Author(s): KRÄTSCHMER, R., SCHWIRTZ, A., Institution: TECHNICAL UNIVERSITY OF MUNICH, Country: GERMANY, Abstract-ID: 1795

INTRODUCTION:
Assessment of neural control of skeletal muscles is achieved by analyzing motor unit (MU) discharge properties. Typical testing scenarios involve force modulation in non-weight bearing tasks, e.g. isolated knee extension in a seated position. However, humans evolved to stand and move in an upright posture where the leg extending muscles generate force against gravity (1). To address this gap, we devised a setup to measure MU activity during force modulation in different knee angles in an upright weight-bearing scenario.
METHODS:
Five participants underwent measurements while standing with both feet on a force plate, with a loaded barbell fixed above their necks, providing immovable resistance. Shin position was standardized to ensure identical ankle angle for all trials. Measurements involved isometric maximal voluntary contractions (MVC) and biofeedback-controlled submaximal force modulation (15 seconds at body weight (BW) followed by 15 seconds at BW+20%MVC) against the immovable barbell in three knee angles (30°, 60°, 90° adjusted by barbell height). High-density EMG signals from vastus lateralis (VL) and vastus medialis (VM) were recorded and decomposed (2,3) into MU discharge rates (DR). Mean DR was calculated for low-threshold (LT, already firing at BW) and high-threshold MUs (HT, start firing above BW). Two-way ANOVA for factors muscle and angle and RM ANOVA for force modulation of LT was performed. Linear regression for relative MVC vs. LT modulation (from BW to BW+20%MVC) was calculated.
RESULTS:
While the MVCs increased with a more upright position (90°: 164±51N/kg; 60°: 238±67N/kg; 30°: 341±106N/kg; p<0.001), there was no difference in DR between angles or muscles for LT. At BW+20%MVC, LT showed higher discharge rate than HT (11.90±2.25 and 10.71±2.08pps; p>0.001). During force modulation from BW to BW+20%MVC, LT increased firing rate from 10.06±2.42 to 11.90±2.25pps (p<0.001). There was a significant correlation between relative MVC (averaged across angles) and the difference of DR in LT from low- to high-force (R=0.8630; p=0.0225).
CONCLUSION:
As the first study to introduce force modulation in a weight-bearing task, we observed that LT are already firing when bearing own bodyweight, which reveals a functional importance not reflected by isolated non-weight bearing setups. Despite the force differences between angles, the firing rate of MUs at submaximal force did not change indicating that the neural control of VL and VM is regulated independently of muscle length. However, we observed that stronger individuals have a higher ability to increase LT firing rate during force modulation, indicating an association between submaximal neural modulation ability and maximal force generation capacity.

REFERENCES:
1) Winter, Gait Posture, 1995
2) Holobar & Zazula, IEEE Trans signal Process, 2007
3) Holobar et al., J Neur Eng, 2014