Author(s): HOWALDT, J., CHETOUANI, A., RAHY, K., RUGGIERO, L., GRUBER, M., Institution: UNIVERSITY OF KONSTANZ, Country: GERMANY, Abstract-ID: 2187

INTRODUCTION:
Ia afferent feedback contributes to the stretch reflex during stretch–shortening cycle (SSC) exercise [1]. However, its contribution to performance in specific SSC tasks remains debated [2,3]. One approach to address this question is to evoke additional Ia input via H-reflex stimulation and examine its influence on performance, especially before the natural short-latency reflex response can modulate stiffness. We therefore tested whether H-reflexes during pre-activity or early ground contact (GC) affect performance during hopping.
METHODS:
Ten healthy adults (2 female, 31 ± 8 years) completed a within-subject repeated-measures study. Participants performed 20 sets of 40 submaximal hops (60 ± 15% of max. hop height) with 2-min rest between sets. For each hop, we randomly applied tibial nerve stimulation (-100 to +30 ms relative to GC) or no stimulation and adjusted stimulation intensity to maximize the Soleus H-reflex while restricting M-wave amplitude to <10% of Mmax. We quantified performance using the Reactive Strength Index (RSI) and analyzed it with a generalized additive mixed model including a smooth interaction of normalized H-reflex amplitude (H/Mmax) and timing, with subject as a random effect and previous flight time and Soleus pre-activity as covariates. Further, we identified connected regions where the modeled stimulation effect differed from zero using 95% simultaneous confidence bands.
RESULTS:
After quality control, we analyzed 4000 hops from nine participants. One participant was excluded because all stimulated trials exceeded the predefined M-wave threshold. A significant H-reflex amplitude x timing interaction predicted RSI (p < .001) explaining 0.09% of additional variance beyond subject effects and covariates. Within this effect, the model identified:
A facilitative region at ~13% of H/Mmax and ~30% of GC (~61 ms after GC), with a median dRSI of +4.2% (range: +3.1% to +6.1%), and
A suppressive region at ~18% of H/Mmax and ~3% of GC (~7 ms after GC), with a median dRSI of -2.0% (range: -2.4% to -1.6%).
The facilitative region was only covered by data of one participant and should be interpreted cautiously.
CONCLUSION:
Overall, externally evoked Ia input during pre-activity or early GC-phase had minimal influence on hopping performance. This suggests that a single H-reflex contribution during hopping is of minor functional relevance or that Soleus activity during early SSC phases may already be optimally aligned with task mechanics, leaving little capacity for performance-relevant integration of externally evoked Ia input. Misaligned Ia input may instead slightly disrupt motor output, indicated by the early suppressive region. In the participant showing facilitation, H-reflex timing coincided with the stretch reflex response, suggesting potential temporal summation of spinal reflex activity.

[1] Zuur et al., 2010, J Physiol, 588(Pt 5):799-807.
[2] Rissmann et al, 2025, J Physiol, 603(14):3987-4004.
[3] Taube et al., 2025, J Physiol, 603(22):7397-7398.