limits of individual ribs. Specimens underwent dead-weight point-loading to assess their elastic deflection capabilities. The measurement protocol included fixing the head of each rib and applying localized loads of 0.75 kiloponds (~7.35 Newtons) to the antero-medial free ends in multiple directions. Outcome measures included the load-deflection response and the maximum displacement in centimeters achieved before the load was removed. The findings demonstrated that this highly concentrated load causes only a temporary, elastic deflection of 3 to 6 centimeters that instantly recoils back to baseline when released, indicating that a permanent, resting structural "flare" of the anterior ribs cannot be achieved without continuous, externally applied direct force.

• Schultz, A. B., Benson, D. R., & Hirsch, C. (1974). Force-deformation properties of human ribs. Journal of Biomechanics, 7(3), 303-309.

Yoganandan and Pintar investigated human cadaveric thoracic ribs to establish the exact biomechanical thresholds for permanent rib deformation and structural failure. Specimens underwent dynamic three-point bending tests to assess the peak forces required to permanently alter the shape of the bone. The measurement protocol included applying a concentrated impact directly to the isolated ribs at varied strain rates until the bone fractured. Outcome measures included the peak force (in Newtons) and absolute tissue deflection at the exact moment of structural failure. The findings demonstrated that pushing a human rib past its elastic limit to the point of permanent physical deformation requires a massive localized force (ranging from 137 to over 250 Newtons per rib depending on the age of the subject).

• Yoganandan, N., & Pintar, F. A. (1998). Biomechanics of human thoracic ribs. Journal of Biomechanical Engineering, 120(1), 100-104.

Muscle Tissue Force

Blazek et al. investigated the existing body of literature to summarize the intra-abdominal pressure (IAP) and intrathoracic pressure (ITP) initiated by the Valsalva maneuver during high-intensity resistance exercises. The researchers underwent a systematic search of major databases (PubMed, Scopus, Web of Science) to capture studies evaluating internal body pressures during heavy lifting. The measurement protocol included extracting data from 16 high-quality studies that utilized intra-rectal or intra-gastric transducers to measure internal pressures during exercises such as the squat, deadlift, and bench press. Outcome measures included the peak IAP and ITP (measured in mmHg) elicited by different loads and body positions. The findings demonstrated that heavy lifting combined with a Valsalva maneuver drastically spikes intra-abdominal pressure, with the highest values recorded during squats (exceeding 200 mmHg) and deadlifts (161–176 mmHg), indicating that the core cylinder generates massive internal compressive forces (equivalent to 15 to 30 kPa) to rigidify the trunk, stabilize the spine, and safely transfer load during athletic movements.

• Blazek, D., Stastny, P., Maszczyk, A., Krawczyk, M., Matykiewicz, P., & Petr, M. (2019). Systematic review of intra-abdominal and intrathoracic pressures initiated by the Valsalva manoeuvre during high-intensity resistance exercises. Biology of Sport, 36(4), 373-386.

How the Ribs Move

Lee investigated the existing body of literature and clinical models to summarize the specific biomechanics and coupled kinematics of the human thorax. The researcher underwent a comprehensive review of osteokinematic and arthrokinematic data regarding the thoracic spine, rib cage, and costovertebral joints during functional movements. The measurement protocol included synthesizing data on the exact translational and rotational patterns of the vertebrae and ribs during forward bending (flexion) and backward bending (extension) of the trunk. Outcome measures included the specific glide and roll mechanics of the costotransverse joints and the resulting displacement of the anterior ribs. The findings demonstrated that sagittal plane motion of the thoracic spine is inextricably coupled with rib motion; specifically, backward bending (thoracic extension) dictates a posterior rotation of the proximal ribs that drives the anterior aspect of the ribs superiorly, indicating that the visual elevation of the anterior costal margins (often misdiagnosed as an isolated "rib flare") is a direct, biomechanically mandated consequence of resting thoracic spinal extension.

• Lee, D. (2015). Biomechanics of the thorax–research evidence and clinical expertise. Journal of Manual & Manipulative Therapy, 23(3), 128-138.

