Unstable Surface and Load Training
Research Review and Practical Applications for Human Movement Professionals
By Tristan J. Rodik, M.AT., ATC
Edited By Brent Brookbush, DPT, PT, COMT, MS, PES, CES, CSCS, ACSM H/FS
- Stability training for performance enhancement
- Stability training for rehabilitation
- Healing tissues
- Balance deficits
- Chronic ankle sprain
- Low back pain
- Special Note: Corrective Exercise
- Stability Training Research by Movement Pattern/Muscle Group:
|Key Points about Stability Training:|
Note on Corrective Exercise:
Lower Body Exercise Summary
Comparing squats, step-ups, lunges, Bulgarian split-squats, single-leg squats and single-leg deadlifts.
Upper Body Exercise
Barbell and Chest Press
- Resistance Training
Human movement professionals have a variety of ways to progress exercise. One method of progressing exercise is challenging stability by introducing unstable loads and/or surfaces. The objective of this course is to review all research pertaining to stability training, transparently present research findings, and develop reasonably objective conclusions about those findings for application in practice.
- Center of mass (CoM) - an object's mean position of mass; that is, a point that is perfectly surrounded by an equal amount mass in all directions.
- Base of support (BoS) - refers to the area beneath an object or person, and the area within the perimeter created by every point of contact that the object or person makes with the supporting surface. This may include the glutes of someone sitting on a chair, or the back of someone leaning against a wall.
- Equilibrium - A state in which opposing forces or influences are balanced.
- Balance - The ability to maintain a body’s center of mass over its base of support. Balance can be static or dynamic.
- Stability - the ability of a body to produce forces that will restore equilibrium when disturbed.
- Stability Training - An exercise or rehabilitation program designed to enhance the body's ability to stabilize; the ability to maintain/restore balance.
- Exercise Progression - Modification of an exercise that increases demand with the intent of promoting adaptation. This can be accomplished by increasing reps, load, tempo, range of motion, complexity, and/or challenge to an individual's stability.
- Experienced Exercisers - Participants in a study who have at least six months of exercise experience.
- Inexperienced exercisers - Participants in a study who have less than six months of exercise experience.
Panjabi (1) proposed a model of stability that separates the spine into three subsystems; neural, active and passive. Although this model was originally proposed to describe spine stability, the Brookbush Institute asserts that the model may be useful in describing stability throughout the human movement system. The neural subsystem is comprised of the central and peripheral nervous system and acts on the active subsystem (muscles) to modify stability. Akuthota et al. summarizes the interaction between the neural and active subsystems; stability of the spine is dependent on sensory input and muscular strength, which allow for constant feedback and sufficient force to refine movement (3). The passive subsystem is comprised of the vertebrae, discs and ligaments of the spine. The passive subsystem may be affected by the active subsystem, provides sensory input for the neural system; however, it functions primarily to provide structural support and limit end range motion. The three subsystems are part of the holistic human movement system and must be considered as interdependent. Panjabi states that a dysfunction of one subsystem will lead to one or more of the following possibilities: compensation from the other subsystem(s), long-term adaptation by the other subsystem(s), and/or an injury to the subsystem(s) (1, 2). Compensation by a subsystem from another subsystem may result in seemingly normal function, but long-term adaptation will result in altered stability (1). Injury to a subsystem may result in pain (1), muscle atrophy (4), passive system injury (e.g. ligament or disc injury) or altered motion. In the case of the spine, altered motion was in reference to changes in vertebral kinematics, such as spondylolisthesis. In summary, Panjabi's model provides a framework for describing how stability of the spine is maintained, and how dysfunction may alter stability and lead to pain. This model may aid in describing how stability is maintained at other joints, and provides a framework for considering the research discussed below.
Comerford and Mottram's Model
Another model of stability proposed by Comerford and Mottram (5) divides the muscular system into global and local systems. The local stabilization system is comprised of single joint muscles and controls segmental stabilization, and the global stabilization system is comprised of multi-joint muscles that control multi-segmental motion. Comerford and Mottram (5) assert that the systems are interdependent, but stability training may preferentially target local stabilizers. Example, the multifidus are local stabilizers whose activity is more affected by unstable environments, and the erector spinae are global stabilizers whose activity is more effected by load. If the local or global system becomes dysfunctional, then higher levels of stress or strain may occur resulting in tissue damage, dysfunction and pain (6). The Comerford and Mottram model of stability accurately predicts the changes in muscle activity that are seen in studies comparing unstable environments and loads. Further, the model aids in describing how dysfunction of one or both systems may increase stress, resulting in tissue damage and pain. Conceptually, the Comerford and Mottram model could be viewed as a division of tthe "active system" described by Punjabi.
- Punjabi's Model: Three Subsystems - passive (connective tissue), active (muscles) and neural (nervous system)
- Comerford and Mottram's Model: Two Subsystems - local muscles and global muscles
- The Comerford and Mottram's model could be viewed as a division of Punjabi's "active system".
- Both models describe the interdependent relationship of the body's systems, are predictive of how stability training may engage specific systems, and how dysfunction of one or more systems could result in tissue stress and pain.
