Analysis of Stretch Reflex Activity—Fast Stretches

We aimed to indicate whether the torque arising due to stretch reflex activity was impacted by the fast 120°/s stretches before and after volitional muscle activation and rest. To do so, the net torque outcome measure, defined in section Quantifying Stretch Reflex Activity—Fast Stretches, was fit to a linear mixed-effects model, with participant as a random effect. We identified whether the net torque depended on the stretch repetition (1–20) and set (first, second) for each testing condition (volitional muscle activation, rest) and muscle group (flexors, extensors). An analysis of variance with a Tukey adjustment identified significant fixed effects.

In addition to identifying effects across all stretches, we also identified effects for specific pairs of stretches. For these pairs, data were analyzed across all participants using a pairwise t-test with a Bonferroni correction.

To begin, we indicated whether the net torque significantly differed between the following pairs of stretches for each testing condition and muscle group. We compared the first set's final stretch and second set's first stretch to determine whether volitional muscle activation and rest had an immediate effect on stretch-induced accommodated reflex activity. Additionally, we compared the first stretch of the first set to the first stretch of the second set to determine the impact of volitional muscle activation and rest on reflex activity when compared to the reflex activity prior to stretching. Moreover, we compared the final stretch of the first set to the final stretch of the second set to determine whether volitional muscle activation and rest affected the extent to which the reflex could be accommodated with the fast 120°/s stretches.

Following, for each muscle group we determined whether the difference in the net torque between each of these pairs of stretches within each session depended on the testing condition.

We aimed to indicate whether the short- and/or long-latency response corresponding to spinal reflexes and potentially cortical circuitry, respectively, were affected by the fast 120°/s stretching after the participant volitionally activated their muscles or relaxed. To do so, we fit the SLR and LLR outcome measures described in section Quantifying Stretch Reflex Activity—Fast Stretches to a linear mixed-effects model, with participant as a random effect. Subsequently, we determined whether the SLR and LLR depended on the stretch repetition (1–20) and set (first, second) for each testing condition (volitional muscle activation, rest) and muscle group (flexors, extensors). An analysis of variance with a Tukey adjustment identified significant fixed effects.