Thoracic Spine Mobility Assessment:

Takatalo et al. investigated subjects with thoracic spine pain to determine the intra-rater and inter-rater reliability of clinical thoracic posture and mobility assessments. Participants underwent physical examination to assess sagittal plane spinal kinematics. The measurement protocol included utilizing gravity-dependent inclinometers and tape measurements to quantify static seated and standing thoracic kyphosis, as well as active sagittal plane excursions. Outcome measures included the reliability coefficients of the various clinical tools used to capture spinal curvature. The findings demonstrated very strong reliability for measuring static and dynamic thoracic posture using inclinometry and tape measures, indicating that clinicians already have access to highly reliable, objective instrumentation to measure the exact sagittal plane spinal extension that drives the visual illusion of a "flared" rib, rendering subjective visual assessments of the anterior costal margins entirely obsolete.

• Takatalo, J., Karppinen, J., Niinimäki, J., Taimela, S., Naylor, J. M., Mutanen, P., ... & Remes, J. (2020). Intra-and inter-rater reliability of thoracic spine mobility and posture assessments in subjects with thoracic spine pain. BMC Musculoskeletal Disorders, 21(1), 1-11.

Macintyre and Asamoah investigated asymptomatic subjects to determine the intra-rater reliability of measuring sagittal plane spinal posture using a simple, clinically accessible inclinometer. Participants underwent physical assessment by a clinician to measure resting thoracic kyphosis, lumbar lordosis, and straight leg raise. The measurement protocol included recording the thoracic angles in a relaxed standing position using an Isomed inclinometer over two separate sessions to reduce investigator memory bias. Outcome measures included intraclass correlation coefficients (ICC) and standard errors of measurement (SEM). The findings demonstrated excellent intra-rater reliability (ICC > 0.90) for the measurement of thoracic kyphosis, indicating that clinicians have access to highly reliable, objective instrumentation to accurately quantify the resting sagittal plane extension of the thoracic spine, further negating the need for subjective visual assessments of the anterior ribs.

• Macintyre, S., & Asamoah, E. (2012). Reliability of measuring thoracic kyphosis angle, lumbar lordosis angle and straight leg raise with an inclinometer. The Open Spine Journal, 4(1), 10-15.

Whetstone et al. investigated healthy adults to establish the reliability and discriminant validity of visually estimating thoracic rotation range of motion using the quadruped lumbar-locked position across clinicians of varying experience levels. Participants underwent bilateral active and passive mobility assessments by an expert clinician, a resident, and a physical therapy student. The measurement protocol included categorizing the patient's mobility as "Unrestricted" (>50°) or "Restricted" (<50°) via pure visual estimation, while a research assistant simultaneously measured the exact motion with a digital inclinometer to serve as the gold standard. Outcome measures included intra-rater reliability, inter-rater reliability, and percent absolute agreement. The findings demonstrated that visual estimation of the lumbar-locked rotation test has moderate to substantial test-retest and inter-rater reliability and can accurately discriminate between active and passive physiological deficits regardless of the rater's experience level, indicating that when the lumbar spine and pelvis are properly locked, even a rapid visual assessment of true thoracic rotation is significantly more reliable and clinically useful than a resting visual assessment of rib flare.

• Whetstone, K. S., Matsel, K. A., Patton, A. S., Gehres, S. E., & Schwartzkopf-Phifer, K. (2024). Reliability and Validity of Visual Estimation in Determining Thorax Rotation Mobility using the Quadruped Lumbar-Locked Position. International Journal of Sports Physical Therapy, 19(5), 583-591.

Odagiri et al. investigated healthy participants to establish the criterion-related validity of common clinical measurement methods for thoracic spinal rotation by comparing them directly against magnetic resonance imaging (MRI). Participants underwent thoracic rotation angle assessments in three functional positions: lumbar-locked, seated, and half-kneeling. The measurement protocol included assessing the rotation angles using an electronic goniometer and immediately comparing those clinical results with corresponding measurements obtained via MRI. Outcome measures included correlation coefficients and Bland-Altman analysis to detect measurement errors between the clinical tools and the imaging. The findings demonstrated a significant positive correlation between the thoracic rotation angle measured by the lumbar-locked test and the MRI, indicating that while clinical tests inherently include some minor compensatory movement from adjacent joints, the lumbar-locked test is a highly valid, evidence-based reflection of true thoracic spine mobility, providing the rigorous diagnostic validity that the rib flare assessment completely lacks.