Unstable Surface Training in a Rehabilitation Setting
Healing tissues (e.g. strains, ruptures, etc.):
Using unstable surface training may be beneficial in the rehabilitation setting for certain injuries that may be exacerbated by increases in force. A literature review by Behm et al. demonstrated that unstable surfaces reduced force output by an average of 29.3%; however, muscle activity was similar or greater when compared to stable surface training. This trend was especially true in novice exercisers (novice exercisers) (21). The reduction in force while maintaining muscle activity may be beneficial for conditions in which an increase in force would increase the risk of further tissue damage (e.g. muscle strain). That is, human movement professionals may be able to progress exercise from more to less stable, increasing motor unit recruitment, without increasing the force on healing tissues.
Research has also investigated unstable surface training in populations with assessed balance deficits. Elderly patients participating in a balance training program demonstrated improvements in balance tests, as well as the timed up-and-go test (22, 23). Note, an additional study has demonstrated that elderly participants without a balance deficit did not improve with an unstable surface training program (24). Research has also shown that children with balance deficits may benefit from unstable surface training, including children with visual impairments, cerebral palsy, Down's syndrome and autism (25-28). Again, further research demonstrates that children without a balance deficit do not demonstrate significant change on balance tests after an unstable surface training program (29-31). In summary, unstable surface training may improve balance and performance on functional tests for those with assessed balance deficits.
Unstable Surface Training for Chronic Ankle Instability
Research has demonstrated that adding balance exercises to a rehabilitation program for chronic ankle instability (CAI) improves functional outcome measures and objective scores on balance assessments (93-96). The tools used in these studies for functional outcomes measures include the Cumberland Ankle instability Tool (CAIT), Foot and Ankle Ability Measure (FAAM), Single Assessment Numeric Evaluation (SANE) or Global Rating of Change (GROC)) (93, 94, 96), and the objective balance scores were attained using the Star Excursion balance Test (SEBT) (94, 96). Kim and Cruz-Diaz et al. demonstrated that strength training with balance exercises elicits greater improvements in CAIT and SEBT scores when compared to strength training alone (93, 94). The balance training included marches, hopping, and unilateral/bilateral throwing exercises with the feet on an unstable surface. Although the balance training did not have an impact on self-reported pain or strength tests, patients demonstrated significant improvements in self-reported perception of function and objective functional outcomes (93, 94). Donovan et al. demonstrated that programs using unstable shoes (Myolux Athletik and Myolux II destabilization devices) and unstable surfaces (foam pad or balance disc) resulted in similar range of motion, self-reported pain, self-reported function and objective function outcomes in those with CAI (96). This study implies that unstable shoes do not provide additional benefit over more versatile and affordable unstable surface products. Interestingly, Kim et al. demonstrated that a 6-week rehabilitation program (balance, strengthening, plyometric and speed/agility exercises) reduced ankle eversion with walking and running, and inversion when landing from a jump in those with CAI (95). However, ankle eversion with walking and running returned to baseline values 24-weeks after exercises was discontinued (95). This implies that clients with CAI may need to continue rehabilitation programs to maintain kinematic improvements, even after function and pain have returned to baseline or better. In summary, research demonstrates that the addition of balance training to a rehabilitation program for CAI improves self-reported and objective functional outcomes, but does not improve strength or self-reported pain. Further, training must continue to maintain benefits beyond 6-weeks of training.
Unstable Surface Training for Low-back Pain
Research demonstrates that adding unstable surface exercises to a rehabilitation program for low-back pain (LBP) improves function, muscle strength, muscle endurance and range of motion outcomes (97-100). The tools used in these studies include the Oswestry Disability Index, visual analog pain scale, the Stork Balance Stand Test and the Beck Depression Inventory questionnaire (97-100). Javadian and Moon et al. demonstrated that a combination of unstable (stability ball and wobble board) and stable rehabilitation exercises elicits greater improvements in self-reported pain, self-reported function, lumbar range of motion, muscle endurance and strength compared to stable surface rehabilitation alone (97, 100). Kang et al. demonstrated that the prone and side plank, curl-up and glute-bridge on an unstable surface (Dynair® ball cushion) elicits greater improvements in self-reported pain, self-reported function, objective function, depression and low-back strength compared to a stable surface rehabilitation program (98). A study by Ko et al. implied that improvements in outcome measures may be equipment specific, as ground based unstable surfaces (stability ball, wobble board or Dynair ball cushion) provide additional improvements in outcomes compared to stable surfaces, but suspension training provided no additional benefit (99). Interestingly, programs using unstable surfaces alone do not appear to alter lumbar lordosis angle, lumbosacral angle and sacral inclination in those with LBP (99), implying that more specific techniques may be necessary to alter motion. Research demonstrates that rehabilitation programs for LBP using unstable surfaces improves function and reduces pain better than programs using stable exercise alone.
