Whether to do static stretching or not can be a very contentious issue.
On the one hand, static stretching (especially intense stretching lasting >60 seconds) is known to reduce force in subsequent exercise. Many strength coaches therefore recommend against doing static stretching before a workout, or before a practice or competition, when working with athletes.
On the other hand, inter-set strength training has been recommended in some popular bodybuilding programs. And although the explanations provided for its efficacy are probably incorrect (fascial deformation), static stretching is essentially passive mechanical loading, and animal models have shown that this also increases muscle size long-term.
Surprising new research has shown that static stretching can increase muscle size (if not strength), even when the stretch is totally passive and involves no muscle activation at all. The key seems to be a fairly intense stretch, with progressively greater loading each workout.
Even though this comes as a surprise to many, there were already clues in the research, as rodent models had found that passive stretching could produce increases in p70S6K phosphorylation, which is part of the mTOR anabolic signaling pathway.
Such studies tend to show that active contractions and passive stretch can both cause increases in anabolic signaling activity, but that combined active and passive mechanical loading together are additive, and can produce the greatest changes.
This is probably why studies comparing strength training at long and short muscle lengths (such as full and partial squats) have typically reported greater hypertrophy in the groups training at long muscle lengths. After all, the muscle contractions at the longer muscle lengths combine the active and passive mechanical loading to produce a greater hypertrophic stimulus.
Even so, this does not indicate that static stretching is always valuable for hypertrophy when performed at any time. Although many bodybuilders use intense static stretching between sets of strength training, this may actually reduce hypertrophy by decreasing workout volume.
After all, strength training will always be the primary stimulus for hypertrophy, and if the static stretching has an adverse effect on the number of reps that can be performed in a workout, then it will be unhelpful overall. This suggests that if static stretching is used in addition to strength training to increase muscle growth, then it should be performed at the end of a workout and not either beforehand or in between sets of strength training.
This study investigated the long-term effects of a passive static stretching program on changes in strength, muscle size, and muscle fascicle length. The passive nature of the stretching was ensured by monitoring muscle activation using EMG.
This older rodent study provides some indications of why passive stretching might produce changes. It showed that passive stretch could trigger hypertrophy-related signaling, and therefore muscular adaptations, so long as the duration of the stretch was quite long (in this case 20 minutes) and the intensity was very high (in this case beyond physiological limits). In contrast, isometric contractions were able to produce comparable signaling events at physiological muscle lengths.
Several studies have shown that strength training with a larger range of motion (ROM) produces greater hypertrophy, as well joint angle-specific strength gains.
However, training with a larger ROM typically involves completing each rep at a longer (more stretched) muscle length than training with a shorter ROM. Therefore, it is unclear from most of the literature whether the superior effects of a large ROM are caused by the ROM itself, or by stretch.
This study investigated the effects of training with the exact same ROM but ending at either a long (stretched) or short muscle length. Both training programs used the exact same muscle force during each exercise, to make sure that the effect of stretch could be identified independently of the mechanical loading.
This study investigated the effects of pre-workout static stretching on training volume and gains in muscular strength and size. Importantly, it showed that there was a reduction in training volume, and that this impaired muscle growth.
Therefore, while static stretching can be used to enhance muscle gains, it probably should not be performed for the prime mover muscle immediately before the first set of an exercise (and possibly also not between sets). This suggests that a better strategy would be to perform stretching of the prime movers after the last set of an exercise, or to stretch antagonist muscles between sets, if hypertrophy is the only goal.