This study has three aims: 1) review how active and passive forces are sensed by the muscle; 2) review research on the effectiveness of implementing inter-set stretching in a resistance training program for hypertrophy; and 3) provide practical recommendations for the application of inter-set stretching.
Mechanical force, exercise-induced muscle damage, and metabolic stress all may be responsible for signaling muscle growth, but the mechanical force is widely considered the most important. Mechanical stimuli trigger the hypertrophic response through mechanotransduction, a process in which the forces are transduced to chemical signals that regulate anabolic and catabolic processes.
This review focused on the sensing of contraction and stretch-induced force as hypertrophy stimuli. Active force sensors sense the force of a muscle contraction while passive force sensors are activated by the passive stretch of a muscle. The discusses the current mechanisms for passive and active sensor stimulation. It also explores whether stretching between sets of resistance training enhances muscle protein synthetic response. Current evidence suggests the lengthened position of the muscle may be the most important range for inducing muscle hypertrophy. Lengthened partials show equivalent hypertrophy to full range of motion contractions and greater responses than shortened partials. The high-load passive stretch increases anabolic signaling, including mTOR. Observed mechanisms don’t always lead to significant outcomes, however. Increased signaling doesn’t guarantee hypertrophy will occur.
The Warnecke lab has demonstrated long duration, intense stretching (1 hour a day for 6 weeks at 8/10 discomfort) can induce significant levels of hypertrophy in the plantar flexors. As interest stretching is a lower dose and is often performed in other regions of the body, we can’t assume the Warnecke trials are generalizable. Some research has shown lower doses (6-minute stretches) don’t induce hypertrophy while pre-exercise stretching may reduce hypertrophy. The latter is likely due to the lower total volume of the stretch group. This is a potential downside to taxing the muscle with aggressive stretching directly before a lift.
There is some preliminary evidence that interest stretching may enhance hypertrophy when performed in a specific manner. The stretch should be performed immediately following the final loaded repetition. There should still be a rest break after the stretch to allow for recovery and preparation of the next resisted set. The stretch should be uncomfortable and last 20–30 seconds. This approach increased the passive tension of the muscle following cross-bridge deactivation than occurs after the eccentric actions. The stretch also increases the time under tension without taxing the body as much as additional eccentric reps would. Keep in mind, this is an add-on that does not replace sound programming. If you are performing sets well short of failure (>4 reps) then you are better off adding repetitions than adding stretching. Perhaps this is a mechanism to assist with people who dislike exercise and pushing close to failure. We need more data to make generalizable recommendations.