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Tuesday, June 6, 2023

Kettlebell Swings and Romanian Deadlifts Preferentially Target Semitendinosus over Biceps Femoris

Brent Brookbush

Brent Brookbush


Research Review: Kettlebell Swings and Romanian Deadlifts Preferentially Target Semitendinosus over Biceps Femoris

By Nicholas Rolnick SPT, MS, CSCS

Edited by Brent Brookbush DPT, PT, COMT, MS, PES, CES, CSCS, ACSM H/FS

Original Citation: Zebis MK, Skotte J, Andersen CH, et al. (2013). Kettlebell swing targets semitendinosus and supine leg curl targets biceps femoris: an EMG study with rehabilitation implications. Br J Sports Med. 47: 1192-1198. - ABSTRACT

Why the Study is Relevant: This 2011 study by Danish researchers investigated the recruitment patterns of the hamstrings (biceps femoris - BF and semitendinosus/semimembranosus -ST/SM ) as well as hip and knee joint kinematics during 14 commonly recommended exercises. In those exhibiting postural dysfunction/movement impairment, the ST/SM are commonly under-active, whereas BF is commonly overactive; predisposing the knee joint to excessive valgus force and increased risk of anterior cruciate ligament injury (1-3). The findings suggest that exercises such as kettlebell swings and Romanian deadlifts, which target the ST/SM over the BF , may be useful in injury prevention and rehabilitation programs.

Kettlebell swings preferentially recruit semitendinosus over biceps femoris.
Caption: Kettlebell swings preferentially recruit semitendinosus over biceps femoris.

Kettlebell swings preferentially recruit semitendinosus over biceps femoris.

Study Summary

Study DesignRepeated Measures Design
Level of EvidenceIII Evidence from non-experimental descriptive studies, such as comparative studies, correlation studies, and case-control studies
Subject Demographics
  • Characteristics of the Subjects:
    • 16 females (8 elite handball players, 8 elite soccer players) with 15.6 ± 4.1 years of experience in their respective sports, 5.4 ± 4.1 years of weight lifting experience, and trained 4.7 ± 0.7 sessions/week
    • Age (mean ± standard deviation) - 23 ± 3 years old
    • Height - 170.2 ± 6.4 cm
    • Weight - 66.2 ± 7.4 kg

  • Protocol of the Study:
    • Subjects came on a separate day (not specified) prior to data collection to familiarize themselves with the exercises and the protocol.
    • Electromyography (EMG) set-up on data collection day.

    • On the day of data collection, subjects performed 3 maximum voluntary isometric contractions (MVIC) for the hamstring muscles after a submaximal warm-up (specifics not mentioned).
      • Subjects were positioned prone in 10° of hip flexion and 45° of knee flexion in a Biodex Medical isokinetic dynamometer to record maximum hamstring activity.
      • Subjects contracted against the machine with strong verbal encouragement.
      • The highest EMG data from the three trials was used for data analysis.

    • Kinematic motion recording sensors were placed on the dominant hip and knee of each subject, which was defined as the leg used for push-off in jumping.
      • Sensors were placed on the iliac crest, the distal femur, and the proximal part of the fibula.
      • Reference angles used for data analysis on hip and knee motion were captured by having the subject stand erect in their normal upright posture.

