Locomotor control of limb force switches from minimal intervention principle in early adaptation to noise reduction in late adaptation

Abstract

During movement, errors are typically corrected only if they hinder performance. Preferential correction of task-relevant deviations is described by the minimal intervention principle but has not been demonstrated in the joints during locomotor adaptation. We studied hopping as a tractable model of locomotor adaptation of the joints within the context of a limb-force-specific task space. Subjects hopped while adapting to shifted visual feedback that induced them to increase peak ground reaction force (GRF). We hypothesized subjects would preferentially reduce task-relevant joint torque deviations over task-irrelevant deviations to increase peak GRF. We employed a modified uncontrolled manifold analysis to quantify task-relevant and task-irrelevant joint torque deviations for each individual hop cycle. As would be expected by the explicit goal of the task, peak GRF errors decreased in early adaptation before reaching steady state during late adaptation. Interestingly, during the early adaptation performance improvement phase, subjects reduced GRF errors by decreasing only the task-relevant joint torque deviations. In contrast, during the late adaption performance maintenance phase, all torque deviations decreased in unison regardless of task relevance. In deadaptation, when the shift in visual feedback was removed, all torque deviations decreased in unison, possibly because performance improvement was too rapid to detect changes in only the task-relevant dimension. We conclude that limb force adaptation in hopping switches from a minimal intervention strategy during performance improvement to a noise reduction strategy during performance maintenance, which may represent a general control strategy for locomotor adaptation of limb force in other bouncing gaits, such as running.

Keywords: motor redundancy; running; spring-mass model; uncontrolled manifold.

Fig. 1.

A: experimental setup: during nonshifted, shifted, and postshift blocks, subjects received visual feedback of a constant target ground reaction force (GRF) and the peak GRF of each hop. The scale of each plot was from 0–5 body weights, although this scale was not visible to the subject. B: experimental protocol, top row: name of the block condition, number of trials in the block, and the shift in visual feedback of the peak GRF vertical bar on the left of the screen. Bottom row: abbreviations for each trial that will be used in subsequent figures. C–F: GRF and ankle, knee, and hip torques for a control trial of a representative subject (average of all hops in trial in black, individual hops in gray).

Fig. 2.

Validation of the single cycle modified uncontrolled manifold (UCM) method. A: R² values of the relationship between endpoint forces calculated from the Jacobian S and experimental vertical GRF data for each hop of a representative subject. B: in steady-state hopping, task-irrelevant deviations squared and averaged across all hops in the trial (○) are nearly equal to goal equivalent variance (x) for the second baseline trial of a representative subject. C: similarly, task-relevant deviations squared and averaged across all hops in the trial (○) are nearly equal to nongoal equivalent variance (x) for the same trial.

Fig. 3.

Magnitude of error between peak GRF and target over time. GRF error decreases in first 3 shifted feedback trials, the performance improvement (PI) phase, and is unchanged in the final trial, the performance maintenance (PM) phase. Peak GRF error magnitude also decreased in the first postshift trial. BW, body weight. Symbols are means ± SE across all 11 subjects. Brackets indicate significant difference between averages of first 10 and last 10 hops, as well as first 10 hops of S1 compared with first 10 hops and last 10 hops of S4. *P < 0.05, **P < 0.01.

Fig. 4.

Task-relevant (A) and task-irrelevant (C) joint torque deviations for every hop, averaged across subjects in the primary experiment. Average task-relevant (B) and task-irrelevant (D) deviations for the first 10 and last 10 hops of each trial in the primary experiment. Average task-relevant (E) and task-irrelevant (F) deviations for the extended control experiment for the first 10 and last 10 hops of each trial. Data are means ± SE across 11 subjects. *P < 0.05, **P < 0.01.

Fig. 5.

Single-cycle index of deviation structure (SCIDS), a measure of coordination of joint torques, across the first and last 10 hops of each trial. Data are means ± SE across 11 subjects. *P < 0.05, **P < 0.01.

Fig. 6.

Total joint torque deviations across the first and last 10 hops of each trial. Data are means ± SE across 11 subjects. *P < 0.05, **P < 0.01.