Author(s): FAZZARI, C., MACCHI, R.2, CHAVET, P.1, CASANOVA, R.1, NICOL, C.1, Institution: AIX-MARSEILLE UNIVERSITÉ, Country: FRANCE, Abstract-ID: 2370

INTRODUCTION:
Runners are often confronted with changing external forces. This requires rapid locomotor adjustments, which can be studied using the experimental paradigm of unweighting and reloading. Recently, we found specific adjustments of muscle synergies when running at 60% body weight, compared to 100% body weight (1). In addition to a greater hamstring contribution to the push-off phase, their temporal activations (motor primitives) were wider and more complex. While these adjustments have been described during stable running phases, they need to be specified during the unweighting and reloading transition phases to better understand how locomotor control reorganizes at each running cycle.
METHODS:
Thirty-eight men (19±1 yrs) ran sequentially on a lower body positive pressure treadmill at 100, 60 and 100% body weight. The analysis focused on the unweighting and reloading transitions (18±3 and 16±2 running cycles, respectively). Each running cycle was normalized to 200 points (100 points for both stance and flight phases). Muscle synergies were extracted from the EMG signals of 11 right lower limb muscles using non-negative matrix factorisation, and divided into motor modules and motor primitives. Linear mixed models were used to test the adjustments of their Centre of Activity (CoA), Full Width at Half Maximum (FWHM) and Higuchi’s Fractal Dimension (HFD) during both transition types, which were each divided into 8 slides of 5% body weight reduction (from S1: [100-95%] to S8: [65-60% body weight]).
RESULTS:
The CoA of the push-off motor primitives increased significantly from S4 during the unweighting transition (ES=1.1, p<.001), whereas it decreased significantly from S6 during the reloading transition (ES=0.8, p<.01). Regardless of transition type, the CoA of the late flight primitives decreased from S7 (ES=1.2, p<.05). Regardless of the slide, the CoA of the braking primitives and the FWHM of the push-off primitives were higher during the unweighting transition than during the reloading transition (+33±11%, ES=0.3, p<0.01 and +9±3%, ES=0.4, p<.001, respectively). The HFD of the braking primitives was lower during the unweighting transition than during the reloading transition (-0.8±0.3%, ES=0.3, p<.01).
CONCLUSION:
During the unweighting transition, the push-off motor primitives shifted later and the late flight primitives shifted earlier in the running cycle. The latter result is attributed to the gradual increase in the contribution of the hamstring muscles to the push-off phase, previously reported at 60% body weight (1,2). Finally, the stance motor primitives were wider and of lower complexity during the unweighting transition than during the reloading transition. As both are indicators of perturbed locomotion (3), the unweighting transition may require a more robust locomotor control due to the unusual sensory and temporal constraints on the musculoskeletal system.

(1) Fazzari et al., Sci Reports, 2024
(2) Fazzari et al., Front Physiol, 2023
(3) Santuz et al., iScience, 2020