Author(s): SCHWENK, M., GRUBER, M., RUGGIERO, L., Institution: UNIVERSITY OF KONSTANZ, Country: GERMANY, Abstract-ID: 1379

INTRODUCTION:
Unloaded or assisted jumping is used in assessment and training to reduce mechanical loading while maintaining functional movement patterns. However, most approaches focus on countermovement jumps, which are suitable for assessing force and power but do not adequately address repetitive stretch-shortening cycle and stretch-reflex behavior [1]. To target the stretch-shortening cycle in populations unable to safely perform hopping tasks, such as older adults or patients in rehabilitation, unloaded repetitive hopping is required. However, validated protocols for unloaded hopping are lacking. Therefore, this pilot study aimed to develop and biomechanically validate an elastic assisted hopping protocol enabling progressive unloading of up to 40% of body weight (BW) while preserving key stretch-shortening cycle characteristics.
METHODS:
Five healthy adults (32.8 ± 9.1 years) performed repetitive hopping on a force plate using a custom-built elastic assistance system providing progressive unloading (0, 10, 20, 30 and 40% BW). For each condition, participants completed 60 consecutive hops using a self-selected frequency. Peak force, contact time, flight time and hopping frequency were derived from ground reaction force data. Activity of the soleus and tibialis anterior muscles was recorded and quantified as the integral of the rectified EMG signal within 100 ms after ground contact (SOL EMG and TA EMG, respectively). For soleus, the short-latency reflex (SLR) was derived as the integration of the electromyographic signal between 30 and 60 ms after ground contact and was additionally normalised to SOL EMG (SLR/SOL EMG). Non-parametric repeated-measures statistics were applied using Friedman tests, with effect sizes reported as Kendall’s W.
RESULTS:
Significant main effects of unloading were observed for peak vertical ground reaction force (p = 0.001, W = 0.90), flight time (p = 0.003, W = 0.79), hopping frequency (p = 0.008, W = 0.70) and SLR (p = 0.048, W = 0.48). Peak vertical ground reaction force decreased progressively with increasing unloading, indicating effective mechanical load reduction. Flight time increased with unloading, whereas hopping frequency showed a systematic decrease with increased unloading. SLR decreased progressively with higher unloading levels. In contrast, contact time, SOL EMG, TA EMG and the SLR/SOL EMG ratio showed no significant main effects (p > 0.18).
CONCLUSION:
Elastic assisted hopping effectively reduces mechanical loading while preserving essential temporal characteristics of hopping. The reduction in SLR, in the absence of changes in normalised SLR/SOL EMG, indicates diminished stretch velocities with decreased loading rather than a loss of stretch-shortening cycle function per se, supporting feasibility for populations unable to tolerate high mechanical loading. The protocol is sensitive to graded unloading and provides a methodological basis for future studies.
References: [1] Tufano JJ et al. J Strength Cond Res, 36(6), 1518–1523, 2022.