• Odagiri, Y., et al. (2024). Assessing validity of thoracic spine rotation range of motion measurement methods: comparison of magnetic resonance imaging and clinical measurements. Journal of Physical Therapy Science, 36(3), 95-101.

Johnson et al. investigated healthy adults to determine the reliability of measuring the isolated thoracic spine rotation range of motion. Participants underwent physical assessment by examiners to assess transverse plane mobility using the lumbar-locked position. The measurement protocol included placing the patient in a quadruped position, having them sit back on their heels to mechanically lock the lumbar spine and pelvis, and using a digital inclinometer placed at the cervicothoracic junction (T1-T2) during active rotation. Outcome measures included intra-rater and inter-rater reliability scores (Intraclass Correlation Coefficients). The findings demonstrated excellent reliability for this method, indicating that when clinicians need to evaluate the structural asymmetry or rotational patterns often mistakenly attributed to unilateral "rib flare," the lumbar-locked test provides an objective, highly reproducible, and clinically validated alternative to subjective visual estimation.

• Johnson, K. D., Kim, K. M., Yu, B. K., Saliba, S. A., & Grindstaff, T. L. (2012). Reliability of thoracic spine rotation range-of-motion measurements in healthy adults. Journal of Athletic Training, 47(1), 52-60.

Somewhat Relevant to Rib Flare as an Assessment (The Best Research Available)

Kim and Weon investigated asymptomatic individuals to determine the reliability of clinical infrasternal angle measurements. Participants underwent a physical assessment of their lower rib cage by examiners. The measurement protocol included assessing the resting infrasternal angle using a standard goniometer, with the xiphoid process aligned to the left and right costal margins. Outcome measures included intra-rater and inter-rater reliability scores between the examiners. The findings demonstrated that baseline measurement reliability could be established under highly controlled conditions, but the researchers did not establish normative databases or investigate functional validity. That is, there is no standard angle or normative range that correlates with pain, dysfunction, or true clinical utility.

• Kim, M. H., & Weon, J. H. (2015). Intra-and inter-rater reliabilities of infrasternal angle measurement. Journal of Korean Physical Therapy, 27(3), 154-158.

Wright et al. investigated patients with adolescent idiopathic scoliosis to establish and validate a novel radiographic metric called the "Rib Flare Distance" (RFD). Participants underwent standing full-length radiography to assess axial plane rib cage deformities. The measurement protocol included assessing the distance from the most posterior aspect of the rib apex at the deformity to the posterior aspect of the adjacent vertebral body. Outcome measures included the intra- and inter-observer reliability and discriminatory ability of the new metric compared to standard clinical methods. The findings demonstrated that RFD is a highly reproducible and discriminative metric for posterior structural deformities, indicating a massive definitional discrepancy in the literature, as this posterior orthopedic deformity is a completely different anatomical concept than the bilateral, anterior infrasternal angle visually assessed by fitness and rehabilitation professionals.

• Wright, C. E., Budd, L. R., Boebel, G. G., Speller, N. C., Sane, E. S., Mohseni, M., ... & Montgomery, B. K. (2025). Rib flare distance: a novel and highly reliable radiographic metric of the rib cage deformity in adolescent idiopathic scoliosis. Spine Deformity, 1-8.

Waldron et al. investigated a single patient with chronic rotator cuff pathology to present a clinical case report on the use of Postural Restoration Institute (PRI) interventions. The participant underwent conservative treatment focusing heavily on repositioning the rib cage to improve respiratory mechanics and alter subsequent scapular kinematics. The measurement protocol included the assessment of rib position and the infrasternal angle as dictated by the proprietary PRI system. Outcome measures included patient-reported pain reduction and functional improvements in shoulder mobility following the intervention. The findings demonstrated symptomatic improvement in this isolated subject, indicating a high risk of confirmation bias, as case reports represent the lowest tier of evidence, lack control groups or randomization, and cannot be used to scientifically validate that assessing rib flare improves intervention selection across a broader population.

• Waldron, J. L., McKenney, M. A., Samuel, M. N., Girouard, T. J., Turner, C. L., & Radzak, K. N. (2020). The use of postural restoration for treatment of chronic rotator cuff pathology: A case report. International Journal of Sports Physical Therapy, 15(5), 832.

