Movement Primitives and Human Arm Movements

Velocity profiles of voluntary unconstrained human arm movements under some conditions, e.g., during accurate movements, are irregular. Specifically, the velocity profiles of the hand paths (Vt) exhibit fluctuations from the typical bell-shaped profile. What is the nature of such fluctuations? Do these fluctuations reveal anything about the nature of the controller of human arm movements?

We recorded the hand path trajectories of 4 unimpaired subjects with a low-impedance planar robot (InMotion2 Inc, Cambridge MA). Subjects performed 300 point-to-point movements at 3 different self-paced speeds (fast, comfortable, slow with peak Vt: 0.40±0.09, 0.16±0.08, 0.07±0.03 m/s, respectively) along the horizontal plane.

We found that the irregularity of Vt profiles increased as movement speed decreased. We described the Vt profiles as combinations of overlapping support-bounded lognormal discrete elementary units of movements (submovements). A typical speed profiles at fast (top) comfortable (middle) and slow (bottom) speed and the corresponding submovement decomposition can be shown. The number of submovements increased as movement speed decreased (1.62±0.56, 2.35±0.75, 3.96±1.15 for the different speeds respectively, p<0.001). Their parameters were modulated to reconstruct the Vt profiles and were similar across subjects.

We suggest the possibility that the occurrence of irregularity in Vt profiles is a by-product of the way human arm movements are generated, i.e., through submovements. Further evidence of this hypothesis is suggested by the apparent inconsistency of this experimental data with the theoretical predictions of alternative hypotheses proposed, during the past few years, to explain human arm movement irregularity.

Specifically, this data appears to be inconsistent with hypotheses that arm movements become irregular when signals generated by a time-continuous controller are degraded either by noise (Gaussian constant or signal-dependent) present in the neuromuscular system or by the mechanics of the musculoskeletal system.