Corrective Exercise First:
There are a few studies investigating muscle activity during pushing that suggest isolated corrective interventions should precede multi-joint stability exercises. Unfortunately, similar studies could not be located for pulling or lower body movement patterns. Individuals with scapular dyskinesis and/or shoulder pain demonstrate increased electromyographic (EMG) activity of the upper trapezius and a decrease in serratus anterior activity during the push-up exercise on an unstable surface (16, 17). Additionally, a study by Park et al. revealed that individuals with scapular winging compared to controls, presented with significantly higher pectoralis major EMG activity and significantly lower serratus anterior EMG activity during a "push-up with plus" (protraction) (18). The decrease in serratus anterior and lower trapezius activity in these studies, is similar to the pattern noted in those with shoulder pain during arm elevation (19, 20). These studies imply that the addition of unstable environments may exaggerate the altered recruitment strategies noted in those individuals exhibiting dysfunction. This may imply that specific interventions should precede general strength training exercises using unstable surfaces or loads.
Stability Training for Performance Enhancement
Research has demonstrated that unstable surface training can increase sport and athletic performance. In novice exercisers, unstable surface training has been shown to improve lower-leg muscle activity during a jump-landing task, stair climbing performance, rate of force development, vertical jump height, and may have a beneficial effects on flexibility (7-9). Kibele et al. demonstrated that novice exercisers improved as much from unstable surface training as a traditional strength training program for sprint times, shuttle run times, standing long jump distance and single-leg hop performance (10). These studies imply that novice exercises may gain a variety of benefits from a balance training program, potentially as much benefits as noted with a traditional strength training program. Experienced exercises may require longer training periods to attain performance benefits from stability training. A study by Yaggie et al. demonstrated that four weeks of balance training improved shuttle run time, but did not increase vertical jump height (11). However, studies investigating longer training programs (6+ weeks) consistently resulted in performance improvements including vertical jump heights, throwing and max strength (12-15). Myer et al. demonstrated an additional benefit for experienced exercisers; balance training may reduce impact forces during a jump-landing (12). In summary, balance training may be as effective as traditional strength training for improving performance in novice exercisers, and experienced exercisers may require longer balance training programs (6+ weeks) to see performance gains.
- Unstable training may be beneficial for progressing exercise for individuals who may benefit from an increase in muscle activity without an increase in force output (e.g. strain, tendon repair, etc.).
- Unstable surface training may improve balance and performance on functional tests for those with assessed balance deficits.
- The addition of balance training to a rehabilitation program for chronic ankle instability improves self-reported and objective functional outcomes, but does not effect strength or self-reported pain.
- Research demonstrates that rehabilitation programs for low back pain (LBP) using unstable surfaces improves function and reduces pain better than programs using stable exercise alone.
- Stability training is unlikely to have a significant effect on altered kinematics.
- Individuals must continue balance training exercises, even after the resolution of symptoms, to maintain benefits beyond 6 weeks.
- There are a few studies that suggest unstable environments may exaggerate altered recruitment patterns in those exhibiting dysfunction. This may imply that specific interventions (e.g. serratus anterior activation) should precede multi-joint exercises using unstable surfaces or loads.
- Balance training may be as effective as traditional strength training for improving performance in novice exercisers.
- Experienced exercises may require longer balance training programs (6+ weeks) to see performance gains.
Stability Training for Performance Research by Movement Pattern/Muscle Group
Prime mover muscle activity during the barbell squat may not be affected by unstable surfaces or loads. Prime movers are defined as the quadriceps and gluteal complex (gluteus maximus and gluteus medius), and research has investigated various unstable surfaces including foam pads, Reebok Core® Boards, balance boards, BOSU® balls and balance cones (32-36). Most research investigating vastus medialis and vastus lateralis activity demonstrate that training surface does not have a significant effect on activity (33-36). However, a study using novice exercisers found a significant increase in vastus medialis and vastus lateralis muscle activity when a foam pad was added to isometric squats (32). This study could be compared to a study using experienced exercises and a similar foam pad, demonstrating no change in muscle activity (35). This may imply novice exercisers exhibit an increase in EMG activity to accommodate unstable surfaces, however experience reduces the need for this increase (32, 35). Only one study was found comparing the glute complex activity of experienced lifters on stable and unstable surfaces, and no significant difference in muscle activity was noted (34). In summary, research suggests unstable surfaces have little effect on Prime mover muscle activity for experienced lifters, but may increase muscle activity in novice lifters. More research is needed to determine how unstable surfaces contribute to the increases in performance noted above; for example, increased coordination, balance, reaction time.
Stabilizer muscle activity during a barbell squat may be affected by unstable loads, but probably not unstable surfaces. Research has investigated lower-extremity stabilizers including the bicep femoris, tibialis anterior, gastrocnemius and soleus, as well as core muscles including the rectus abdominis, external obliques, erector spinae, and multifidus (33-38). A study by Lawrence et al. demonstrated that weight plates suspended from a barbell by bands resulted in a significant increase in external oblique, rectus abdominis and soleus muscle activity (37), and Ditroilo et al. demonstrated that squats using a water-filled tube significantly increased external oblique and multifidus muscle activity (38). The research studies mentioned above also demonstrated that stabilizer and core muscle activity were not significantly affected by unstable surfaces (33-36). One study by Saeterbakken et al. demonstrated an increase in soleus activity using a BOSU® ball during a squat, but the increase in activity did not reach clinical significance (36). These studies suggest that unstable loads have a much larger effect on Stabilizer and core muscle activity then unstable surfaces, and if the goal is an increase in muscle activity then unstable loads may be preferred. Again, more research is needed on unstable surface and balance training to determine the factors contributing to the increases in performance.
