Neuromuscular-skeletal constraints upon the dynamics of unimanual and bimanual coordination

In the first of three experiments, 11 participants generated pronation and supination movements of the forearm, in time with an auditory metronome. Th e metronome frequency was increased in eight steps (0.25 Hz) from a base fr equency of 1.75 Hz. On alternating trials, participants were required to co ordinate either maximum pronation or maximum supination with each beat of t he metronome. In each block of trials, the axis of rotation was either coin cident with the long axis of the forearm, above this axis, or below this ax is. The stability of the pronate-on-the-beat pattern, as indexed by the num ber of pattern changes, and the time of onset of pattern change, was greate st when the axis of rotation of the movement was below the long axis of the forearm. In contrast, the stability of the supinate-on-the-beat pattern wa s greatest when the axis of rotation of the movement was above the long axi s of the forearm. In a second experiment, we examined how changes in the po sition of the axis of rotation alter the activation patterns of muscles tha t contribute to pronation and supination of the forearm. Variations in the relative dominance of the pronation and supination phases of the movement c ycle across conditions were accounted for primarily by changes in the activ ation profile of flexor carpi radialis (FCR) and extensor carpi radialis lo ngus (ECR). In the Final experiment we examined how these constraints impac t upon the stability of bimanual coordination. Thirty-two participants were assigned at random to one of four conditions, each of which combined an ax is of rotation configuration (bottom or top) for each limb. The participant s generated both inphase (both limbs pronating simultaneously, and supinati ng simultaneously) and antiphase (left limb pronating and right limb supina ting simultaneously, and vice versa) patterns of coordination. When the pos ition of the axis of rotation was equivalent for the left and the right lim b, transitions from antiphase to inphase patterns of coordination were Freq uently observed. In marked contrast, when the position of the axis of rotat ion for the left and right limb was contradistinct, transitions From inphas e to antiphase patterns of coordination occurred. The results demonstrated that when movements are performed in an appropriate mechanical context, inp hase patterns of coordination are less stable than antiphase patterns.