Effect of voluntary vs. artificial activation on the relationship of muscle torque to speed

Abstract

The speed-torque relationship of the right knee extensor muscle group was investigated in eight untrained subjects (28 +/- 2 yr old). Torque was measured at a specific knee angle during isokinetic concentric or eccentric actions at nine angular velocities (0.17-3.66 rad/s) and during isometric actions. Activation was by "maximal" voluntary effort or by transcutaneous tetanic electrical stimulation that induced an isometric torque equal to 60% (STIM 1) or 45% (STIM 2) of the voluntary isometric value. Torque increased (P less than 0.05) to 1.4 times isometric as the speed of eccentric actions increased to 1.57 rad/s for STIM 1 and STIM 2. Thereafter, increases in eccentric speed did not further increase torque. Torque did not increase (P greater than 0.05) above isometric for voluntary eccentric actions. As the speed of concentric actions increased from 0.00 to 3.66 rad/s, torque decreased (P less than 0.05) more (P less than 0.05) for both STIM 1 and STIM 2 (two-thirds) than for voluntary activation (one-half). As a result of these responses, torque changed three times as much (P less than 0.05) across speeds of concentric and eccentric actions with artificial (3.4-fold) than voluntary (1.1-fold) activation. The results indicate that with artificial activation the normalized speed-torque relationship of the knee extensors in situ is remarkably similar to that of isolated muscle. The relationship for voluntary activation, in contrast, suggests that the ability of the central nervous system to activate the knee extensors during maximal efforts depends on the speed and type of muscle action performed.

Vastus lateralis fatigue alters recruitment of musculus quadriceps femoris in humans

“…First, force loss during EMS was marked; thus the perception for EMS was comparable for Ex1 and Ex2. Second, we have shown that the nature of the in vivo speed torque relation for the QF is comparable when muscle actions are evoked by EMS vs. voluntary effort and that these relations are similar to those found for affected skeletal muscle in clinically complete spinal cord-injured patients or skeletal muscle in situ or in vitro (19). Third, at least for the tetanic superimposition technique, EMS actually decreases voluntary force (25).…”

Modelling the maximum voluntary joint torque/angular velocity relationship in human movement

“…As a consequence the seven parameter function of Subject 1 shows a pronounced plateau for low concentric velocities whereas that of Subject 2 is closer to a hyperbola with concave curvature for all concentric velocities (Figures 3a, 4a). This plateau in the torque data of Subject 1 at low concentric velocities is evident in previous studies (Dudley et al, 1990;James et al, 1994;King and Yeadon, 2002) and a seven parameter function is required in order to fit such data reasonably. On the other hand the data of Subject 2 shows no such plateau and in such a case a reasonable fit should be possible using a five parameter function in which the maximum eccentric to isometric torque ratio is a free parameter.…”

“…It could be argued that the value of 1.4 indicated by the data of Dudley et al (1990) would be more appropriate since it based on in vivo knee extensor torques of human subjects. The particular value used for this constant will not affect the overall seven parameter function greatly although it will affect the level of a min .…”

The torque–velocity relationship in large human muscles: Maximum voluntary versus electrically stimulated behaviour

“…Maximum torque expressed at the joint level is a complex integration of the muscle fibre contractile properties with the in vivo architecture of multiple muscle fibres, connective tissue and neural input. In vivo measurements of maximum voluntary contraction's (MVC) forcevelocity show differences to the in vitro tetanic profile, with eccentric forces not increasing much above isometric and tending to decline with increasing lengthening velocity (Westing et al, 1988;Dudley et al, 1990;Weber and Kriellaars, 1997;Kellis and Baltzopoulos, 1998;Forrester and Pain, 2010). Consequently, maximum voluntary eccentric strength is much lower than one might expect based on maximum isometric measurements and in vitro tetanic force-velocity behaviour.…”

“…Transcutaneous electrical stimulation to supplement maximum voluntary contractions has been found to increase eccentric knee extension torque to above the maximum voluntary levels, but to have no significant effect on concentric torque (Dudley et al, 1990;Westing et al, 1990). However, subjects are not able to tolerate the development of maximum torques through transcutaneous electrical stimulation as the sole source of knee extensor activation.…”