By Brad Schoenfeld, Ph.D. Flex
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:
BOTH THE STIFF-LEG DEADLIFT AND THE LYING LEG CURL ARE EXCELLENT EXERCISES FOR TARGETING THE HAMSTRINGS
Ten college-aged men who were experienced in resistance training performed the lying leg curl and the stiff-leg deadlift to failure at a load equating to their 8RM. These exercises were chosen because they targeted the two primary actions of the hamstrings (knee flexion and hip extension) in single-joint fashion. To determine muscle activation during exercise, we hooked each subject up to a device called an electromygraph, (EMG) which measures the electrical activity in muscles; greater EMG amplitude corresponds to greater muscle activation. Electrodes were placed on the upper and lower aspects of the medial and lateral hamstrings.
The results may surprise you.
While activation of the upper hamstrings was similar between exercises, the lying leg curl produced significantly greater lower hamstrings activity. The differences in activation of the lower hamstrings was off-the-charts huge, with the lying leg curl showing greater lower lateral hamstrings activity of ~170% and lower medial hamstrings activity of ~65% compared to the stiff-leg deadlift. Now, the increased activation in the lateral aspect of the hamstrings were somewhat expected. As mentioned, the short head of the biceps femoris doesn’t cross the hip joint, so a knee-dominant exercise such as the lying leg curl would necessarily be the only way to directly target this muscle. On the other hand, the semitendinosus and semimembranosus are biarticular muscles, meaning they cross both the hip and knee joints. Therefore, the findings highly suggest that the partitioning of these muscles may allow for greater regional-specific activation in their lower aspect.
From a practical standpoint, there are a couple of take-home messages here. For one, both the stiff-leg deadlift and the lying leg curl are excellent exercises for targeting the hamstrings. Probably not breaking any new ground here. However, the findings of this study clearly indicate that the lying leg curl is essential for maximizing development of the lower part of the hamstrings, both laterally and medially; it’s an exercise that should be a staple in your routine. That said, there is reason to include hip-extension exercises in your routine as well.
Here’s why. There were substantial inter-individual differences between subjects in their activation patterns; some displayed substantially higher upper-hamstring activation with the stiff-leg deadlift while others had markedly greater recruitment from the lying leg curl. So performing a combination of knee flexion and hip extension is the best way to ensure recruitment of the full spectrum of fibers in the hamstrings complex.
IF YOU WANT YOUR HAMMIES TO REALLY BULGE, SINGLE-JOINT MOVEMENTS ARE A MUST.
One thing is patently clear when it comes to hamstrings training: You can’t simply rely on compound lower-body exercises to maximize muscle development. Contrary to popular belief, the hamstrings are only moderately active during exercises such as squats, lunges, and leg presses; the quads and glutes are much more dominant. So if you want your hammies to really bulge, single-joint movements are a must.