Overhead Squat Assessment: Reliability, Accuracy, and Validity

Post et al. investigated college students to determine the reliability and discriminative ability of observational screening for medial knee displacement during the overhead squat. Participants underwent video-recorded overhead squat assessments, which were evaluated by three certified athletic trainers on two different occasions. The measurement protocol included calculating kappa coefficients for intra- and inter-rater agreement and directly comparing observational classifications with 3D electromagnetic motion analysis. Outcome measures included intra-rater and inter-rater reliability scores, sensitivity, specificity, and the correlation between visual classifications and true kinematic displacement. The findings demonstrated substantial average intra-rater (0.70) and inter-rater (>0.60) reliability, alongside high specificity and a significant positive correlation with motion analysis (r=0.48), indicating that the overhead squat assessment is a highly reliable and valid observational tool for clinicians to accurately assess the presence of medial knee displacement when standardized screening guidelines are utilized.

• Post, E. G., et al. (2017). The reliability and discriminative ability of the overhead squat test for observational screening of medial knee displacement. Journal of Sport Rehabilitation, 26(1).

Rabin and Kozol examined healthy adults to assess the utility of the overhead squat and forward arm squat tests for screening limited ankle dorsiflexion range of motion. Participants underwent overhead squat and forward arm squat assessments, alongside bilateral weight-bearing and non-weight-bearing goniometric measurements of ankle dorsiflexion. The measurement protocol included classifying participants as having limited dorsiflexion (defined as falling more than 1 standard deviation below the sample mean) and evaluating the predictive accuracy of the squatting tests based on visual movement compensations. Outcome measures included sensitivity, specificity, and positive and negative likelihood ratios for detecting limited ankle mobility. The findings demonstrated that the overhead squat possessed excellent sensitivity (1.00) and a negative likelihood ratio of 0.00, indicating that the overhead squat is a highly valid and accurate initial screening tool; if a patient successfully performs the overhead squat without compensations, a clinician can confidently rule out limited ankle dorsiflexion range of motion.

• Rabin, A., & Kozol, Z. (2017). Utility of the overhead squat and forward arm squat in screening for limited ankle dorsiflexion. The Journal of Strength & Conditioning Research, 31(5), 1251-1258.

Mauntel et al. investigated matched pairs of healthy young adult males and females to determine the biomechanical and kinematic differences between the sexes during an overhead squat. Participants underwent overhead squat assessments using an electromagnetic motion-tracking system interfaced with a force platform. The measurement protocol included quantifying peak lower-extremity kinematics and kinetics during the descent phase of the squat, along with static range-of-motion measurements assessed with a standard goniometer. Outcome measures included peak joint angles (e.g., knee valgus, hip flexion, ankle dorsiflexion), ground reaction forces, and joint moments. The findings demonstrated that males displayed significantly greater peak knee valgus and hip flexion angles with less active ankle dorsiflexion, whereas females displayed greater ankle dorsiflexion and hip rotation, indicating that the overhead squat assessment is a highly accurate tool that reliably captures and reflects underlying, sex-specific differences in range of motion and joint kinematics.

• Mauntel, T. C., Post, E. G., Padua, D. A., & Bell, D. R. (2015). Sex differences during an overhead squat assessment. Journal of Applied Biomechanics, 31(4), 244-249

Bell, Padua, and Clark investigated healthy subjects to determine the specific muscle strength and flexibility characteristics of individuals displaying excessive medial knee displacement. Participants underwent overhead squat assessments to be visually categorized into a medial knee displacement group or a control group. The measurement protocol included assessing peak isometric force using a hand-held dynamometer and passive range of motion using a goniometer across the hip and ankle joints. Outcome measures included normalized peak isometric strength and passive flexibility values. The findings demonstrated that individuals with visually identified medial knee displacement exhibited significantly less ankle plantarflexion strength, greater hip external rotation and extension strength, and increased hip external rotation range of motion, indicating that the visual observation of medial knee collapse during an overhead squat is a valid predictor of underlying neuromuscular deficits, specifically pointing toward tight and weak ankle musculature.

• Bell, D. R., Padua, D. A., & Clark, M. A. (2008). Muscle strength and flexibility characteristics of people displaying excessive medial knee displacement. Archives of Physical Medicine and Rehabilitation, 89(7), 1323-1328.

