Since the discovery of muscle fibers, people have wondered how their recruitment varies from exercise to exercise. From side-to-side and top-to-bottom, some recruitment patterns make perfect sense while others are a mystery.
Previously it was thought a muscle almost certainly couldn’t be worked differently along the direction its fibers run. However, a recent study by Bret Contreras and Brad Schoenfeld from the Journal of Strength and Conditioning Research cast more light on this often misunderstood aspect of training.
Study Design
There were ten participants, all well trained in resistance exercise. They had about 4.5 years of experience on average, and their average stiff-legged deadlift (SLDL) was about 300lbs, which is pretty decent. For the study, they each performed both an SLDL and a lying leg curl. Half of the subjects performed one exercise first and the other half did the other first. They used their eight-rep-max for the test and performed as many reps as they could with a fixed cadence.
During the tests, the participants were hooked up to electrodes to take EMG readings and measure muscle activity. Four regions of the hamstrings were selected, two on the upper hamstrings, near the glutes, and two on the lower hamstrings, closer to the knee. On both the upper and lower portions, one electrode was on the inside part of the leg and the other was on the outside at the same level.
Results
The results may come as a surprise. In general, the leg curl yielded the strongest response in the hamstrings. The SLDL was stronger only in the upper-inside EMG results and only by an insignificant margin.
Of particular interest was that the upper and lower parts of the hamstrings had substantially different responses to each exercise. Specifically, the researchers found it was possible to emphasize an upper or lower part of the hamstring depending on the exercise.
Now that may be blasphemy for some coaches, but it’s not as crazy as it sounds. While the results are useful for muscle building, and particularly for the superficial muscles, they aren’t absolute, stand-alone evidence that muscle fibers can be preferentially recruited along their length. This is because the EMG readings were for portions of muscle and included readings from several muscle heads simultaneously in some cases.
For example, part of the reason for the elevated response shown in the leg curl was because of the activity in the short head of the biceps femoris. This muscle only activates significantly during knee flexion.
Still, this doesn’t fully explain why the medial electrodes picked up stronger signals near the origin of the muscle compared to near the knee. It could be because each of the heads of the hamstring has an attachment on the medial side of the back leg, and thus the EMG data reflected the work of a larger portion of the hamstring.
Conclusions
We can say two important things based on this data. First, for people working to build muscle for aesthetic reasons, regions of the leg can be preferentially selected, even from top to bottom. The results suggest regional muscle development does seem to be possible.
Second, and more important, working biarticular muscles like the hamstrings in a variety of ways is important. If a muscle crosses multiple joints (e.g. the biceps, triceps, hamstrings, quads, and calves), spend time working all the functions of the muscle for complete strength development.
References:
1. Brad Schoenfeld, et. al., “Regional Differences in Muscle Activation During Hamstrings Exercise,” Journal of Strength and Conditioning Research, DOI: 10.1519/JSC.00000000000005
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