Abstract
Little is known about the ability to precisely regulate forces or torques during unexpected disturbances, as required during numerous tasks. Effective force regulation implies small changes in force responding to externally imposed displacements, a behavior characterized by low limb impedance. This task can be challenging, since the intrinsic impedance of muscles increases when generating volitional forces. The purpose of this study was to examine the ability to voluntarily reduce limb impedance during force regulation, and the neural mechanisms associated with that ability. Small displacement perturbations were used to quantify elbow impedance during the exertion of volitional elbow torques from 0% to 20% of maximum voluntary contraction. Subjects were instructed either to not intervene with the imposed perturbations or to explicitly intervene so as to minimize the influence of the perturbations on the elbow torque. Our results demonstrated that individuals can reduce the low frequency components of elbow impedance by 35%. Electromyographic analysis suggested that this behavior is mediated by volitional and possibly long-latency reflex pathways with delays of at least 120 ms. These results provide a context for understanding how feedback altered by aging or injuries may influence the ability to regulate forces precisely.
Highlights
Effective force regulation implies that external perturbations of limb posture should result in only small changes in the force being produced
Feedforward strategies are certainly appropriate for certain tasks, they cannot be used to reduce limb impedance below what can be attributed to the intrinsic force-dependent stiffness of the muscles within the limb, which is necessary for force regulation under unpredictable perturbations
Subjects were able to complete the torque control task, reducing the change in elbow torque due to the applied perturbations relative to that measured during the do not intervene” (DNI) tasks
Summary
Effective force regulation implies that external perturbations of limb posture should result in only small changes in the force being produced. Muscle and limb stiffness increase with increasing levels of activation[9,10,11,12,13] This provides a simple mechanism to maintain a desired posture in the presence of unexpected disturbances[14,15,16], but it would inevitably limit the ability to keep limb stiffness low, as is desirable during force regulation tasks. Feedforward strategies are certainly appropriate for certain tasks, they cannot be used to reduce limb impedance below what can be attributed to the intrinsic force-dependent stiffness of the muscles within the limb, which is necessary for force regulation under unpredictable perturbations. In addition to the intrinsic stiffness of muscle, stretch reflexes opposing external perturbations of posture increase with volitional activation, potentially further increasing the stiffness of the limb and reducing the ability to regulate force precisely[25,26,27]. It remains unclear to what extent humans can reduce limb impedance during tasks that require active force generation
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