Maximal muscle development is predicated on recruiting all the fibers in a given muscle. This is basic physiology: If you don’t recruit a fiber, there is no stimulus for it to adapt.
The best way to ensure complete recruitment of a given muscle, is by varying exercise selection. It’s well known that different movements can selectively target aspects of muscles that have multiple heads. A prime example is performing front raises for the anterior delt, side raises for the middle portion, and reverse flyes for the posterior aspect. Similarly, fat and incline presses target the sternal and clavicular heads of the pecs, respectively. Okay, so that’s probably nothing new. What’s less clear, however, is whether you can target specific portions of a given muscle that does not segment into different heads.
A recent study conducted by my lab says you can, at least with respect to the hamstrings!
Before getting into the nitty gritty of the research, let’s first review a little basic anatomy and kinesiology. Anatomically, the hamstrings are comprised of three separate muscles: semitendinosus, semimembranosus, and biceps femoris. The semitendinosus and semimembranosus are located medially (toward the midline) on the back of the upper leg and the biceps femoris is located laterally (toward the outside). Furthermore, the biceps femoris has a long head and a short head. The short head is the only aspect of the hamstrings that does not cross the hip joint, meaning that it is only involved at actions taking place at the knee joint.
For years it had been thought that muscle fibers always spanned from origin to insertion. Based on this premise, prevailing theory stated that fibers were activated as an entire unit along the full length of the muscle. This claim has recently been challenged, however. A compelling body of evidence shows that many muscles do not span from origin to insertion, but rather are compartmentalized so that fibers terminate within the fascicle. Importantly, the fiber subdivisions are often innervated by their own nerve branch. This partitioned structure provides a mechanism by which exercises can conceivably target the individual subdivisions within the muscle. Interestingly, it just so happens that the hamstring muscles are in fact partitioned in a manner that would potentially allow for such regional-specifc activation.
And this provided the basis for the study in question. Here’s what we did:
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