Fiber length and range of motion

[Shion]

This isn't keeping me up at night or making me ponder my training program, I still squat deep and do the lifts over the longest range of motion possible, but my question is the following. In the book you mention that if muscles get trained over a partial range of motion, then they will get strong only over that range of motion. This is obvious when you compare bottom bench presses to full RoM ones - the pecs do the bulk of the work without giving the triceps the chance to participate more by locking out at the top.

I'm referring more to your example in the book with leg extensions. If muscle fibers run the entire length of the muscle, from origin to insertion, then how is it possible that a muscle will only get strong over a portion of the RoM and not the whole thing?

Again, just something I was thinking about.

[Mark Rippetoe]

For a full-ROM movement, it would be a combination of the anatomical contributions of the various muscles that participate in the movements as the skeletal position changes, and the additive effect of the overlap of the actin and myosin filaments at points within the different muscle bellies at their respective strong positions during the movement. This is why we rely on full-ROM for the squat, so that all the muscles that are anatomically capable of contributing to the movement get worked in the part of the ROM where they can participate.

Your example of the single-joint "open-chain" leg extension is probably explained by the existence of the "strength curve" in measured efforts on the equipment, the result of differences in intra-sarcomeric overlap and mechanical advantage at various angles of attack in the ROM. Different positions within the ROM involve different amounts of actic/myosin overlap, and if the muscle is only trained at one position in isolation, the other positions of actin/myosin interaction and overlap -- and therefore contractile force production -- will not be trained or developed. Why an isotonic-isometric contraction performed at the specific peak of the strength curve would not strengthen that muscle belly over it's entire contractile potential, I don't know precisely. We'll ask. But don't post a bunch of stupid shit I'll have to delete -- just deal with the question.

[Jonathon Sullivan]

It may clarify the question if we change the language, and think in terms not of training a particular muscle or muscles, but of training a movement.

[Mark Rippetoe]

Thank you, Sully. I hate questions that make me feel like I'm back in ExPhys.