Impact of Exercise Selection on Hamstring Muscle Activation

By Alex Howard PT, DPT, CSCS

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

Original Citation: 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. ABSTRACT

Why is this relevant?:

It is important to restore optimal motion post hamstring injury, both for activities of daily living and to reduce the risk of future injury. Knowing the recruitment pattern and level of activation during various commonly performed hamstring exercises may be beneficial for refining exercise selection during training and/or treatment. This study used normalized electromyography (nEMG) and functional magnetic resonance imaging (fMRI) to determine which commonly used hamstring exercises selectively activated the biceps femoris .

Caption: Dr. Brookbush instructs personal trainer, Vinnie Laspina, on how to properly perform a stability ball glute bridge.

Stability Ball Bridge

Study Summary

Study Design	Cohort study
Level of Evidence	Level III: Evidence from non-experimental descriptive studies, such as comparative studies,correlation studies, and case-control studies
Subject Demographics	Characteristics: 24 subjects - 18 subjects part 1, 10 subjects part 2 Gender: Male Age: 24.4 (+ 3.3 years) Weight 85.2 kg (+ 13.4 kg) Height 181.8 cm (+ 6.1 cm) Inclusion Criteria: Free of soft tissue and orthopedic injuries to the trunk, hip and lower limbs at time of testing No history of cardiovascular, metabolic, or neurological disorders No history of hamstring strain injury in the previous 18 months No history of anterior cruciate ligament injury
Outcome Measures	Part 1: nEMG amplitudes and ratios of Biceps Femoris (BF) to Medial Hamstring (MH) during 10 exercises Exercises: bilateral stiff leg deadlift, unilateral stiff leg deadlift, hip hinge (swing), lunge, unilateral and straight knee bridges, leg curl, 45° hip extension (pictured below), glute-ham raise (pictured below), Nordic exercise (pictured below) Unless the exercise was explosive (hip hinge) or eccentric-only (nordic and glute ham raise), it was performed using a 12 RM load at a constant pace ( 2 sec up/2 sec down) Electrodes were placed on BF and MH: the muscles were identified with palpation and observation of active external and internal rotation of knee in 90° flexion; the reference electrode was placed on the fibula head Data was sampled from a randomly selected limb Reference MVIC: prone with ankles secured, forceful knee flexion with strong verbal cues, contraction with highest amplitude for BF and MH used Mean nEMG for BF and MH obtained from 6 reps for eccentric and concentric phases Activation ratio = BF nEMG/MH nEMG, ratios >1.0 = BF activity > MH activity Part 2: fMRI investigation of two exercises identified in Part 1: The "Nordic Exercise" resulted in the most equal amounts of BF and MH activity (least selective recruitment of the of BF), and the 45° hip extension selectively activated the BF the most during eccentric contractions Two separate exercise sessions separated by at least 6 days fMRI of both thighs taken before and immediately after the exercise 5 sets of 10 reps with 1 min rest 45° hip extension performed unilateraly with load adjusted to their 12 RM, Nordic performed bilaterally with body weight only fMRI T2 relaxation times measured at 5 axial slices (30, 40, 50, 60, and 70% of thigh length) fMRI T2 relaxation times measured in each slice at circular region of intersect placed in homogeneous region of contractile tissue Percentage change in T2 relaxation time for each hamstring muscle calculated
Results	Part 1 BF activity varied from 21.4%(lunge) to 99.3% (unilateral straight knee bridge) of MVIC during the concentric phase, and from 10.7% (hip hinge) to 71.9%(nordic) of MVIC during eccentric phase. Concentric: BF activity was greatest during the leg curl and 45° hip extension exercises when compared to lunge and bent knee bridge. Eccentric: BF activity was greatest during 45° hip extension, more than Nordic, bent knee bridge, leg curl and glute ham raise. Part 2 % change in T2 relaxation time following 45° hip extension: BF short head significantly lower than at BF long head, semitendinosus, and semimembransosus. T2 changes at semitendinosus sigificantly greather than semimembranosus. No difference between BF longhead and semimembranosus or semitendinosus % change in T2 relaxation time following Nordic hamstring exercise: semitendinosus significantly larger than BF long head, BF short head and semimembranosus. T2 change at BF short head significantly greater than BF long head and semimembranosus During 45° hip extension significantly greater hamstring activation vs the Nordic exercise when comparing BF long head vs semitendinosus and the semimembranosus vs semitendinosus During Nordic exercise there was significantly greater hamstring activation ratio than during the 45° hip extension, when comparing BF shorthead to semitendinosus
Conclusions	This study provides evidence that hamstring muscle activation patterns differ significantly between exercises.
Conclusions of the Researchers	nEMG activity findings show different hamstring muscle activation patterns during exercises and fMRI T2 relaxation time findings suggest that hip extension exercises target the BF longhead while Nordic hamstring exercise targets the semitendinosus

Caption: Image of Trainer and Client performing the Noric Curl a.k.a the Nordic Hamstring Curl

The Nordic Curl - http://willlevy.com/wp-content/uploads/2014/08/Nordic-Curl-1024x705.jpg

Review & Commentary:

The authors of this study examined hamstring muscle activity, using nEMG, during ten common exercises. Further, they compared two of these exercise hamstring using fMRI .

