Similar movements are associated with drastically different muscle contraction velocities

Abstract

We investigated how kinematic redundancy interacts with the neurophysiological control mechanisms required for smooth and accurate, rapid limb movements. Biomechanically speaking, tendon excursions are over-determined because the rotation of few joints determines the lengths and velocities of many muscles. But how different are the muscle velocity profiles induced by various, equally valid hand trajectories? We used an 18-muscle sagittal-plane arm model to calculate 100,000 feasible shoulder, elbow, and wrist joint rotations that produced valid basketball free throws with different hand trajectories, but identical initial and final hand positions and velocities. We found large differences in the eccentric and concentric muscle velocity profiles across many trajectories; even among similar trajectories. These differences have important consequences to their neural control because each trajectory will require unique, time-sensitive reflex modulation strategies. As Sherrington mentioned a century ago, failure to appropriately silence the stretch reflex of any one eccentrically contracting muscle will disrupt movement. Thus, trajectories that produce faster or more variable eccentric contractions will require more precise timing of reflex modulation across motoneuron pools; resulting in higher sensitivity to time delays, muscle mechanics, excitation/contraction dynamics, noise, errors and perturbations. By combining fundamental concepts of biomechanics and neuroscience, we propose that kinematic and muscle redundancy are, in fact, severely limited by the need to regulate reflex mechanisms in a task-specific and time-critical way. This in turn has important consequences to the learning and execution of accurate, smooth and repeatable movements—and to the rehabilitation of everyday limb movements in developmental and neurological conditions, and stroke.

A cross-species neural integration of gravity for motor optimisation

“…Next, we examine muscular activation patterns to understand the production of direction-dependent arm kinematics further. Because of the redundancy between the muscular level and the kinematic level (Bernstein, 1967), many different muscular activation patterns can give rise to the same velocity profile (Burdet et al, 2001; Hagen and Valero-Cuevas, 2017). Does the neural integration of gravity truly blossom into muscular activation patterns that discount muscle effort?…”

“…This limitation is problematic because of the redundancy between the muscular level and the kinematic level (Bernstein, 1967). Because many different muscular activation patterns can give rise to the same trajectory, using kinematic data exclusively to infer central processes is insufficient (Burdet et al, 2001; Hagen and Valero-Cuevas, 2017). Establishing that direction-dependent kinematics truly reflects motor commands that discount muscle effort requires additional evidence from muscle dynamics.…”

Errors in Predicting Muscle Fiber Lengths from Joint Kinematics Point to the Need to Include Tendon Tension in Computational Neuromuscular Models

“… However, moment arms are not constant, and the equations for l MT and v MT must account for changes in moment arms with respect to joint angles. As a first approximation, previous work evaluated equation (20b) with posture dependent moment arms ( r i ( θ i ) equation (21)) [18]. Integrating equation (21) with respect to time reveals that this approximation equates si to the integral of the moment arm function ( r i ( θ i )) across some joint rotation from neutral ( θ i − θ i,o , equation (22)).…”

“…The constant moment arm equation [5–7] is the simplest approximation but clearly the arc length ( dashed purple ) does not accurately convey the true MT excursion ( black ). This was extended in [18] where the true arc length was approximated by integrating the posture-specific moment arm function (orange). Even for sufficiently small Δθ, this approach does not completely capture the true MT excursion as it ignores the change in moment arm with respect to the joint angle .…”

On neuromechanical approaches for the study of biological and robotic grasp and manipulation

“…However, the same is not true when we consider movement. The rotation of that single joint defines the lengths of all muscles that cross it [183,57,197]. While in principle muscles can go slack, muscles with tone will shorten appropriately.…”