Influence of Different Wrist Positions on Lumbrical Muscle Activity in the Hand: A Surface Electromyographic Investigation.

Background: The lumbrical muscle is crucial for fine motor control by balancing the activity of the extrinsic flexor and extensor muscles. However, it remains unclear whether wrist position affects lumbrical muscle activity. This study aimed to investigate the effect of wrist joint position on lumbrical muscle activity using surface electromyography. Methods: The activities of the first lumbrical, first dorsal interosseous, flexor digitorum profundus and extensor digitorum muscles were measured in 19 right-handed healthy adults (mean age: 25.0 ± 3.4 years). Participants performed two tasks: (1) isometric metacarpophalangeal (MP) joint flexion at 30% maximal voluntary contraction and (2) isotonic proximal/distal interphalangeal (PIP/DIP) joint flexion-extension. These tasks were conducted in five wrist positions (60° flexion, 30° flexion, 0°, 30° extension and 60° extension). Muscle activities were compared amongst wrist positions for each task. Results: In the isometric MP joint flexion task requiring lumbrical muscle strength, wrist joint position partially influenced lumbrical muscle activity, with a significant difference observed only between the 0° and 30° extension. In the isotonic PIP/DIP flexion-extension task, which requires rapid fine finger movements, there was no difference in the lumbrical muscle activity between the wrist joint positions. Conclusions: In both isometric and isotonic motor tasks, the effect of the wrist position on lumbrical muscle activity was minimal, indicating a constant level of muscle activity. These results suggest that the lumbrical muscles maintain balance between the flexor and extensor muscles, enabling humans to grasp and manipulate objects, regardless of wrist joint position.