Author(s): VIEIRA, D., SILVA, M.A., BABAULT, N., BOTTARO, M., Institution: UDF - CENTRO UNIVERSITÁRIO, Country: BRAZIL, Abstract-ID: 1978

INTRODUCTION:
Dynamic stretching (DS) is a widely recognized warm-up practice that induces acute adaptations in muscle performance, potentially through acute neural and/or mechanical changes [1]. However, the magnitude of acute neuromuscular changes during DS may vary with movement velocity and volume [2]. Therefore, this study aimed to investigate how DS volume and movement velocity affect homosynaptic depression and spinal reflex activity.
METHODS:
Eight young men (22 ± 5.24 years) performed four experimental sessions in random order: 1) Control Session; 2) DS with high volume and moderate velocity (7 sets of 10 repetitions at 60 beeps per minute - bpm; DS-HM); 3) DS with moderate volume and moderate velocity (4 sets of 10 repetitions at 60 bpm; DS-MM); 4) DS with moderate volume and high velocity (4 sets of 10 repetitions at 90 bpm; DS-MH). The protocols were conducted on the plantar flexors of the right ankle, with participants lying on the isokinetic machine with the knee fully extended. The rest intervals between the sets were 30 seconds. The homosynaptic depression and spinal reflex activity were assessed using tibial nerve stimulation associated with paired Hoffman reflex ratio (Hcon/Href), and maximal Hoffman reflex normalized to maximal Mwave (Hmax/Mmax) of the soleus muscle before (PRE), immediately after (POST), and 10 minutes (P10) after each protocol.
RESULTS:
The Generalized Mixed Model showed no significant main effect of condition (p = 0.106), but significant main effects of time (p = 0.005) and a time × condition interaction (p = 0.042) for homosynaptic depression. Tukey post hoc analysis indicated that the Hcon/Href ratio was significantly reduced immediately after the protocol only in the DS-HM condition (p = 0.007). However, this reduction was transient, with values returning to baseline 10 minutes after the DS protocol (p > 0.05). For the spinal reflex, the Generalized Mixed Model revealed no significant main effect of time (p = 0.157) but significant main effects of condition (p = 0.001) and a time × condition interaction (p = 0.001). Tukey post hoc analysis demonstrated a significant reduction in the Hmax/Mmax ratio at the P10 time point compared with PRE (p = 0.001) and POST (p = 0.001) in the DS-MM condition.
CONCLUSION:
A high volume of dynamic stretching reduces homosynaptic depression. Additionally, moderate movement velocity may attenuate spinal reflex activity, but only when performed with a moderate stretching volume. Collectively, these findings indicate that both stretching volume and movement velocity interact to modulate spinal excitability during dynamic stretching.