There were many strengths to the methodology of this study. First, the authors recruited a sample size that was sufficient to show statistical significance with a 10% difference in ratio between the biceps femoris (BF) to the medial hamstrings (MH) . Second, the authors selected a variety of commonly used open- and closed-chain hamstring exercises. Third, the figures utilized to present data allowed the reader to easily identify nEMG levels of specific exercises and make comparisons of the BF to the MH activation ratio.

There were limitations to this study that should be discussed before application to practice. First, the reference maximal voluntary isometric contraction (MVIC) may not represent a "true max" (although may still serve as a reasonable reference marker), as can be seen in the nEMG levels of several exercises reaching levels of activation beyond 100%. To obtain the MVIC the subjects were positioned in prone with resistance at their ankles, creating a significant moment arm on a muscle already shortened at its proximal attachment (active insuffeciency?). Second, the reference electrode used during the nEMG readings was placed on the fibular head; the insertion of the BF muscle. Although the BF is likely tendon, with little or no contractile tissue, the electrode could have been placed in an area less likely to pick-up interference - for example, the bony tibial crest. Last, the exercises selected for fMRI were based on results from the nEMG data; however, consideration of the most commonly used exercises may have resulted in a 3rd and potentially 4th exercise. Although all the exercises used in this study are common, the bridge and lunge exercise are so common as to be found in almost every routine across a wide variety of settings.

Caption: 45 Degree Hip Extension Exercise

45 Degree Hip Extension Exercise

Why is this study important?

Understanding hamstring activation patterns of various exercises may be used to optimize exercise selection. This includes reducing or eliminating exercises that may reinforce compensation patterns, eliminating exercises that place excessive load on a muscle at risk for further injury, or adding exercises that may preferentially recruit commonly under-active muscles. Relative to this study, the commonly becomes synergistically dominant relative to an inhibited gluteus maximus (1-4). Reducing the amount of exercise that preferentially recruits the biceps femoris is likely beneficial for optimizing motion and reducing the risk of future injury.

Caption: Glute Ham Raise Exercise

Glute Ham Raise Exercise

How does it affect practice?

The findings of this study demonstrate that the lunge and 45° hip extension exercise have a greater concentric BF /MH ratio (BF much greater BF activity) compared to the leg curl and bent knee bridge. This may imply that the lunge and 45° hip extension exercise (commonly used for "back strengthening") should not be part of the initial phases of a rehabilitation program post hamstring injury. Further, the 45° hip extension exercise had a greater eccentric BF /MH ratio compared to the nordic, bent knee bridge, leg curl and glute ham raise, implying that the 45° hip extension exercise may be a poor choice of exercise for any individual looking for ideal hamstring and gluteus maximus recruitment. In general, compound movement had better BF /MH ratios (lunges, deadlifts, swings, single leg deadlifts, bridges).

How does it relate to Brookbush Institute Content?

Based on research (1-4), clinical findings, and the predictive models of Lower Leg Dysfunction (LLD) , Lumbo Pelvic Hip Complex Dsyfunction (LPHCD) , and Sacroiliac Joint Dysfunction (SIJD) , the Brookbush Institute does not recommend isolated strengthening for the hamstring muscles, specifically the biceps femoris . As a general rule, resistance, strength and performance training is comprised of compound movements, and single joint and muscle specific exercises are considered "corrective/therapeutic exercise." This is due to the affect exercise has on increasing neuromuscular activity in target muscles (regardless of fatigue). As the biceps femoris has a propensity toward over-activity, this muscle is not targeted with strengthening or activation techniques, but rather release techniques. Further, as many of the exercises demonstrated in this study resulted in a significant increase in biceps femoris activity, they would also be excluded from both rehabilitation and performance programming (45° hip extension, nordic, leg curl and glute ham raise). Those exercises with a better BF /MH ratio may be used for core and performance training programs (lunges, deadlifts, swings, single leg deadlifts, bridges). Interesting note, the one exercise to have a high BF /MH ratio (significantly more BF activity) both concentrically and eccentrically, 45° hip extension, was also discussed as a "questionable exercise" due to increased activation of the erector spinae , with little to no activation of the gluteus maximus in a separate study (5).

Below are some sample techniques used by the Brookbush Institute:

Sagittal Plane Lunge

Dead Lift Progressions Cueing and Questionable Variations

Single Leg Touchdown

Biceps Femoris Self Administered Dynamic Release

Biceps Femoris Active Stretch

Bibliography

Questions, comments, and criticisms are welcomed and encourage

Impact of Exercise Selection on Hamstring Muscle Activation

Impact of Exercise Selection on Hamstring Muscle Activation

Bibliography

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