Exercise, Studies

July 6, 2014

Can you target different aspects of the hamstrings?

In this post I want to delve into the specifics of a study I recently published in the Journal of Strength and Conditioning Research that investigated muscle activation in two popular hamstrings exercises: the stiff leg deadlift and the lying leg curl. While other studies have evaluated differences between these exercises in activation of the medial hamstrings (semitendinosus and semimembranosus) versus the lateral hamstrings (biceps femoris), our study is unique in that we also looked at activity in the upper and lower aspects of the individual muscles.

My interest in undertaking this work revolves around an emerging body of research showing that, contrary to popular belief, muscle fibers do not necessarily span from origin to insertion. Rather, many muscles are compartmentalized so that fibers terminate intrafascicularly (within the fascicle) with each subdivision innervated by its own nerve branch. This structure provides a mechanism by which different exercises can conceivably target different portions of a given muscle. It just so happens that several studies have shown that the hamstrings muscles are in fact partitioned in a manner that would potentially allow for such regional-specific activation. I thus decided to test this hyptothesis in the lab under controlled conditions.

What We Did
Ten young men were recruited to participate in the study. All of the subjects were experienced with resistance training, defined as lifting at least 3 times per week for a year or more. We employed a counter-balanced design where each subject performed both the stiff-leg deadlift and the leg curl. This design provides a high-degree of statistical power, thereby improving the possibility of detecting a significant difference if one in fact does exist. Testing took place over two sessions. In the first session we tested subject’s 8RM on both exercises. The second session involved assessing activation during performance of the movements at the subject’s 8RM.

Muscle activation was determined by a technique called electromyography (EMG). Electrodes were placed on the upper and lower aspects of the subject’s medial and lateral hamstrings; care was taken in placement to make sure that cross-talk between muscles did not confound results. The subjects then performed one of the exercises followed by a lengthy rest period and then performed the other exercise. The exercises were counterbalanced so that Subject 1 performed the leg curl first, Subject 2 performed the stiff-leg deadlift first, etc. This ensured that the order of performance did not confound results. All sets were performed at the subject’s 8RM to muscular failure.

What We Found
Activation of the upper hamstrings was similar between exercises. Interestingly, however, activation of the lower hamstrings, both medially and laterally, was significantly greater in the lying leg curl. The differences in activation of the lower hamstrings was stark, with the leg curl showing greater lower lateral hamstrings activity of ~170% and lower medial hamstrings activity of ~65% compared to the stiff-leg deadlift. The data for the lateral hamstrings was not unexpected; the short head of the biceps femoris does not cross the hip joint, so a knee-dominant exercise such as the leg curl would necessarily be the only way to directly target this muscle. However, the data for the medial hamstrings was somewhat surprising since both the semitendinosus and semimembranosus are biarticular muscles. Results suggest that the partitioning of these muscles may allow for greater regional-specific activation in their lower aspect.

Practical Implications
The findings suggest that performing both a hip-dominant hamstrings exercise (such as the stiff-leg deadlift) and a knee-dominant exercise (such as the leg curl) are beneficial to maximize activation of the muscle complex. Given that muscle hypertrophy is predicated on recruiting as many motor units as possible in the target muscles and achieving high firing rates in these motor units for a sufficient length of time to fully stimulate the fibers, it stands to reason that greater activation achieved should lead to greater regional-specific muscle growth. Several recent studies have in fact shown this to be the case, with non-uniform hypertrophy correlating to the region of greatest muscle activation. However, the evidence to date remains correlational and more research is needed to draw cause-effect conclusions. In the meantime, the present study provides interesting insight into how different exercises elicit different responses in a given muscle and lends support to the potential benefits of varying exercise selection to optimize muscle development.

Reference
Schoenfeld BJ, Contreras B, Tiryaki-Sonmez G, Wilson JM, Kolber MJ, Peterson MD. Regional Differences in Muscle Activation During Hamstrings Exercise. J Strength Cond Res. 2014 Jun 24. [Epub ahead of print]