Q: Isn’t it true that you activate the most muscle fibers in the flexed position, like at the top of a leg extension or leg curl? I know that [Nautilus creator] Arthur Jones said that. So shouldn’t I really flex at the top of those isolation-type exercises? Come to think of it, why should I do compound exercises [like squats] at all? Isolating the muscle appears to be the logical way to stimulate the muscle growth best.
A: We respect the late Arthur Jones and value much of his research, theories, and opinions (Steve met him many years ago in Florida); however, newer research states the contrary—that the contracted, or flexed, position of an exercise is not where the most force/fiber activation occurs—it’s the bottom, where the target muscle is somewhat stretched. Here’s a quote from respected scientists Steven J. Fleck, Ph.D., and William J. Kraemer, Ph.D., that makes the point (we’ll put their statements in simpler terms in a moment):
There is an optimal length at which muscle fibers generate their maximal force. The total amount of force developed depends on the total number of myosin crossbridges interacting with active sites on the actin. At the optimal length there is potential for maximal crossbridge interaction and thus maximal force…. With excessive shortening there is an overlap of actin filaments so that the actin filaments interfere with each other’s ability to contact the myosin crossbridges. Less crossbridge contact with the active sites on the actin results in a smaller potential to develop tension.
In other words, in the peak-contracted position, the fibers are very bunched up, so much so that they can’t produce as much tension/force as when the muscle is in a more lengthened state. More force equals more muscle stimulation. Since tension/force is a key hypertrophic trigger, that means optimal fiber activation occurs when the muscle is slightly stretched, such as near the bottom of a chinup.
That’s not to say that the contracted position isn’t important. In fact, it’s one of the best spots on the stroke to emphasize occlusive hypertrophy (not force)—because the fibers are so bunched up, blood flow is impeded to the maximum, and that can spur more growth via capillary expansion and mitochondria development—not as important as force generation, but contributors to mass nevertheless.
It’s one of the reasons we developed the X Fade, one of our X-hybrid techniques discussed in the Beyond X-Rep Muscle Building e-book that helps maximize occlusion with a fiber-activation finish. It makes each set three to five times more effective at building muscle than stopping short. You use this technique on contracted-position exercises like leg extensions, leg curls, concentration curl, etc. Here’s how to do it…
At exhaustion on leg extensions, have your partner help you into the contracted, or flexed, position to do top-end X-Rep partials—do short pulses, squeezing your quads and extending occlusion time. After four to six of those, move down into the semi-stretch position and do as many X-Rep partials as possible, blowing out any of the fast-twitch fibers left standing (remember, that’s the max-force position for optimal muscle fiber activation). You may need some help from your partner on those bottom X Reps.
Now, the reason you need compound midrange exercises (like squats, bench presses, etc.) for maximum muscle growth is because those multijoint moves use muscle synergy, or teamwork, to move the load. That’s the way the body is designed to work, and that ergonomic synergy produces the most force overload on the target muscle—end-of-set X Reps in the semi-stretch position amplify that overload.
Max force is the key to optimal fiber activation and hypertrophy acceleration, so compound exercises are best for size stimulation. Isolation exercises are just icing on the muscle-building cake—more for occlusive hypertrophy than force/fiber activation.
Note: For more details and explanation of occlusion, muscle fiber activation, and X-hybrid techniques you can add to your next workout, take a look at Beyond X-Rep Muscle Building.
Till next time, train hard—and smart—for BIG results.
—Steve Holman and Jonathan Lawson
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