    • Each subject performed each of the 14 exercises in a randomly-designed order with the same individual overseeing all of the exercises.
      • One-leg jump onto balance mat - Subject performed a one-legged jump onto a balance mat (Airex Pad) and maintained stability (knee over toe position) on landing for 3 seconds.
      • One-leg landing from box on balance mat - Subject stepped down from a 37.5 cm high box with one leg onto a balance mat and maintained stability on landing for 3 seconds.
      • One-leg drop jump on balance mat - Subject stepped down from a 37.5 cm high box with one leg onto a balance mat and immediately jumped as high as possible and landed on the balance mat and maintained stability for 3 seconds.
      • One-leg forward jump - Standing on one leg, the subject jumped forward and landed, maintained stability for 3 seconds and jumped backwards to the starting position.
      • One-leg side jump - Standing on one leg, the subject jumped to the side to a fixed point and maintained stability for 3 seconds and jumped back to the starting position.
      • Two-handed kettlebell swings - Subjects used either a 12 kg or 16 kg kettlebell (based on ability to properly swing it 20 times without form breakdown). Subjects stood in front of the kettlebell with their feet parallel, shoulder-width apart. The subjects flexed their hips and knees while keeping the spine in a neutral position as they grabbed the kettlebell with two hands. The subjects kept the knees slightly flexed (10-15°) and forcefully swung the kettlebell back between the legs and reversed its direction with explosive hip extension where the kettlebell ended at chest height with the subject standing upright with the knees fully extended.
      • Nordic hamstring lowers (curls) - The subject kneeled on a balance mat while a partner held the ankles. The subject leaned forward using the hamstrings to resist the motion of the trunk to the ground.
      • Supine one-leg curls - Subject was positioned supine on the floor with their arms by the side and hips in extension, lifted off the floor with one leg straightened. The subject curled the leg positioned on the ground inward and outward.
      • Supine pelvis lifts - Subject was positioned supine on the floor with the arms by the side, with one leg bent on the floor, the other leg is bent and crossed over the working leg. The subject lifted the hips off the floor until the shoulders, hip, and knee were in alignment in a straight line.
      • Romanian Deadlift - The subject stood close to the barbell (with a weight corresponding to 12RM) with feet parallel and shoulder width apart. The subject flexed the hips and kept the upper body straight while grabbing the bar. The knees were slightly flexed (10-15°) at the start of the exercise. The subject extended the hips until the body was upright and returned the bar to the ground to complete the exercise.
      • Hyperextensions off table - The subject was positioned in prone with the legs fixed on a table and the upper body hanging off the table. The subject flexed the torso until a stretch was felt in the hamstrings and then extended the torso to complete the exercise.
      • Hyperextensions off table with barbell - The exercise was performed the same as HE, but with a barbell (13.3 kg) placed in the wide grip of the subject.
      • Seated leg curl (SeLC) - The subject was seated in a Biodex Medical isokinetic dynamometer with 80° of hip flexion. The subject's calf pushed against the pad of the dynamometer set to 60° maximum voluntary contraction (MVC) per second throughout 80° of knee flexion (90° - 10°).
      • Prone leg curl - The subject was prone in a Biodex Medical isokinetic dynamometer with no hip flexion. The subject's calf pushed against the pad of the dynamometer set to 60° MVC per second through 80° of knee flexion (90° - 10°).

    • Statistical Analysis
      • A priori power analysis was performed to determine statistical significance with 80% power at a minimal relevant difference between exercises of 10% with p < 0.05.
        • 16 subjects was determined to be the minimum number of subjects needed.

      • Two-way ANOVA was used to determine if differences existed between the peak nEMG, and knee and hip joint angles at peak nEMG between exercises and muscles.
        • BF and ST muscle activity were analyzed across the 14 different exercises as well as the interactions between these.

      • Post-hoc testing was performed when a significant main effect was found to determine the differences between BF and ST and was expressed as delta (Δ).
      • Intraday reliability was assessed using Intraclass coefficient correlations (ICCs).

  • Inclusion Criteria:
    • No previous history of knee or hamstring injury

  • Exclusion Criteria: N/A
Outcome Measures
  • Intraday reliability (ICC) for each exercise
  • Level of BF and ST muscle activation in the 14 exercises
  • BF/ST muscle activity balance in the 14 exercises
  • Knee and hip angles at peak nEMG
  •  ICCs ranged from 0.783-0.941 (p < 0.05) suggesting very good to excellent intra-tester reliability for calculation of peak nEMG activity.
  • Level of BF and ST muscle activation in each of the 14 exercises
  • BF/ST muscle activity balance in the 14 exercises

Post-hoc analysis:

  • Preferential activation of ST over BF during Romanian deadlift (Δ 17.3 ± 9.7% peak nEMG, p < 0.05) and kettlebell swings (Δ 22.5 ± 9.7% peak nEMG, p < 0.05)
  • Preferential activation of BF over ST during supine leg curl (Δ 22.9 ± 9.4% peak nEMG, p < 0.05) and hyperextensions off the table (Δ 20.4 ± 8.8% peak nEMG, p < 0.05)
  • The rest of the exercises did not show post-hoc significance between nEMG of ST and BF

