Initial information prior to movement onset influences kinematics of upward arm pointing movements

Abstract

To elaborate a motor plan and perform online control in the gravity field, the brain relies on priors and multisensory integration of information. In particular, afferent and efferent inputs related to the initial state are thought to convey sensorimotor information to plan the upcoming action. Yet it is still unclear to what extent these cues impact motor planning. Here we examined the role of initial information on the planning and execution of arm movements. Participants performed upward arm movements around the shoulder at three speeds and in two arm conditions. In the first condition, the arm was outstretched horizontally and required a significant muscular command to compensate for the gravitational shoulder torque before movement onset. In contrast, in the second condition the arm was passively maintained in the same position with a cushioned support and did not require any muscle contraction before movement execution. We quantified differences in motor performance by comparing shoulder velocity profiles. Previous studies showed that asymmetric velocity profiles reflect an optimal integration of the effects of gravity on upward movements. Consistent with this, we found decreased acceleration durations in both arm conditions. However, early differences in kinematic asymmetries and EMG patterns between the two conditions signaled a change of the motor plan. This different behavior carried on through trials when the arm was at rest before movement onset and may reveal a distinct motor strategy chosen in the context of uncertainty. Altogether, we suggest that the information available online must be complemented by accurate initial information.

Keywords: arm movement; gravitational force; initial information; motor planning.

Fig. 1.

A and B: experimental protocol illustrating the Vertical Active (A) and Vertical Rest (B) conditions of the main experiment. Arrow indicates a 45° upward arm movement. Gray T (B) represents the support for the outstretched arm. Black disks indicate the initial and final positions of the target. C: experimental protocol illustrating control experiment 3. Arrow indicates a 45° rightward arm movement. Gray cube represents the masses used to generate vertical shoulder torques. Black disks indicate the initial and final positions of the target.

Fig. 2.

Velocity profiles normalized in time in the Vertical Rest and Vertical Active conditions. Slow, Medium, and Fast speeds are depicted by thick, normal, and thin lines, respectively. Shaded areas correspond to SE between participants.

Fig. 3.

A: acceleration duration normalized in time in the Vertical Rest and Vertical Active conditions for Slow, Medium, and Fast speeds. Error bars represent SD across participants. B: acceleration duration normalized in time in the Vertical Rest and Vertical Active conditions averaged between subjects and represented for the first 15 trials in Slow (left), Medium (center), and Fast (right) speeds. Error bars represent SE across participants.

Fig. 4.

RMS signal of EMG for the Vertical Rest and Vertical Active conditions for Slow (left), Medium (center), and Fast (right) speeds. RMS signal profiles are averaged between subjects, and shaded areas correspond to SE between participants. Vertical line corresponds to movement onset.

Fig. 5.

Acceleration duration normalized in time in the Vertical Rest, Vertical Active, and Vertical Rest Cocontract conditions for control experiments 1 (left) and 2 (center) and acceleration duration normalized in time in the Horizontal Active and Horizontal Rest conditions for control experiment 3 (right). Error bars represent SD across participants.