It is well established through research that agonist stretching reduces repetition output and performance. Agonists, aka the "working" muscles" are oftentimes stretched mid-set in an attempt to loosen up and get more work done. How often do you see someone in the gym finish a set of bench press only to stand up and stretch their chest? We now know, that it may be more ideal to do the opposite and stretch your back instead.
Antagonist muscle pairs are opposing muscle groups. The most basic example I usually give for laymen purposes involve the biceps and triceps. Biceps flex the elbow, the triceps extend the elbow. Not only does the brain signal for the biceps to flex during elbow flexion (agonistic action), but it must also signal for the muscle fibers of the triceps to relax in order to lengthen (antagonistic action).
An interesting study from Brazil tells us that antagonistic stretching mid-set is beneficial in increasing reps output:
Acute Effects of Antagonist Static Stretching in the Inter-Set Rest Period on Repetition Performance and Muscle Activation
The purpose of this study was to investigate the effects of antagonist passive static stretching (AS) during the inter-set rest period on repetition performance and muscle activation. Ten trained men (22.4 ± 0.9 years) participated in this study. Two protocols were adopted: Passive recovery (PR) – three sets to repetition failure were performed for the seated row (SR) with two-minute rest interval between sets without pre-exercise stretching; AS – forty seconds of stretching was applied to pectoralis major prior to each set of SR. Significant increases in the number of repetitions were noted under AS compared with PR (p < 0.05). Significant increases on latissi- mus dorsi (p = 0.002) and biceps brachii (p = 0.001) muscle activity were noted inter-sets under the AS compared with the PR
So in this study, participants completed a seated row with or without static chest stretching during inter-set rest periods. The participants that stretched their chest were able to complete more repetitions than those who did not stretch.
We can speculate the reasoning for this, based on the information above - thinking of the actions on a given joint as a "tug of war" between two muscles. Again, agonists shorten or pull, while the antagonist must simultaneously relax and lengthen. If we spend time stretching the antagonist in between sets, it appears to aid in the relaxation of the opposing muscle group to allow for more activation in the agonist, or working muscle group.
Tyler Robbins B.Sc. CSCS
Director of Fitness
Head of CrossFit