- Adding unstable surfaces to a squat may increase prime mover activity for inexperienced lifters.
- Adding unstable surfaces to a squat may not alter EMG activity for stabilizers or prime movers in experienced lifters.
- Adding unstable loads to a squat significantly increases stabilizer and core muscle activity for experienced and inexperienced lifters.
- More research is needed to determine how unstable surfaces contribute to the increases in performance noted above; for example, increased coordination, balance, reaction time.
Comparing the Squat, Lunge and Bulgarian Split-Squat
Muscle activity during squats has been compared to the lunge and Bulgarian split-squat (35, 39-43). Stuart et al. compared Muscle activity during squats and lunges using a 50 pound barbell; demonstrating higher quadriceps and hamstring Muscle activity during the lunge (39). However, a study by DeForest et al. demonstrated that when lunges were performed with half the weight used for the squat, lunges exhibited similar Muscle activity for the quadriceps and lower activity for the biceps femoris (40). In studies comparing squats to the Bulgarian split-squat, EMG activity for the vastus medialis and vastus lateralis were similar (40, 41), even when performed on an Airex pad (unstable surface) (35). Squat, lunges, and Bulgarian split-squats demonstrated similar erector spinae activity, however, the Bulgarian split-squat elicited higher external oblique, gluteus medius and gluteus maximus activity (35, 40-43). Biceps femoris activity was similar for the Bulgarian split squat and squats when using heavier external loading for the squat, but when using loads of 3-10 repetition maximus (RM) for each exercise, more biceps femoris activity was exhibited during the Bulgarian split squat (35, 41-43). One study by DeForest et al. (40) compared Muscle activity during the Bulgarian split-squat and lunges, with the Bulgarian split-squat requiring similar quadricep Muscle activity and higher hamstring Muscle activity. These studies imply that careful attention to the load used during the study is important for developing sound conclusions. Prime mover activity is likely similar during all exercises when loads are adjusted to ensure failure in the same rep range. Hip and trunk stabilizer Muscle activity is higher during lunges and Bulgarian split-squats when compared to squats. Bulgarian split squats exhibited the most biceps femoris activity, and the lunge exhibited the least, which may imply Bulgarian split-squats should be avoided due to the propensity of the biceps femoris to be over-active in those individuals exhibiting signs of Lower-Extremity Dysfunction (LED). In summary, the reduction in biceps femoris activity and increase in core and hip Muscle activity during lunges may imply this is an ideal progression from squats.
Comparing the Squat, Step-up and Single-leg Squat
Muscle activity during the squat has also been compared to the step-up and single-leg squat (44, 45). These exercises are discussed separate from the Bulgarian split-squat and lunge, because the step-up and single-leg squat are truly unilateral; performed on one limb without support from the other. When no external load is added, the step-up requires twice as much glute complex activity as the squat, and the single-leg squat requires significantly more glute complex activity than the step-up (44). When a barbell was used to add external load to the squat and single-leg squat, the squat exhibited similar gluteal complex and semitendinosus activity, but higher quadricep, erector spinae and bicep femoris Muscle activity (45). It is possible that during these loaded studies the gluteal complex reached a recruitment threshold, that reduced the differences noted when larger loads were used (44, 45). Interestingly, band resisted abduction around the knee increased gluteal complex muscle activity during the unweighted squat, but decreased gluteal complex muscle activity during the step-up and single-leg squat (44). This is likely due to the step-up and single-leg squat being open chain movements, and the gluteal complex being unable to contribute to abduction when the pelvis was not fixed (by the other leg on the ground). That is, the only means of resisting a valgus force at the knee during an "open-chain" lower extremity exercise is ankle eversion, because a planted foot on the ground creates a stable base to "push" from. In summary, an increase in gluteal complex muscle activity may be achieved by progressing from squats to step-ups to single-leg squats; however, difference in EMG activity decrease when external loads are used. Further, band around the knees resisted abduction only increases gluteal complex activity during close chain activities.
Comparing the Unilateral Lower Body Exercise on Stable and Unstable Surfaces
EMG activity has been compared during various unilateral lower-extremity exercises, on stable and unstable surfaces (35, 46, 47). Krause et al. demonstrated EMG activity during a body-weight lunge was 9 - 22% maximal voluntary isometric contraction (MVIC) for the rectus femoris, hamstrings, gluteus medius, gluteus maximus and adductor longus (46). Another study by Krause et al. demonstrated EMG activity during a body-weight single-leg squat was 50% MVIC for the gluteus medius (47). These same studies demonstrated that when an unstable surface was introduced, %MVIC increased for the lunge, but not for the single-leg squat (46, 47). A study by Andersen et al. reported that %MVIC for a loaded Bulgarian split squat was 140% MVIC for the bicep femoris, rectus abdominis and external oblique, and between 70-90% MVIC for the rectus femoris, vastus medialis, vastus lateralis and erector spinae (35). The much higher %MVIC in these studies is likely due to the use of external load; however, it is interesting to note that adding an unstable environment to the loaded Bulgarian split-squat actually reduced %MVIC of the erector spinae and biceps femoris. The reduction in activity when adding an unstable environment is likely due to an inability to produce the same amount of force, i.e. lift the same load, or lift the same load at the same speed. In summary, progression from a lunge to single leg squat may increase glute complex activity, adding an unstable surface to body-weight exercise is likely to increase EMG activity; where as, adding unstable surfaces to loaded exercise may reduce the capacity to produce force. Consideration of the phase of training and the client's goal may dictate whether stability or load is the appropriate progression. (Note, the Bulgarian split squat was omitted for the reasons mentioned above).
