Dynamic response of human muscle spindle afferents to stretch during voluntary contraction

148

179

Mottram, Carol J., Jennifer M. Jakobi, John G. Semmler, and Roger M. Enoka. Motor-unit activity differs with load type during a fatiguing contraction. J Neurophysiol 93: [1381][1382][1383][1384][1385][1386][1387][1388][1389][1390][1391][1392] 2005. First published October 13, 2004; doi:10.1152/jn.00837.2004. Despite a similar rate of change in average electromyographic (EMG) activity, previous studies have observed different rates of change in mean arterial pressure, heart rate, perceived exertion, and fluctuations in motor output during the performance of fatiguing contractions that involved different types of loads. To obtain a more direct measure of the motor output from the spinal cord, the purpose of this study was to compare the discharge characteristics of the same motor unit in biceps brachii during the performance of two types of fatiguing contractions. In separate tests with the upper arm vertical and the elbow flexed to 1.57 rad, the seated subjects maintained either a constant upward force at the wrist (force task) or a constant elbow angle (position task) for a prescribed duration. The force and position tasks were performed in random order at a target force equal to 3.5 Ϯ 2.1% (mean Ϯ SD) of the maximal voluntary contraction (MVC) force above the recruitment threshold of the isolated motor unit. Each subject maintained the two tasks for an identical duration (161 Ϯ 96 s) at a mean target force of 22.2 Ϯ 13.4% MVC (range: 3-49% MVC). The dependent variables included the discharge characteristics of the same motor unit in biceps brachii, fluctuations in motor output (force or acceleration), mean arterial pressure, heart rate, and rating of perceived exertion. Despite similar increases in the amplitude of the averaged EMG (% MVC) for the elbow flexor muscles during both tasks (P ϭ 0.60), the rates of increase in mean arterial pressure (P Ͻ 0.001), rating of perceived exertion (P ϭ 0.023), and fluctuations in motor output (P ϭ 0.003) were greater during the position task compared with the force task. Consistent with these differences, mean discharge rate declined at a greater rate during the position task (P ϭ 0.03), and the coefficient of variation for discharge rate increased only during the position task (P ϭ 0.02). Furthermore, more motor units were recruited during the position task compared with the force task (P ϭ 0.01). These findings indicate that despite a comparable net muscle torque, the rate of increase in the motor output from the spinal cord was greater during the position task.

Section: Discussionmentioning

confidence: 99%

Motor-Unit Activity Differs With Load Type During a Fatiguing Contraction

Mottram¹,

Jakobi²,

Semmler³

et al. 2005

148

179

“…To date, no recordings of human spindles have been made during locomotion, and thus, the firing rate can only be estimated from models mainly based on cat studies (Յ130 Hz) (Prochazka 1999) or from human studies on sinusoidal movements (rarely Ͼ50 Hz) (Grill and Hallet 1995;Kakuda and Nagaoka 1998). At the slow walking velocity of the present study, namely 0.85 m/s, 70-Hz vibration is thus believed to be sufficiently higher than the normal Ia firing rate to induce a significant artificial afferent input.…”

Section: Discussionmentioning

confidence: 99%

Vibration-Induced Changes in EMG During Human Locomotion

Verschueren

Swinnen

Desloovere³

et al. 2003

Verschueren, Sabine M. P., Stephan P. Swinnen, Kaat Desloovere, and Jacques Duysens. Vibration induced changes in EMG during human locomotion. J Neurophysiol 89: 1299 -1307, 2003; 10.1152/jn.00863.2002. The present study was set up to examine the contribution of Ia afferent input in the generation of electromyographic (EMG) activity. Subjects walked blindfolded along a walkway while tendon vibration was applied continuously to a leg muscle. The effects of vibration were measured on mean EMG activity in stance and swing phase. The results show that vibration of the quadriceps femoris (Q) at the knee and of biceps femoris (BF) at the knee enhanced the EMG activity of these muscles and this occurred mainly in the stance phase of walking. These results suggest involvement of Ia afferent input of Q and BF in EMG activation during stance. In contrast, vibration of muscles at the ankle and hip had no significant effect on burst amplitude. Additionally, the onset time of tibialis anterior was measured to look at timing of phase transitions. Only vibration of quadriceps femoris resulted in an earlier onset of tibialis anterior within the gait cycle, suggesting involvement of these Ia afferents in the triggering of phase transitions. In conclusion, the results of the present study suggest involvement of Ia afferent input in the control of muscle activity during locomotion in humans. A limited role in timing of phase transitions is proposed as well.

“…The differential equations of the model were solved in MATLAB 2013a with the function "ode15s." Given a muscle spindle density in human hand muscles of ϳ12/g (Banks 2006) and assuming a muscle weight of 4.5 g (Jacobson et al 1992), the number of muscle spindles in each of the two antagonist muscles was set to 54, each supplied by one static and one dynamic gamma motor neuron (Kakuda and Nagaoka 1998;Matthews 1962). During dynamic contractions gamma motor neuron discharge rates can be divided into two subgroups on the basis of their behavior.…”

Section: Methodsmentioning

confidence: 99%

The optimal neural strategy for a stable motor task requires a compromise between level of muscle cocontraction and synaptic gain of afferent feedback

Dideriksen

Negro

Farina

2015

Dideriksen JL, Negro F, Farina D. The optimal neural strategy for a stable motor task requires a compromise between level of muscle cocontraction and synaptic gain of afferent feedback. J Neurophysiol 114: 1895-1911, 2015. First published July 22, 2015 doi:10.1152/jn.00247.2015.-Increasing joint stiffness by cocontraction of antagonist muscles and compensatory reflexes are neural strategies to minimize the impact of unexpected perturbations on movement. Combining these strategies, however, may compromise steadiness, as elements of the afferent input to motor pools innervating antagonist muscles are inherently negatively correlated. Consequently, a high afferent gain and active contractions of both muscles may imply negatively correlated neural drives to the muscles and thus an unstable limb position. This hypothesis was systematically explored with a novel computational model of the peripheral nervous system and the mechanics of one limb. Two populations of motor neurons received synaptic input from descending drive, spinal interneurons, and afferent feedback. Muscle force, simulated based on motor unit activity, determined limb movement that gave rise to afferent feedback from muscle spindles and Golgi tendon organs. The results indicated that optimal steadiness was achieved with low synaptic gain of the afferent feedback. High afferent gains during cocontraction implied increased levels of common drive in the motor neuron outputs, which were negatively correlated across the two populations, constraining instability of the limb. Increasing the force acting on the joint and the afferent gain both effectively minimized the impact of an external perturbation, and suboptimal adjustment of the afferent gain could be compensated by muscle cocontraction. These observations show that selection of the strategy for a given contraction implies a compromise between steadiness and effectiveness of compensations to perturbations. This indicates that a task-dependent selection of neural strategy for steadiness is necessary when acting in different environments.