QUANTIFYING THE CONTRIBUTION OF PASSIVE STRUCTURES IN FINGER INDEPENDENCE

Abstract

Mechanical and neural factors have been suggested to limit finger independence. Fingers producing involuntary movement or force production during intended actions of another finger are considered “enslaved” to that finger. The purpose of this thesis was to quantify the contribution of passive mechanical factors to this enslaving effect, in particular, the contributions of the intertendinous connections between extensor tendons. Twenty participants (10 men, 10 women) performed Master and Slave Tasks in three wrist (30° extension, neutral, 30° flexion) and two metacarpophalangeal (MCP) (straight and 90° flexion) postures. During the Master Task, the ring finger was the intended or “master” finger. Three 10 s isometric ring finger extensions were performed at 25% of maximum voluntary contraction. Finger force and surface electromyography of the 4 extensor digitorum (ED) bellies were recorded. In the Slave Tasks, the middle and little fingers (“slave” fingers during the Master Task) each performed three 10 s isometric finger extensions at their mean activation levels during the Master Task. Hypothetical mechanical contribution (HMC) was determined for the middle and little fingers. The HMC was defined as the difference between the involuntary force (from the Master Task) and the voluntary force (from the Slave Task) relative to the involuntary force. A small proportion of the HMC values (Middle: 39%; Little: 15%) were within the expected range of 0 to 100%, suggesting that the equation developed in this study provided a limited representation of the contribution of passive intertendinous structures. Index finger forces increased with MCP flexion, suggesting the importance of juncturae tendineii in finger independence. Higher ED activity during wrist extension, than neutral or flexed postures, with straight MCP supports previous evidence in the literature. The complex phenomenon of enslaving in different wrist and MCP positions warrants further research for quantifying the mechanical contribution in finger independence.