Maintenance tendency in co-ordinated rhythmic movements: relative fluctuations and phase

Evidence from the oscillatory behavior of fish fins and the crayfish swimmeret system suggests that local rhythmic-pattern generators preserve their characteristic properties over the various locomotory co-ordinations in which they participate. This maintenance tendency, as von Holst termed it, was investigated in an experiment in which human subjects swung, through motions at the wrists, hand-held pendulums of variable mass and length. In the experiment (comprising six sessions over 21 months with the same three subjects) the context for the maintenance tendency was steady-state absolute co-ordination: two rhythmic units oscillating at a single, common period and at a bounded phase relation. The experimental methodology permitted systematic control of (a) the characteristic periods of the individual rhythmic units and (b) the deviations from these periods. Relative fluctuations in periodic timing and amplitude were least when a rhythmic unit's period in absolute co-ordination approximated its characteristic period and increased with departures from the characteristic period. Rates of increase in timing fluctuations were approximately the same for deviations on either side of the characteristic period; the rate of increase in spacing fluctuations was substantially greater for the range in which periods were less than the characteristic period. The phase relation between two co-ordinated rhythmic movement units in absolute co-ordination depended on the difference between their characteristic periods. The intended phase relation of 180 degrees was attained only when the characteristic periods were identical. When the characteristic periods differed, the departure from 180 degrees increased systematically with the difference. The fluctuation results are discussed in terms of the relation between relaxation and harmonic dynamics in producing rhythmic movements, with particular emphasis on the harmonic tuning of relaxation oscillations. The phase results are discussed in terms of whether or not the very many stable phase relations in absolute co-ordination are reflective of the nervous system or of differences in response latencies in left and right muscle systems induced by different degrees of inertial compensation.