Task-specific stability in muscle activation space during unintentional movements

Abstract

We used robot-generated perturbations applied during position-holding tasks to explore stability of induced unintentional movements in a multi-dimensional space of muscle activations. Healthy subjects held the handle of a robot against a constant bias force and were instructed not to interfere with hand movements produced by changes in the external force. Transient force changes were applied leading to handle displacement away from the initial position and then back towards the initial position. Inter-trial variance in the space of muscle modes (eigenvectors in the muscle activations space) was quantified within two sub-spaces, corresponding to unchanged handle coordinate and to changes in the handle coordinate. Most variance was confined to the former sub-space in each of the three phases of movement, the initial steady state, the intermediate position, and the final steady state. The same result was found when the changes in muscle activation were analyzed between the initial and final steady states. Changes in the dwell time between the perturbation force application and removal led to different final hand locations undershooting the initial position. The magnitude of the undershot scaled with the dwell time, while the structure of variance in the muscle activation space did not depend on the dwell time. We conclude that stability of the hand coordinate is ensured during both intentional and unintentional actions via similar mechanisms. Relative equifinality in the external space after transient perturbations may be associated with varying states in the redundant space of muscle activations. The results fit a hierarchical scheme for the control of voluntary movements with referent configurations and redundant mapping between the levels of the hierarchy.

Intentional and unintentional multi-joint movements: Their nature and structure of variance

“…Note that F PERT increased smoothly over 0.5 s to avoid inducing so-called triggered responses (Hammond 1954; Tatton et al 1978; reviewed in Shemmel et al 2010). A recent study of the electromyographic patterns in arm muscles using similar perturbations (Falaki et al 2014) has shown no visible phasic muscle reactions that would resemble triggered responses.…”

Anticipatory postural adjustments and anticipatory synergy adjustments: preparing to a postural perturbation with predictable and unpredictable direction

“…ΔM and ΔCOP AP data were computed from IEMG norm and integrated COP AP (within 50-ms time windows) data from the body sway task. Both ΔM and ΔCOP AP , were filtered with a 5 Hz, low-pass, 4th order, zero-lag Butterworth filter (Falaki et al 2014, 2016). Multiple regression analysis was performed for each subject separately: $$\mathrm{\Delta }{\mathrm{italicCOP}}_{AP}=k_1\mathrm{\Delta }{M}_1+k_2\mathrm{\Delta }{M}_2+k_3\mathrm{\Delta }{M}_3+k_4\mathrm{\Delta }{M}_4$$…”

Unintentional changes in the apparent stiffness of the multi-joint limb

“…F PERT increased smoothly over 0.5 s to avoid triggered (pre-programmed, long-loop reflex) responses (Hammond 1954; Tatton et al 1978; Shemmel et al 2010). In an earlier study, similar perturbations showed no visible phasic muscle reactions (Falaki et al 2014). During the final phase, Recovery , the robot force returned to F BIAS , also over 0.5 s.…”

“…In an earlier study, we recorded arm muscle electromyographic signals (EMGs) using surface electrodes in a somewhat similar setup (Falaki et al 2014). In that study, the pilot analysis of EMG changes during the dwell time was disappointing: The EMG signals were relatively low (note that the external forces used in both studies were not very high to avoid fatigue), and we could not detect their consistent changes over the dwell time.…”

“…A recent series of studies have presented examples of violations of equifinality across tasks and systems of analysis: Multi-finger force production (Ambike 2014; Reschechtko et al in press) and multi-joint positional tasks analyzed within the space of joint configurations (Zhou et al 2014; 2015a) and muscle activations (Falaki et al 2014). In the current study, we also observed violations of equifinality when a transient force perturbation was interrupted by a time interval (dwell time) between the force application and removal.…”