The targeted force steps developed by a human wrist: Force trajectories under conditions of repetition or random alternation of different target levels

The targeted isometric force steps developed by a human wrist after an acoustic signal under conditions of regular repetition and random alternation (the Reg and Rnd modes) of three visually presented target levels (25 N, 35 N, and 50 N) were studied. The main components (MC) and postcorrections could be easily differentiated in the majority of the force trajectories (FT). Analysis of MC parameters has shown that the Rnd mode in comparison with the Reg one is characterized by a noticeable increase in mean amplitude errors of MC with respect to the target level; as a rule, mean errors were positive for the 25 N target level and negative for the 50 N level. An increase in the amplitude variability and a weaker correlation of the force amplitude and the peak value of the first force derivative by time (the maximum rate of the force increase) were also typical of the Rnd mode. It is concluded that when duration of force development is not limited from above, the motor program of such reaction (especially in the Rnd mode) is more liberal than in the case of limitations [5, 12]: values of the force increase rate, specified by the central program, determine the force amplitude in a less rigid manner. Program transferring, i.e., the use of the central program of a fast ballistic movement to make a fast isometric force step, is regarded as a possible reason for the above-mentioned discrepancy; the rigidity of the program of the fast movement is regarded to be determined by corresponding biomechanical limitations. Correlations of the amplitudes and durations of the FT MC varied a great deal for different tested persons and in different modes, but there was a substantial positive correlation in the majority of cases. This finding contradicts the widespread notions about principles of scaling, when scaling of some parameters and maintenance of other parameters of targeted motor reactions as invariants are postulated. Correspondence of the resulting data to the concept of the generalized motor programs and its modifications is discussed.Neirofiziologiya/Neurophysiology, Vol. 25, No. 5, pp. 389–399, September–October, 1993.