Investigating Muscle Coordination during Dynamic Functional Reach Assessment

Introduction The Functional Reach (FR) is used to evaluate dynamic balance by measuring how far one can reach forward beyond arm’s length while standing, beginning with the arm at 90° of shoulder flexion [1]. In everyday activities, however, reaching may also involve lifting the arm from a resting position to reach an object, requiring higher coordination and motor control. The understanding of muscle coordination mechanisms underlying FR with arm elevation and lowering is missing. This study aims at providing a baseline understanding of muscle coordination patterns by investigating muscle synergies in healthy young individuals, which lays the groundwork for future research on elderly population and on age-related changes in muscle synergies. Methods Seventeen healthy volunteers (7F, 27.53 ± 3 y.o.) completed 10 repetitions of standardised bilateral (BiFR) and unilateral (UniFR) FR, using both arms for BiFR and their dominant arm for UniFR. Bilateral activity was recorded from 16 muscles by surface electromyography (EMG): Anterior Deltoid (AD), Erector Spinae (ES), Latissimus Dorsi (LD), Vastus Lateralis (VL), Tibialis Anterior (TA), Biceps Femoris (BF), Gastrocnemius Medialis (GastM), and Soleus (Sol). Kinematic data enabled segmentation of each repetition, which was then applied to pre-processed EMG data. Segmented, time normalised EMG data from all repetitions were concatenated and averaged to emphasize consistent activation patterns and reduce noise. Muscle synergies were extracted for each subject from the mean EMG dataset using a non-negative matrix factorization. The number of synergies was identified by [2,3]. Similarity of weight coefficients within subject groups and between UniFR and BiFR was evaluated using the scalar product (DOT) [4]. Similarity of synergies’ activation profiles was assessed with a non-parametric two-sample t-test using statistical parametric mapping. Results Three muscle synergies were necessary to reconstruct the EMG data for both tasks (Figure 1): Syn1 pertains to FR execution, involving ankle stabilizer muscles alongside AD and LD; Syn2 relates to braking during movement control and returning to upright posture, featuring ES and BF; Syn3 acts as a stabilizing synergy during arm elevation and lowering. No significant differences in synergies’ activation profiles were found. Discussion This study demonstrated that FR exhibits a typical muscle coordination pattern regardless of whether it is performed with one or both arms. Moreover, young individuals exhibited a preference for an ankle strategy in transitioning from a static to an unbalanced forward position. Future research is needed to elucidate if age-related decline in balance ability changes these patterns in elderly.