Effect of co-contraction and muscle strength on muscle-tendon dynamics

Abstract

I. INTRODUCTION

Muscular co-contraction may be necessary for maintaining stability on uneven surfaces. However, co-contraction analyses are limited due to a sole reliance on electromyography (EMG) measurements, infrequent analysis on uneven terrain, and a focus on muscles stabilizing the hip, knee, and ankle joints—overlooking the 33 joints of the foot and ankle [1]. One such joint is the subtalar joint, crucial for stability in the medial and lateral directions. Recent evidence indicates that co-contraction may influence muscle-tendon dynamics [2]. Yet, the impact of co-contraction on muscle-tendon dynamics, particularly in stabilizing the subtalar joint under mediolateral perturbations, remains unclear. In addition, studies indicate that co-contraction may be essential for populations with reduced muscle strength [3]. However, no study investigates how co-contraction impacts muscle-tendon dynamics while humans with reduced muscle strength walk on mediolaterally unstable surfaces.

The first aim studies how muscles crossing the subtalar joint contribute to foot stabilization and how co-contraction strategies affect muscle-tendon dynamics in young healthy adults. The second aim mirrors the first but employs modelling walking behavior in a reduced strength scenario.

II. METHODS

20 young participants walked on a force-instrumented treadmill at a slow, preferred, and fast walking speed wearing five different 3D printed footwear conditions promoting or limiting foot pronation (Fig. 1). 30 reflective markers were placed over body landmarks and tracked using a 12 – camera motion capture. EMG activity of the tibialis anterior (TA), peroneus longus, and peroneus tertius (PT) was measured using surface electrodes. The TA and PB were imaged with two B-mode ultrasound probes.

Co-contraction was assessed for the TA/PL and TA/PT muscle pairs through two metrics: 1) the ratio of total antagonist to agonist muscle activity and 2) the ratio of total antagonist to agonist muscle moments. For the reduced strength model muscle-driven simulations will be created where maximal isometric forces will be reduced by 30% [4].

Fig. 1 3D – printed footwear conditions. III. RESULTS AND EXPECTED RESULTS

Using metric 1, no significant differences (p < 0.05) were found in co-contraction of the TA/PL and TA/PL muscle pairs across all footwear conditions. Using metric 2, we anticipate increased co-contraction on uneven surfaces and at higher walking speeds. Co-contraction is expected to cause muscle-tendon units to operate more optimally, either by operating at their optimal length or isometrically.

For the reduced strength model, it is expected that co-contraction will increase on uneven surfaces with both metrics and relative to younger adults. Co-contraction will also not increase with increased walking speeds, relative to the slowest walking speed. Compared to younger adults, co-contraction may cause muscles to not function at their optimal length and/or isometrically.

ACKNOWLEDGEMENTS

NSERC awarded to M.J. Asmussen funded this study.

REFERENCES

1. Banks C. L et al. (2017) Electromyography exposes heterogeneity in muscle co-contraction following stroke. Front Neurol 8:699
2. Dick T. J et al. (2021) Series elasticity facilitates safe plantar plexor muscle-tendon shock absorption during perturbed human hopping Proc Royal Soc B 288:1947
3. Ebisu S et al. (2022) Decrease in force control among older adults under unpredictable conditions. Exp Gerontol 158:111649
4. Webber S. C et al. (2009) Modelling age-related neuromuscular changes in humans. Appl Physiol Nutr Metab 34:732-744