The wrist extensor muscles of the forearm exhibit relatively greater muscle activity than the wrist flexors during most hand/wrist tasks. Since the extensors operate at a greater percentage of maximum to balance wrist joint moments, this could contribute to their higher incidence of overuse injury. However, current knowledge of forearm muscle function comes primarily from isometric research or from studies examining isolated motor tasks. Conclusions derived from this work may not translate to tasks of daily living, which are typically dynamic and performed by multiple muscle actions simultaneously. Additionally, while fatigue develops more rapidly in the extensors than the flexors, the consequences of fatigue between these two muscle groups are presently unclear.
The objectives of this thesis were broken into two parts. Part 1: Quantify forearm muscle recruitment during the simultaneous execution of various handgrip and wrist forces (Chapter 3) and during dynamic wrist exertions (Chapter 4). Part 2: Characterize the effects of sustained isometric wrist flexion and wrist extension contractions on hand-tracking accuracy (Chapter 5) and investigate the underlying central mechanisms that may contribute to accuracy impairments (Chapter 6).
In Part 1, we identified that the muscle activity of the wrist flexors was highly sensitive to changes in dual-task parameters (grip and wrist exertions), while the activity of the extensors was consistently greater than the flexors during both dual-task and dynamic contractions. In some conditions, the wrist extensors exceeded flexor activity even during pure wrist flexion contractions. In Part 2, it was found that inducing fatigue separately through sustained wrist extension and wrist flexion contractions significantly impaired hand-tracking accuracy. However, there were no differences in hand-tracking accuracy between the two methods of inducing fatigue. This was surprising, given that follow-up work demonstrated both muscle activity and corticospinal differences between the muscle groups following sustained contractions.
This thesis provides a robust examination of the factors that can influence forearm muscle recruitment. It is also the first work to document the consequences of fatigue in opposing muscle groups of the forearm. The conclusions drawn from this research are essential in furthering our understanding of overuse injury development in the distal upper-limb