Knee and hip angles at peak nEMG:

  • Peak normalized ST EMG activity occurred in a more extended knee position compared with BF during the "one-leg jump onto balance mat" and "one-leg drop jump onto balance mat" exercises. No other significant differences were observed in any of the other exercises.
ConclusionsKettlebell swings and romanian deadlift preferentially activate ST over BF, whereas hyperextensions off the table and supine single leg curls preferentially activate BF over ST.
Conclusions of the ResearchersSpecific therapeutic exercises preferentially activate different hamstring muscles . These varied recruitment patterns can have important implications for injury prevention training and rehabilitation.

The variation to the Romanian Deadlift that the Brookbush Institute recommends in place of the Romanian Deadlift in a strength and conditioning program.
Caption: The variation to the Romanian Deadlift that the Brookbush Institute recommends in place of the Romanian Deadlift in a strength and conditioning program.

The variation to the Romanian Deadlift that the Brookbush Institute recommends in place of the Romanian Deadlift in a strength and conditioning program.

Review & Commentary:

The purpose of this study was to investigate hamstring muscle recruitment patterns and hip and knee kinematics in commonly used strength and rehabilitation exercises in elite female athletes. The authors sought to determine appropriate exercise selection for prophylaxis (injury prevention) and rehabilitation purposes based on the normalized electromyography (EMG) results. The results of the current study have potentially important implications for hamstring strengthening programs.

The study had many methodological strengths, including:

  • The experimental protocol was clearly described, using appropriate exclusion criteria, and detailed descriptions and images of all the exercises were included.
  • Comprehensive data sets showed the results, reducing reporting bias.
  • The kinematics of the hip and knee were measured in each exercise, to investigate whether changes in kinematics influence peak muscle recruitment patterns. These findings may have clinical significance in programming for individuals with overactive hamstring musculature.
  • The exercises used in the study are commonly performed in fitness and rehabilitation programs to reduce the risk of, or recover from, anterior cruciate ligament and hamstring tears; this choice increases the ease of application for the study's findings.

Weaknesses that should be noted prior to clinical integration of the findings include:

  • Elite, young, female athletes with no histories of significant lower extremity pathologies were used as subjects. Future studies should include a more heterogenous sample size.
  • The two exercises that produced the highest normalized EMG activity of ST required external load. It's unclear whether the results were due to the additional external loading or the demands of the movements. Future studies should address whether a similar pattern of hamstring activity would be observed in the absence of external loads.
  • Using surface EMGs makes it challenging to evaluate small differences in muscle activities between exercises. Future studies should include fine-wire EMGs, which could better elucidate the changes observed in hamstring muscle activity.
  • The study was confined to a one-day testing period. It's unclear whether longer term performance of the ST - or BF-dominant exercises will lead to changes in relative strength or reduce injury risk. Longitudinal data is needed.

Why is this study important?

The current study indicates that the biceps femoris - BF and semitendinosus/semimembranosus -ST/SM can be selectively recruited during various exercises commonly used in rehab, fitness and performance settings. Preferentially targeting ST over BF  could theoretically reduce knee valgus during functional tasks.

How does it affect practice?

Specific exercises can be classified as either ST or BF dominant. Kettlebell swings and Romanian deadlift specifically targets ST , and at intensity levels high enough to stimulate muscle gains. The preferential firing of ST over BF in these exercises may be due to ST anatomy. This muscle is composed of long, parallel muscle fibers with a high number of sarcomeres in series. The higher number of sarcomers in series of the ST increases its total shortening capacity and absolute velocity of contraction, allowing it to shorten more effectively at long muscle lengths than the BF . Both ST -dominant exercises require a large range of motion, which could explain the increased recruitment of ST over BF . The authors recommend kettlebell swings and Romanian deadlift to enhance ST 's ability to forcefully contract from large angles of hip flexion. For training purposes, the kettlebell swing may provide the most transfer to sport, as this has a larger power component than the Romanian deadlift , due to the high-velocity component.

