Waveform analysis of forearm muscle activity during dynamic wrist flexion and extension: Effects of forearm posture and torque direction

Abstract

Background and Aim: For both isometric and dynamic movements at the wrist, a popular analysis technique for forearm muscle activation includes averaged time-series data that may not represent changes in muscle activity throughout the task. Changes in muscle fiber length and environmental stimuli can alter forearm/upper arm muscle activity during dynamic tasks (D. A. Forman et al., 2020a). The purpose of this study was to determine the effects of forearm posture and torque on forearm muscle activity using waveform analysis. Methods: 12 participants performed a controlled wrist flexion/extension (±40°) tracking task using a wrist manipulandum. Participants were positioned in a neutral, 30° pronated, or 30° supinated forearm posture and the manipulandum applied a constant torque that resisted either wrist extension or flexion. Posture-torque combinations were performed once each, with six flexion/extension repetitions completed per condition. Wrist kinematics were tracked using the manipulandum and the movement cycle was time normalized. Surface electromyography from eight forearm/upper arm muscles were normalized to maximum voluntary contractions. Statistical non-parametric mapping analyzed waveforms for each muscle using a two-way repeated measures ANOVA for main/interaction effects (p=0.05), with post-hoc t-tests. Results: All muscles showed main effects for both posture and torque direction. Decreases in activity were observed in non-neutral forearm postures (flexors: 53-70%, extensors: 5-23% of the cycle). Flexion torque increased muscle activity for FCR, FDS, and FCU during 0-56% and 75-100%, 9-81%, and 22-51% of the movement cycle, respectively. ED and ECU had significantly increased activity during 0-26% and 70-100% of the movement cycle during the extension torque direction. During the neutral-flexion condition, FCR activity increased compared to all other conditions during 58-70% of the movement. Conclusion: When evaluating the entire waveform, non-neutral forearm postures decreased activity for all muscles during specific ranges. The extension torque increased ED and ECU activity at the start and end of the movement, while the flexion torque increased FCR and FDS activity for the majority of the movement. Also, FCR was important in supporting wrist extension during the neutral-flexion condition. Waveform analysis demonstrated complex forearm muscle activity patterns that could provide insight into neuromuscular control, performance, and fatigue progression.