Muscle activity during the single-leg deadlift has been compared to the lunge, step-up and single-leg squat. Single-leg deadlifts elicits similar glute activity, but higher hamstring activity than the step-up or lunge (48, 50), and elicit similar hamstring and glute activity but less quadriceps activity than the single-leg squat (50). These findings should not be surprising, as the larger moment arm created by the trunk during deadlifts requires more recruitment of hip extensors, but the lack of significant knee flexion reduces the amount of quadriceps recruitment necessary at the knee. Boren et al. demonstrated that single-leg deadlifts elicited less glute activity than the single-leg squats (49); however, this is likely due to the study allowing the non-weight bearing leg to extend behind the individual and act as a counter-balance. Note, the Brookbush Institute does not recommend allowing the non-weight bearing leg to extend posteriorly, as it reduces load and may contribute to compensation. The findings of these studies are congruent with practical application which suggests the "level of difficulty" progresses from step-up, to lunge, to unilateral exercise like the single leg deadlift and single-leg squat, and further that deadlifts place greater emphasis on hip extensor strength.
- The reduction in biceps femoris activity and increase in core and hip muscle activity may imply lunges are a better progression from squats than Bulgarian split-squats.
- Gluteal complex and core muscle activity increases as you progress from squats to step-ups to lunges to single-leg squats/deadlifts; however, difference in EMG activity decrease when external loads are added.
- Single-leg deadlifts increase recruitment of the hamstrings and glutes, but reduce quadriceps activity when compared to squats to step-ups to lunges.
- Adding an unstable environment to body-weight exercise is likely to increase EMG activity; where as, adding unstable environments to loaded exercise may reduce the capacity to produce force.
- Band resisted abduction around the knees only increases gluteal complex activity during squats.
Single Leg Touch Down with Posterior Pull
The Push-up on Stable and Unstable Surfaces
Muscle activity has been compared during push-ups on stable and unstable surfaces (51-75). The unstable surfaces investigated include the wobble board, suspension trainer, stability ball and BOSU® ball (51-75). Push-ups on unstable surfaces resulted in greater Muscle activity for prime movers (pectoralis major, anterior deltoid and triceps brachii), and core muscles (rectus abdominis, internal and external oblique, erector spinae and lumbar multifidus )(61, 64-68, 70-75). Latissimus dorsi Muscle activity was only greater during the push-up on a suspension trainer, and only if the arms were adducted below 90 degrees (74).
Serratus anterior activity was greater during push-ups on unstable surfaces, or with increased upper body load (feet elevated) (61, 62, 67, 69, 70, 73). The unstable surfaces that increase serratus anterior activity include the wobble board, BOSU® ball (hands and feet) and suspension trainer with the feet elevated to the same height as the hands (61, 62, 67, 69, 70). Two studies found that serratus anterior activity was greater on a stable surface when compared to having hands on a suspension trainer, wobble board, or feet elevated, but these studies included multiple push-up variations in which fatigue may have influenced muscle activity (63, 68). Torres et al. (77) found that anterior and posterior deltoid muscle activity was greater during the push-up on a stable surface compared to the push-up with the hands and feet on a BOSU® ball. Conversely, a study by de Araujo et al. demonstrated that upper trapezius muscle activity was greater during the push-up with the hands on a wobble board (61). These studies demonstrating a decrease in serratus anterior activity, decrease in deltoid muscle activity, and increase in upper trapezius muscle activity match the altered activity exhibited by those with shoulder dysfunction (101). This may be further evidence of unstable surfaces exaggerating altered recruitment patterns, as discussed above. In summary, adding instability or changing body position during a push-up increases upper body muscle activity; however, more research is needed to investigate the potential for the increased activity to exaggerate compensation patterns.
Chest Press on Stable and Unstable Surfaces
Muscle activity has been investigated for the chest press with stable and unstable surfaces and loads (76-86). The unstable surface tested was torso on a stability ball (76-82), and the unstable load tested was weights suspended from a BandBell™ Earthquake Bar or barbell (83-86). Lawrence et al. demonstrated that prime mover Muscle activity was greater during a chest press with a stable load, then a chest press with an unstable load when performing 1-set of 5-repetitions (84, 86); however, multiple sets resulted in similar prime mover Muscle activity for stable and unstable loads (83, 85). Similarly, the barbell chest press on a stable surface resulted in more prime mover Muscle activity when compared to a barbell on an unstable surface (77). (Note, performing barbell presses on a stability ball is not recommended). These studies may imply that prime mover activity is greater for stable exercises when loads are attempted that could not be performed with unstable loads or surfaces; however, unstable loads may be appropriate when loads are low to moderate. The dumbbell chest press requires similar prime mover Muscle activity when the torso is on a stable or unstable surface (80, 82), and anterior trunk Muscle activity is similar during high loads regardless of surface type (78, 80, 81). However, anterior trunk Muscle, and potentially prime mover activity is greater on an unstable surface when loads are low to moderate (76, 77, 79, 82). Again, this implies that anterior trunk Muscle activity is similar when loads are high, but more anterior trunk activity is noted when low to moderate loads are performed on an unstable surface. The chest press with unstable loading results in greater biceps brachii, middle trapezius and middle deltoid Muscle activity (84-86). In summary, a trend is noted toward increased prime mover and core Muscle activity with unstable surfaces and loads when load is low to moderate. prime mover and core Muscle activity may be higher on stable surfaces using high loads, likely due to the increase in load that can be lifted.
