High specific torque is related to lengthening contraction-induced skeletal muscle injury

Abstract

Black CD, Elder CP, Gorgey A, Dudley GA. High specific torque is related to lengthening contraction-induced skeletal muscle injury. J Appl Physiol 104: 639-647, 2008. First published December 13, 2007 doi:10.1152/japplphysiol.00322.2007.-Animal models implicate multiple mechanical factors in the initiation of exercise-induced muscle injury. Muscle injury has been widely studied in humans, but few data exist regarding the underlying cause of muscle injury. This study sought to examine the role of torque per active muscle volume in muscle injury. Eight subjects performed 80 electrically stimulated [via electromyostimulation (EMS)] eccentric contractions of the right and left quadriceps femoris (QF) through an 80°arc at 120°/s. Specific torque was varied by applying 25-Hz EMS to one thigh and 100-Hz EMS to the contralateral thigh. Transverse relaxation time (T2) magnetic resonance images of the QF were collected before and 3 days after the eccentric exercise bouts. Injury was assessed via changes in isometric force and ratings of soreness over the course of 14 days after exercise and by determining changes in T2 and muscle volume 3 days after exercise. The 100-Hz EMS induced greater force loss (P Ͻ 0. 05), soreness (P Ͻ 0.05), change in muscle volume (P ϭ 0.03), and volume of muscle demonstrating increased T2 (P ϭ 0.005) than the 25-Hz EMS. In addition, injury was found to be similar across the QF in all but the most proximal regions of the QF. Our findings suggest that, in humans, high torque per active volume during lengthening muscle contractions is related to muscle injury. transverse relaxation time magnetic resonance images; electrical stimulation; muscle activation; soreness IT IS WELL ESTABLISHED THAT novel exercise, especially exercise involving lengthening muscle actions, can evoke skeletal muscle injury. Data from animal studies have implicated several mechanical factors as the potential underlying cause or initial event in muscle injury. Contraction force (27, 45), contraction velocity (8, 45), initial muscle length (42), muscle strain (8, 23), and number of eccentric contractions (27,42,46) have been found to contribute to eccentric exercise-induced muscle injury. These studies were performed on a variety of animal species (rabbits, mice, rats) and used multiple in vitro and in situ protocols to induce skeletal muscle injury. It has been suggested that these differences may account for the disparate findings regarding the relative contribution of each factor to muscle injury (11).Few human studies have focused on the underlying mechanisms of exercise-induced muscle injury. As shown in animals, performing contractions at long muscle lengths (29) and increasing the number of eccentric contractions (9) have been shown to cause greater injury. However, the role contraction force plays in muscle injury remains unclear. Eccentric actions recruit fewer motor units to generate a given absolute force (10), and it has been suggested that this is the primary reason eccentric actions cause greater injury than the...

Effects of Electrical Stimulation Parameters on Fatigue in Skeletal Muscle

“…16,17 However, increasing the frequency of NMES has been shown to increase evoked torque by increasing the torque per active muscle area of skeletal muscle. 9,17 Because increasing the frequency and pulse duration increase the evoked torque per unit of activated muscle, 9,17 it causes an increased energy demand that cannot be supplied by the muscle and thus leads to muscular fatigue. 4,28 These findings may illustrate that fatigue during NMES is not necessarily related to peak muscle torque production, but may in fact be related to the metabolic demand placed upon each activated motor unit.…”

“…Torque per Active Area The torque per active area was calculated by dividing the highest torque (Nm) achieved for each NMES protocol by the total activated skeletal muscle area (cm 2 ). 9,16,17 The torque and the activated CSA values in response to the 4 NMES protocols were previously measured and published. 17 Considering the clinical purpose of the current study, we recalculated these values and presented them as torque relative to the activated CSA (Nm/cm 2 ): torque per active area = peak torque of the first contraction/the activated knee extensor CSA.…”

“…At a constant frequency, the increase in current amplitude and pulse duration enhances the evoked torque by recruitment of additional motor units, suggesting that the number of cross-bridge attachments remains constant across different muscle fibers. When the frequency was reduced, the evoked torque decreased without changing the activated area, 9,17 suggesting that the number of cross-bridge attachments per unit of active tissue was reduced. The current findings suggest that initiation of the cross-bridge attachments is far more costly at 100 Hz compared to 25 Hz (ie, more cross-bridges at 100 Hz than 25 Hz).…”

“…Lowering the stimulation frequency results in less fatigue, by possibly reducing the evoked torque relative to the activated muscle area. J Orthop Sports Phys Ther 2009;39 (9): 684-692. doi:10.2519/jospt.2009.3045 amplitude, frequency, NMES, pulse duration resulting in a higher rate of muscle fatigue.…”

Localization And Quantification Of Intramuscular Damage Using Statistical Parametric Mapping And Skeletal Muscle Parcellation

“…In most of these studies, the averaged T 2 value was calculated in regions of interest within a given muscle thereby ignoring any spatial information. Although local T 2 changes were assessed along muscles 19,22 , no study provided information on the accurate localization and extent of intramuscular damage into the three dimensions (3D) of a skeletal muscle.Yet, accurate localization and quantification of muscle damage should provide more robust indices in diagnosis and longitudinal follow-ups of diseases or injuries. So far, only one study has reported information related to the distribution of intramuscular damage in dystrophic boys 7 .…”