In Part 1 we discussed misconceptions that present a barrier to the understanding of muscle growth of even the most well-read strength coaches and personal trainers, which is how the degree of motor unit recruitment affects subsequent hypertrophy.
Fortunately, we can fix this problem by comparing the effects of high-velocity strength training programs, such as those that involve jumping or plyometrics, and conventional bodybuilding programs.
In Part 2 we’ll discuss practical implications and the takeaway…
What are the practical implications?
Importantly, the size of the weight used does not affect the bar speed of the final rep when training to failure. Regardless of what weight we use on an exercise, we end up moving at the same speed by the end of the set.
Given that this is the same speed as we move in a 1RM effort, it probably happens because it is the speed that allows the maximum force-producing capacity of the recruited motor units.
However, since muscle growth can be achieved without training to failure, there must be a (slightly faster) threshold bar speed at which a set begins to trigger muscle growth, because this corresponds to a threshold level of tension experienced by each muscle fiber.
Before this point, we are either training with a submaximal bar speed, and therefore using mainly low-threshold motor units that will not grow substantially after training, or we are training with maximal bar speed but moving too quickly for the working muscle fibers to achieve the necessary levels of mechanical tension that stimulate muscle growth.
This threshold “hypertrophy velocity” will probably correspond to the speed we can move when lifting slightly heavier weights than have traditionally been used for bodybuilding, because such weights are lifted under fatiguing conditions. Since full motor unit recruitment is typically reached at 85–90% of 1RM, we might speculate that the speed we can move without fatigue with this weight in a given exercise is the threshold speed we need to reach in order to trigger muscle growth, although whether motor unit recruitment increases and bar speed decreases in exactly the same way is unclear.
Since training to failure leads to more muscle damage than avoiding failure, monitoring bar speed during a set could be a valuable way of stopping a set after the hypertrophy stimulus has been triggered but before too much muscle damage accumulates, thereby allowing faster recovery post-workout, and a higher training frequency. Naturally, this would only work if all reps were performed with maximal effort.
Once we identify it, this “hypertrophy velocity” will likely correspond to a given number of reps in reserve, given the close relationship between bar speed and reps in reserve that has been observed, and this would be the easiest way of implementing this finding in practice.
What is the takeaway?
Muscle fibers increase in size when they are activated and shorten at a slow contraction velocity. Only this state allows enough actin-myosin crossbridges to form and produce a high enough level of mechanical tension to stimulate the mechanoreceptors on the muscle cell membrane, which then trigger the molecular signaling cascades that lead to elevated muscle protein synthesis, and therefore an increase in the protein content of the muscle fibers of high-threshold motor units.
This state can be reached by strength training with either heavy loads or light loads under fatiguing conditions, but not by high-velocity strength training or plyometrics, which involve high levels of motor unit recruitment but fast muscle contraction velocities.
Since we can achieve muscle growth without training to failure, and since the bar speed of the final rep in a set to failure is the same regardless of the relative load we use, there must be a threshold bar speed below which hypertrophy is stimulated (so long as maximal effort is used on all reps). Since training to failure and stopping short of failure produce similar muscle growth, and since training to failure takes longer to recover from than avoiding failure because it causes more muscle damage, this could be a valuable way to increase training frequency.
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