Influence of Plantarflexor stretch training on fascicle length and strain, H-Reflex amplitude and muscle force production

Chronic stretching training is commonly performed by athletic and patient populations to improve joint range of motion (ROM), reduce injury risk and, possibly, enhance movement performance. In this study, we examined changes in resting gastrocnemius medialis (GM) fascicle length and fascicle strain during stretch (ultrasound imaging) and soleus H-reflex amplitude (stimulation of tibial nerve) after 3 weeks of twice-daily static stretches in 12 healthy men; 10 men acted as non-training controls. These changes were then related to changes in peak active joint moment, rate of torque development (RFD) and the moment-angle relation (MArel) of the plantarflexors. In each stretch training session, four 30-s static stretches were performed to maximum stretch tolerance with the knee extended. Self reported training adherence was 92%. All dependent variables except H-reflex were measured pre- and post-training at 5 joint angles: 20 deg (angle 1) and 10 deg (angle 2) of plantarflexion, and 3 further equidistant angles (angles 3-5) where angle 5 was defined as the angle where 80% of the peak passive moment was developed (~3-4 deg from maximum ROM). H-reflex was assessed at joint angles 2, 3 and 4. There were no changes in any measure in the control group. Stretch training elicited a 20% (8.6 deg; p<0.001) increase in ankle ROM. This could not be explained by a change in resting GM fascicle length. However fascicle strain measured from angle 1-5 increased significantly (28.8% increase, p<0.05). This increased strain was associated with a significant post-training depression of the resting (2% MVC) H/M-wave ratio at joint angles 2 and 3, but not 4. Increases in fascicle compliance were related to (p<0.05) decreases in H/M ratio at angles 2 (r=0.71) and 3 (r=0.54), but not 4 (r=0.18). The decrease at angles 2 and 3 may partly explain the increased fascicle strain as a reduced reflex sensitivity, but the lack of change at angle 4 does not. These data are suggestive that the increased strain with stretch training resulted from decreased fascicle strain sensitivity, in which the observed decrease in spinal reflex input might have been influential. Despite these changes, there were no changes in RFD measured from contraction onset to 50, 100, 200 and 400 ms or from 200-400 ms at any joint angle and no shift occurred in MArel. These data are symptomatic of a lack of change in SEC compliance. There was also no change in peak active moment at any joint angle. Chronic stretch of the plantarflexors is associated with an increased ROM due largely to an increased fascicle strain, which may be partly linked to increased parallel elastic, but not SEC, compliance. It may also be partly linked to a decreased spinal excitability. These changes did not affect peak active moment, RFD or alter the moment-angle relation. It is not yet clear whether dynamic force is affected, or whether performance changes might occur with longer periods of stretch training.