Should the neural–mechanical behaviour of a muscle be coupled or co‐vary?

Abstract

muscle (FDI) of the hand. By using this particular muscle, the authors were able to circumvent many of the limitations traditionally associated with making the neural‐mechanical comparisons at joints controlled by many muscles. The results from that study led the authors to conclude that the neural drive to the FDI muscle and the mechanical advantage of the muscle are coupled when the thumb changes position. The remainder of this note will focus on the impact of the study and the interpretation of the results, specifically with regard to whether the neural drive to the muscle and the muscle mechanics should co-vary or be coupled when performing motor tasks. The study by Hudson et al. (2009) recorded end-point forces produced (i) by ulnar nerve stimulation and (ii) during static index finger flexion efforts, with the thumb positioned in a number of ‘thumb up’ and ‘thumb down’ postures. Although a seemingly minor postural adjustment, ulnar nerve stimulation resulted in a 60% larger end-point twitch force in the thumb down position compared to thumb up. Furthermore, ultrasound results indicated that, relative to thumb up, the thumb down posture caused a 65% increase in the distance between the FDI tendon and the lateral tubercle of the second metacarpal, which was used to estimate FDI moment arm magnitude. Thus, the mechanical advantage for FDI force transmission across the metacarpophalangeal (MCP) joint was significantly increased with the thumb down, regardless of muscle force. Based on the mechanics, end-point force is directly proportional to the MCP joint torque, via the geometric configuration of the system. In other words, sincetheindexfingerwassplintedandMCP jointangleremainedconstant,momentarm magnitude was the main geometric factor determining how effectively FDI muscle force was transformed into end-point force. The end-point forces generated by supramaximal ulnar nerve stimulation (i.e. constant neural input) illustrate the proportionality between MCP joint torque and end-point force (a 65% increase in moment arm → 60% increase in end-point force). However, the mechanics are only one aspect of generating joint torques. Can, or does, the central nervous system