Neural and biomechanical specializations of human thumb muscles revealed by matching weights and grasping objects

1. Human manual dexterity has been linked by some to biomechanical adaptations of the hand and by others to neural adaptations. To investigate neural adaptations, the present study using the performance of four muscles acting on the index and thumb, quantified weight matching and electromyography. 2. The accuracy (i.e. reproducibility) of weight matching was used to investigate whether thumb muscles (i.e. flexor pollicis longus (FPL) and adductor pollicis (AP)) perform differently from index muscles (i.e. flexor digitorum profundus (FDP) and first dorsal interosseous (FDI)), and whether intrinsic hand muscles (AP and FDI) perform differently from extrinsic ones (FPL and FDP). 3. Subjects lifted reference weights on the right which represented predetermined percentages of a force generated in a maximum voluntary contraction (MVC) ranging from 2.5% to 35% MVC (and to 50% MVC in two muscles) and matched them with a variable weight lifted in the same way on the left. 4. Analysis of the coefficients of variation (c.v., expressed as a percentage) and the standard deviations calculated for repeated estimates of perceived heaviness, revealed significant differences in the accuracy of weight matching between different muscles and between reference weights. Based on the c.v., subjects lifted more accurately with FPL and AP (the two thumb muscles) than with the two index muscles. The two intrinsic hand muscles (FDI and AP) were equally accurate, and significantly more accurate than FDP which was the least accurate muscle. The high accuracy for FPL remained when accuracy was expressed in terms of the torque produced by the muscles when lifting the reference weights, and also when the torques were converted to absolute intramuscular forces. 5. Accuracy (based on c.v.) decreased significantly with light weights and increased with heavy weights for all muscles except FPL, which was equally accurate over a very wide range of weights (< 2.5% to 50% MVC). When data from all muscles were pooled, the c.v. increased from 12.9 to 19.1 as the weights lifted decreased from 35% to 2.5% MVC. 6. To examine the functional implications of the weight-matching study, electromyographic activity (EMG) was recorded with fine-wire electrodes from the same four muscles while subjects lifted cylinders of different widths (17-50 mm) and weights (15-1000 g). For recordings in which integrated EMG was linearly related to force up to maximal levels, the amplitudes of the EMGs at 'lift off' and at the mid-point of the 'hold' phase of the task were expressed relative to the maximal EMG during a MVC.(ABSTRACT TRUNCATED AT 400 WORDS)