The supine leg curl and hip hyperextensions were found to be the most BF -dominant exercise. Isolated use of these exercises, without incorporating other ST -dominant exercises, have the potential to reinforce postural dysfunction (Lower Extremity Dysfunction , Lumbo Pelvic Hip Complex Dysfunction ). These exercises should be performed in moderation (if performed at all), and likely only when balanced by exercises targeting muscles that have a propensity toward under-activity.

An alternative exercise to strengthen the BF that could be performed instead of hip hyper-extension or supine leg curls is the Nordic hamstring curl (4). While the Nordic hamstring curl was shown to activate both ST and BF equally, the eccentric component makes it a desirable exercise for BF hamstring strain injury prevention.

Many exercises activated both BF and ST equally, and were proclaimed by the authors as "universal hamstring exercises." These plyometric-type exercises may be important to incorporate when trying to modify existing motor programs, such as reducing dynamic knee valgus. Kinematic analysis of the hip and knee and EMG data reveal that ST peaked earlier in the landing phase in the one-leg drop jump and the one-leg jump onto balance mat, whereas BF peaked later. This finding suggests that the medial compartment provides a protective contraction as a dynamic restraint against valgus forces.

How does it relate to Brookbush Institute Content?

ST has a propensity to become long/under-active in those exhibiting lumbopelvic hip complex dysfunction (LPHCD) , sacroiliac joint dysfunction (SIJD) , and lower leg dysfunction (LLD) ; where as the BF commonly adopts a pattern of long/overactive. In all models of postural dysfunction, activating and integrating under-active musculature is indicated to restore optimal length/tension relationships, better joint alignment, and promote increases in movement quality.

The Brookbush Institute (BI) uses variations of both kettlebell swings and Romanian deadlift exercises as advanced progressions in a fitness, strength and performance programs.

Kettlebell swings are used primarily for development of power (muscle force/time) in a max strength/power program. Based on the results of the current study, kettlebell swings can be used as an advanced "integration" exercise for under-active medial hamstrings without increasing the relative activity of BF . The BI recommends performing 2-handed kettlebell swings for sets of 8-12 repetitions, in a similar set-up to the variation performed in the current study. Once 2-handed swings are mastered, 1-handed swings can be introduced, as this progression increases the rotational forces on the spine, requiring additional lumbopelvic stabilization.

A Brookbush Institute preferred progression from the traditional Romanian deadlifts , is the deadlift with an anterior-posterior pull) to facilitate recruitment of the gluteal complex (also typically under-active in postural dysfunction along with the posterior oblique subsystem ), stimulate more gluteal complex hypertrophy and improve neuromuscular control.

Other progressions of the kettlebell swing and the deadlift variants can be viewed in detail in the course Deadlift Progression .

The following videos depict some of the exercises performed in the current study as well as advanced BI-specific progressions.

Brookbush Institute Videos

Kettlebell Swings

Kettle Bell Clean (Progression of Kettlebell Swings)

Romanian Deadlift

Single Leg Touchdown

Deadlift with Anterior to Posterior Pull

Single Leg Deadlift with Anterior to Posterior Pull


  1. Padua, D. A., Bell, D. R., & Clark, M. A. (2012). Neuromuscular characteristics of individuals displaying excessive medial knee displacement. Journal of athletic training, 47(5), 525
  2. Bell, D. R., Oates, D. C., Clark, M. A., & Padua, D. A. (2013). Two-and 3-dimensional knee valgus are reduced after an exercise intervention in young adults with demonstrable valgus during squatting. Journal of athletic training,48(4), 442-449.
  3. Hewett, T. E., Myer, G. D., Ford, K. R., Heidt, R. S., Colosimo, A. J., McLean, S. G., & Succop, P. (2005). Biomechanical measures of neuromuscular control and valgus loading of the knee predict anterior cruciate ligament injury risk in female athletes A prospective study. The American journal of sports medicine, 33(4), 492-501.
  4. Bourne, M., Williams, M., Opar, D., Najjar, A., Kerr, G., & Shield, A. (2016) Impact of exercise selection on hamstring muscle activity. British Journal of Sports Medicine, 0, 1-9

© 2016 Brent Brookbush

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