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December 6, 2021

Understanding Mechanical Tension, Part II: The Light Load Paradox?

In Part 1 of this series, I operationally defined mechanical tension and discussed how sensors in the working muscles detect the magnitude of tension from a given resistance to carry out the muscle-building process. However, as mentioned in that post, this doesn’t necessarily mean that heavier is better for gaining muscle. In fact, a compelling body of research indicates that within wide limits, you build as much muscle from training with relatively light weights as you do from heavier loads. While this may seem counterintuitive, there are several possible explanations for the apparent paradox.

First and foremost, mechanical tension inevitably increases as you approach muscle failure in a set. For example, say you are curling a weight that you can lift 20 times (i.e., your 20RM). The first few reps of the lift will be very easy to perform, and thus the tension imposed on the working muscles necessarily will be low. However, as you continue to curl the load, muscle fibers begin to fatigue causing increasingly greater tension on the remaining pool of available fibers. By the last few reps, the working muscle fibers are under a great deal of stress in their effort to complete the movement. In support of this theory, research indicates that fast-twitch fibers are progressively activated as a light-load set nears muscle failure, thus indicating that tension is specific to the level of exerted effort.

So does that mean that only the last few reps of a set matter when training with higher rep schemes?

Not necessarily.

Mechanical tension is present throughout a lighter-load set, even during the initial repetitions, and it is conceivable that other factors may play a synergistic role in the hypertrophic process under conditions of lower tension. For example, metabolites are produced during high rep training that may contribute to hypertrophic gains. Moreover, blood vessels are compressed during repeated contractions, and the corresponding ischemia/hypoxia may be involved in anabolic signaling. We are just beginning to scratch the surface in our understanding of the mechanisms of muscle hypertrophy, with much still to be determined. If you’re interested in learning more on the topic, check out our review paper that discusses what we currently know about potential sensors and stimuli.

Another possibility is that there may be a fiber-type specific response to loading. Some evidence suggests a preferential growth of type I fibers when training with lower loads and a preferential growth of type II fibers from heavier loads. If true, this would potentially “even out” the magnitude of growth when comparing training with higher versus lower loads. It also would suggest the possibility that combining heavier and lighter loads may optimize hypertrophy by promoting maximal growth of both fiber types. I’d note that our recent study did not indicate a fiber type-specific response between moderate- (~6 to 10 reps) and lighter- (~20 to 30 reps) load training of the calf muscles, but certainly more research is warranted to draw stronger conclusions on the topic.

Based on what we’ve discussed, the question then arises: Is there an ideal time under tension in a set to promote gains? Perhaps such a scheme would provide a means to harness the benefits of sufficient mechanical tension while achieving higher volume loads, stimulating the spectrum of muscle fibers, and perhaps taking advantage of other mechanistic anabolic factors? Stay tuned for Part 3 of the series where I delve into the evidence on this topic.