The Overhead Press Exercise on Stable and Unstable Surfaces
Research has compared EMG activity with unstable loads and unstable surfaces, during an overhead press (80, 81, 87, 88). Research by Williams et al. demonstrated that an overhead press with weights suspended from a Bandbell® Earthquake bar (unstable load) elicited greater pectoralis major, scapular stabilizer (serratus anterior and rhomboids), trunk (rectus abdominis and erector spinae) and antagonist (latissimus dorsi and bicep) muscle activity when compared to stable loads (88). Prime mover (deltoid and tricep), external oblique and rotator cuff muscle activity was similar when using the same weight with stable and unstable loads (88). However, Kohler et al. (87) demonstrated that weights too heavy to be performed with unstable loads, resulted in the greatest Prime mover and external oblique muscle activity. This implies that stabilizer muscle activity increases with unstable loads, Prime mover and external oblique muscle activity are similar when similar weight is used for stable and unstable loads, and Prime mover activity may be greatest when performing stable loads that could not be performed with unstable loads.
Stable and unstable loads with moderate to heavy weight elicited large amounts rotator cuff muscle activity in all condition (88). This may imply a "threshold" activation level that was reached, in which no higher level of activity can be achieved without the significant increases in load seen during stable 1-5 rep max (RM) training. This tendency is seen throughout the body of research comparing stable and unstable exercise, and may imply that an increase in muscle activity is only one of several methods the body uses to accommodate stable/unstable training.
Studies by Kohler et al. (87), Uribe et al. (80) and Lehman et al. (81) compared dumbbell overhead press seated on a stability ball to sitting on a stable surface. The studies demonstrated similar muscle activity from prime movers and anterior trunk muscles (rectus abdominis and external oblique), with the exception of the study by Kohler et al. that noted small differences in rectus abdominis and erector spinae activity (87). Based on these findings it is likely that unstable surfaces do not have as large an effect on muscle activity as unstable loads do.
The Inverted Row with Stable and Unstable Environments
Muscle activity has been investigated during the inverted row with a barbell and suspension trainer, and with the feet on stable and unstable surfaces (65, 89-92). Research suggests that Muscle activity increases when hanging from an unstable apparatus like the TRX® suspension trainer, but unstable surfaces for the feet have little effect on Muscle activity. Prime mover (latissimus dorsi, posterior deltoid and middle trapezius) Muscle activity is similar when hanging from a barbell or TRX®, but only when the load is decreased by bending the knees to 90º (feet flat on the floor) (90, 91). McGill et al. (89) demonstrated that Prime mover Muscle activity is higher with the TRX® row, when the load is increased by straightening the legs. This implies that Prime mover activity is greatest when higher loads can be done on unstable apparatus. A potentially counter-intuitive finding, biceps brachii Muscle activity was higher with a pronated grip when using a barbell (90, 91), but was highest with a supinated grip when using a TRX® (91). Further, anterior trunk Muscle activity was greater during the TRX® row (65, 89), but erector spinae Muscle activity was greater during a hanging barbell row (89). An interesting finding, the primary scapular stabilizer - serratus anterior exhibited similar activity in stable and unstable environments in novice exercisers (65), but exhibited more activity during a TRX® row in experienced exercises (89). Although the study findings appear to suggest complex relationships, there does appear to be a general trend toward prime movers exhibiting higher activity in stable environments with larger loads, and stabilizing muscles exhibiting more activity in unstable environments. The only exception may be that Prime mover activity can be greater if large loads can be performed in less stable environments. Youdas et al. is the only study that could be found comparing feet on a stable and unstable surfaces (92). This study demonstrated that Prime mover, scapular stabilizer and anterior trunk Muscle activity were similar during the inverted row with one foot on the ground or feet on a BOSU® ball (92). In summary, unstable loads will increase stabilizer Muscle activity, and may increase Prime mover Muscle activity if load is sufficient. Placing the feet on unstable surfaces does not seem to have an effect on Muscle activity.
