CONTRIBUTION OF VELOCITY CONGRUENCY AND APPARENT STIFFNESS TO INTERFERENCE OF BIMANUAL RHYTHMIC-DISCRETE MOVEMENTS.

Abstract

Many actions feature rhythmic and discrete movements, individually or in combinations. Rhythmic movements are defined as those having no clearly defined start and end-point, while discrete movements have a definite starting and ending posture. Performing a discrete movement against a base rhythm by the contralateral limb typically speeds up the rhythmic movement -- indicating the presence of bimanual coupling. While bimanual rhythmic/rhythmic interaction has been studied extensively in the field, understanding of the interaction between discrete and rhythmic movements has been less represented. In this thesis, I examined two potential sources of interlimb interference during rhythmic-discrete bimanual actions: 1) velocity congruence of the limbs, 2) co-contraction level of the synergist muscles. Hypothesis 1 predicted that the extent of rhythmic-discrete interference depends on the discrepancy between the velocities of the rhythmic and discrete movements, such that faster discrete movement would speed up the rhythmic movement and vice versa. Hypothesis 2 predicted that speeding up the rhythmic movement following the discrete response would be associated with increased muscle co-contraction in the rhythmic limb because higher apparent stiffness typically increases the natural frequency of oscillations. To address these hypotheses, I used a computational model of upper limb movements proposed by Ronsee et al. that allows simulation of both unimanual and bimanual rhythmic and discrete movements using the central pattern generator (CPG) concept. Discrete movement with an amplitude of 60 deg was simulated in two velocity conditions: Slow (peak velocity: 163 deg/s) and Fast (249 deg/s), these values differed by approximately 20% from the peak velocity of the rhythmic movement. Phase discrete movement initiation within rhythm was also manipulated. Dependent measures included rhythmic movement period, amplitude, phase, and level of co-contraction following the initiation of the discrete response were examined. The results showed that 1) the velocity of the discrete movement does not account for the changes of rhythmic behavior and (2) the level of co-contraction and the period shift of the rhythmic arm do not co-vary. However, the results suggested that the response of the rhythmic arm is dependent of the rhythmic movement's phase during which the discrete movement is initiated.