Figure 1. Testing apparatus and experimental set up.

The testing apparatus used consists of a rotary stepper motor (a) with a digital stepper drive/controller (b), an aluminium rod (c) instrumented with two pairs of strain gauges (d) and an auxiliary piece for thumb placement (e), with an analog encoder (f). When the adductor pollicis contracts, the thumb presses against the rod and thumb adduction force is measured by the calibrated strain gauges. The forearm and the four digits of the left hand are restrained with a clinical cast (g) and secured on the apparatus using velcro straps. The thumb is placed and aligned on the rod as shown in the pictures. This experimental set up was designed to measure thumb adduction force during voluntary and electrically elicited, static and dynamic contractions.