Only a couple of studies were located comparing additional back/pulling exercises and muscle EMG activity. Fenwick et al. and Saeterbakken et al. demonstrated that standing unilateral rows increased activity of muscles involved in rotational control of the lumbar spine when compared to standing bilateral rows (102, 103). Further, Fenwick et al., comparing 3 exercises, demonstrated that bent over horizontal rows resulted in the greatest upper back muscle activity, suspension rows resulted in the greatest latissimus dorsi and gluteus maximus activity, and again, unilateral standing rows resulted in the greatest activity for muscles involved in rotational control of the spine (e.g. external obliques) (102). The bent-over horizontal rows also resulted in the greatest erector spinae activity, likely due to the large moment arm on the lumbar spine and forward bending position; where as, the suspension rows resulted in the least erector spinae activity and most gluteus maximus activity (102). These findings support two reccomendations made by the Brookbush Institute. Bent-over rows should likely be avoided unless their is a goal specific reason the exercise is better than other options, and suspension rows are a wonderful recommendation for those individuals recovering from low back pain and returning to a strength training program.
Upper Body Exercise Summary
- Instability and changing body position (elevating fee) during a push-up increases upper body muscle activity.
- Some research suggests that unstable surfaces/loads may exaggerate compensation patterns correlated with dysfunction; implying corrective interventions should precede stability training.
Barbell and Chest Press
- Prime mover activity is greater for stable exercises when loads are attempted that could not be performed with unstable loads or surfaces; however, unstable loads may be appropriate when loads are low to moderate.
- Core muscle activity is similar for unstable and stable surfaces when loads are high, but greater core muscle activity is noted when unstable surfaces are compared to stable surfaces and loads are low to moderate.
- Unstable loading during a chest press may increase the activity of ancillary muscles including the biceps brachii, middle trapezius and middle deltoid.
- Unstable loads have a larger impact on prime mover muscle activity than Unstable surfaces.
- Unstable loads may increase muscle activity of antagonists, scapular stabilizer and trunk muscles.
- Heavier loads only possible on stable surfaces elicit the greatest prime mover muscle activity.
- Rotator cuff muscle activity is similar in all condition, due to high levels of recruitment throughout (88).
- Unstable loads will increase activity of stabilizing muscles.
- Prime mover muscle activity may be highest when high loads can be performed from an unstable apparatus.
- Placing the feet on unstable surfaces does not have an effect on muscle activity.
- Inverted rows are helpful for those recovering from low back pain, as these progressions result in smaller amounts of posterior to anterior force on the lumbar spine and lower erector spinae activity then many other back exercises (102).
The Front-Plank and Side-Plank on Stable vs Unstable Surfaces:
Muscle activity during the prone- and side-plank have been compared on stable and unstable surfaces (51-56). Two studies demonstrated similar Muscle activity for the prone-plank, prone-plank with feet on a stability ball, and prone-plank with forearms on a BOSU® ball (53, 56). However, an increase in rectus abdominis and external oblique activity was noted during more challenging progressions including prone-plank with forearms on a suspension trainer or forearms on a stability ball (52, 55, 56). Czaprowski et al. demonstrated forearms on a stability ball increased activity of the internal oblique and transverse abdominis (56), while Lehman et al. investigated only the internal obliques for the same exercise and demonstrated no change (55). This may imply this progression increases transverse abdominis activity, but not internal oblique activity. Imai et al. demonstrated an increase in the activity of several core muscles when a prone-plank was performed with forearms on a stability ball and feet on a balance disc (54); however, this progression was compared to prone-plank on the floor. There does appear to be a limit to the amount of additional Muscle activity that can be expected from adding unstable surfaces; a study by Byrne et al. demonstrated that a prone-plank with forearms in a suspension trainer resulted in greater Muscle activity than forearms on the floor, but putting the forearms and feet in a suspension trainer did not increase activity more than forearms in a suspension trainer (52). The study by Imai et al. also demonstrated that side-planks require more rectus abdominis Muscle activity than prone-planks (54). Further, a side-plank with forearms on a BOSU® ball, or foot on a BOSU® ball and forearms on a balance disc, increased rectus abdominis and external oblique activity more than a side-plank on a stable surface (54, 56). In summary, research suggests that side-planks require more rectus abdominis recruitment than the prone-plank, that a significant stability challenge is required to increase core Muscle activity (forearms on stability ball versus feet on stability ball), and there may be a limit to the increase in Muscle activity that may be expected from continuing to progress stability.
The Glute-Bridge on Stable vs Unstable Surfaces
Muscle activity has been compared during the glute-bridge and single-leg glute-bridge on stable and unstable surfaces (54-60). Experienced exercises exhibit similar muscle activity during a glute-bridge with feet on a stability ball when compared to feet on a stable surface (55). However, inexperienced exercisers exhibit greater Muscle activity with feet on a stability ball or Aero-step pad when compared to a stable surface (57). Note, inexperienced exercises did not exhibit an increase in muscle activity when glute-bridges were done with feet on a BOSU® ball (54, 56). This may imply that Muscle activity of experienced exercises is not effected by unstable surfaces; however, Muscle activity of inexperienced exercisers increases when unstable surfaces are sufficiently challenging. Several studies have demonstrated greater gluteus maximus and internal oblique Muscle activity during a single-leg glute-bridge when compared to a bilateral glute-bridge (58-60). Further, two of these studies also demonstrated a significant increase in gluteus medius and hamstring Muscle activity during the single-leg glute-bridge (59, 60). Interestingly, Youdas et al. (59) found that gluteus maximus Muscle activity decreased and hamstring Muscle increased when unstable surface was added to a single leg bridge (59). This may imply that this progression is not recommended due to the decrease in prime mover activity, and increased activity of a Muscle prone toward over-activity in those exhibiting dysfunction. Ekstrom et al. (60) found that rectus abdominis Muscle activity was similar when comparing the single-leg glute-bridge to a bilateral glute-bridge with the arms on the ground (60), and Yoon et al. found that rectus abdominis Muscle activity was greater during the single-leg glute-bridge with the arms folded across the chest (58). These studies suggest that inexperienced exercisers may benefit from sufficiently challenging unstable surfaces, that all exercises may benefit from progressing to a single-leg bridge (without an unstable surface), and that folding the arms across the chest may further increase core Muscle activity.