Bell et al. investigated healthy adults to determine differences in range of motion, isokinetic strength, and muscle activity between individuals with and without excessive medial knee displacement during a double-leg overhead squat. Participants underwent double-leg squat assessments and were categorized into an excessive displacement group or a control group. The measurement protocol included assessing ankle and hip range of motion, hip isokinetic peak torque, and surface electromyography (EMG) of the gluteus maximus and adductor complex during the descending and ascending phases of the squat. Outcome measures included peak joint mobility, isometric strength, and normalized EMG activation amplitudes. The findings demonstrated that the excessive displacement group exhibited significantly decreased dorsiflexion range of motion (indicating gastrocnemius tightness) and significantly increased hip adductor muscle activation during the squat, indicating that the overhead squat assessment possesses strong construct validity, as visually observed movement faults accurately reflect true underlying muscle overactivity and joint restrictions.

• Bell, D. R., Vesci, B. J., DiStefano, L. J., Guskiewicz, K. M., Hirth, C. J., & Padua, D. A. (2012). Muscle activity and flexibility in individuals with medial knee displacement during the overhead squat. Athletic Training and Sports Health Care, 4(3), 117-125.

Sever and Kır investigated national-level long-distance runners to determine the association between selected biomechanical variables during an overhead squat assessment and those observed during treadmill running. Participants underwent 3D motion analysis while performing both the overhead squat and high-speed treadmill running at 16 km/h. The measurement protocol included the use of a 9-camera motion analysis system to capture and correlate the kinematic orientations of the ankle, knee, hip, pelvis, and torso across both tasks. Outcome measures included Pearson correlation coefficients comparing peak angles and joint positioning between the static assessment and dynamic running. The findings demonstrated significant positive correlations between overhead squat compensations and running mechanics (e.g., pelvic anterior tilt during the squat strongly correlated with anterior tilt during running foot strike and mid-stance), indicating that the overhead squat assessment possesses strong ecological validity and can accurately detect injury-related compensation patterns that transfer directly to high-impact athletic performance.

• Sever, O., & Kır, M. A. (2023). Overhead squat assessment reflects treadmill running kinematics. Journal of Men's Health, 19(4), 45-52.

Eckard et al. investigated incoming collegiate athletes to determine the predictive validity of the double-leg squat and single-leg squat for future lower extremity injury incidence. Participants underwent baseline movement screening utilizing the double-leg overhead squat and single-leg squat prior to their athletic seasons. The measurement protocol included grading the squats based on standardized movement faults (e.g., knee valgus, forward trunk lean) and tracking the athletes prospectively to record all non-contact lower extremity injuries during the season. Outcome measures included movement quality scores (poor versus non-poor movers) and unadjusted lower extremity injury incidence rates. The findings demonstrated that athletes classified as "poor movers" during squat assessments experienced a significantly higher incidence of lower-extremity injuries than "non-poor movers," indicating that the overhead squat and single-leg squat possess strong predictive validity and clinical utility for identifying athletes at heightened risk of musculoskeletal injury.

• Eckard, T. G., Padua, D. A., Mauntel, T. C., Frank, B. S., & Pietrosimone, B. G. (2018). Association between double-leg squat and single-leg squat performance and injury incidence among incoming NCAA Division I athletes: A prospective cohort study. Physical Therapy in Sport, 34, 192-200.

Clifton, Grooms, and Onate investigated collegiate athletes to determine if the overhead deep squat assessment could accurately predict composite scores on the full Functional Movement Screen (FMS). Participants underwent the full 7-test FMS battery, which includes the overhead deep squat as a primary component. The measurement protocol included scoring the overhead squat on a standardized 0-to-3 scale and comparing those individual scores against the total composite FMS scores. Outcome measures included predictive values, sensitivity, and specificity for identifying individuals who scored below the established injury-risk threshold (≤ 14 points). The findings demonstrated that athletes who scored less than a 2 on the overhead deep squat consistently achieved composite FMS scores of 14 or less, indicating that the overhead squat is a highly valid, time-efficient standalone assessment that accurately reflects total kinetic chain dysfunction and global injury risk without the need to perform a lengthy, multi-test screening battery.