Core Exercise Summary
- Side-planks require more rectus abdominis recruitment than prone-planks
- A significant stability challenge is required to increase core muscle activity (forearms on stability ball versus feet on stability ball)
- There is a limit to the increase in muscle activity that may be expected from increasing instability.
- Inexperienced exercisers exhibit increased muscle activity during glute bridges with sufficiently challenging unstable surfaces.
- Progressing a bilateral glute bride to a single-leg bridge (without an unstable surface) will increase muscle activity.
- Folding the arms across the chest during a single-leg glute bridge may further increase core muscle activity.
- Gluteus maximus muscle activity decreased and hamstring muscle increased when unstable surface were added to a single leg bridge. This may imply that this progression is not recommended due to the tendency for the hamstrings to become over-active.
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- Local and Global Stability Systems
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- Stability Training for Rehabilitation:
- Corrective Exercise First:
- Stability Training for Rehabilitation:
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- Unstable Surface Training
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- The Glute-Bridge on Stable vs unstable Surfaces
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- The Push-up on Stable and Unstable Surfaces
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- The Chest Press Exercise on Stable and Unstable Surfaces
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- The Inverted Row with Stable and unstable Environments
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- Balance Training for Chronic Ankle Instability
- Kim, K. J., Kim. Y. E., Jun, H. J., Lee, J. S., Ji, S. H., Ji, S. G., Seo, T. H. and Kim, Y. O. (2014) Which treatment is more effective for functional ankle instability: strengthening or combined muscle strengthening and proprioceptive exercises? Journal of Physical Therapy Science, 26, 385-388
- Cruz-Diaz, D., Lomas-Vega, R., Osuna-Perez, M. C., Contreras, F. H. and Martinez-Amat, A. (2015) Effect of 6 weeks of balance training on chronic ankle instability in athletes: a randomized controlled trial. International Journal of Sports Medicine, 36, 754-760
- Kim, E., Choi, H., Cha, J. H., Park, J. C. and Kim, T. (2017) Effects of neuromuscular training on the rear-foot angle kinematics in elite women field hockey players with chronic ankle instability. Journal of Sports Science and Medicine, 16, 137-146
- Donovan, L., Hart, J. M., Saliba, S. A., Park, J., Feger, M. A., Herb, C. C. and Hertel, J. (2016) Rehabilitation for chronic ankle instability with or without destabilization devices: a randomized controlled trial. Journal of Athletic Training, 51(3), 233-251
- Unstable Training for Low-Back Pain
- Javadian, Y., Behtash, H., Akbari, M., Taghipour-Darzi, M. and Zekavat, H. (2012) The effects of pain on disability of patients with lumbar segmental instability. Journal of Back and Musculoskeletal Rehabilitation, 25, 149-155
- Kang, T. W., Lee, J. H., Park, D. H., Cynn, H. S. (2018) Effect of 6-week lumbar stabilization exercise performed on stable versus unstable surfaces in automobile assembly workers with mechanical chronic low back pain. Work, 60(3), 445-454
- Ko, K. J., Ha, G. C., Yook, Y. S. and Kang, S. J. (2018) Effects of 12-week lumbar stabilization exercise and sling exercise on lumbosacral region angle, lumbar muscle strength, and pain scale of patients with chronic low back pain. The Journal of Physical Therapy Science, 30, 18-22
- Moon, H, J., Choi, K. H., Kim, D. H., Kim, H. J., Cho, Y. K., Lee, K. H., Kim, J. H. and Choi, Y. J. (2013) Effect of lumbar stabilization and dynamic lumbar strengthening exercises in patients with chronic low back pain. Annals of Rehabilitation Medicine, 37(1), 110-117
- Additional Research
- Scovazzo, M. L., Browne, A., Pink, M., Jobe, F. W., & Kerrigan, J. (1991). The painful shoulder during freestyle swimming: an electromyographic cinematographic analysis of twelve muscles. The American journal of sports medicine, 19(6), 577-582.
- Fenwick, C. M., Brown, S. H., & McGill, S. M. (2009). Comparison of different rowing exercises: trunk muscle activation and lumbar spine motion, load, and stiffness. The Journal of Strength & Conditioning Research, 23(5), 1408-1417.
- Saeterbakken, A., Andersen, V., Brudeseth, A., Lund, H., & Fimland, M. S. (2015). The effect of performing bi-and unilateral row exercises on core muscle activation. International journal of sports medicine, 94(11), 900-905.