Quantitative analysis of muscle hardness in tetanic contractions induced by electrical stimulation in rats

Abstract

The purpose of this study was to investigate the in vivo relation between muscle hardness during an electrically induced contracting state and neuromuscular functions (M-wave and developed tension). Sixteen Sprague-Dawley rats were deeply anesthetized with urethane. Muscle hardness was measured quantitatively at the mid-portion of the gastrocnemius (GS) muscle during tetanic contractions induced by electrical stimulation (50 Hz, 100 micros duration) of the sciatic nerve or of the muscle directly. The M-wave was recorded with a pair of wire electrodes inserted into the muscle, and the developed tension was monitored with a push-pull gauge. Muscle hardness, M-wave amplitude and developed tension increased rapidly with the onset of nerve stimulation. Similar but intensity-dependent increases in muscle hardness and tension were observed following direct tetanic stimulation of the muscle. The hardness measured during nerve stimulation was correlated with the amplitude of the M-wave (r = 0.62, P < 0.0001) and the developed tension (r = 0.85, P < 0.0001). These phenomena were suppressed by pancuronium treatment (2 mg/ml, i.v.). These results suggest that muscle tension might be the most important factor for transcutaneously measured muscle hardness induced by tetanic muscle contraction.

Assessment of individual muscle hardness and stiffness using ultrasound elastography

“…Evidence from studies that did not use ultrasound elastography (e.g., using push-in meter) suggests that the increase in muscle stiffness for contractions with up to moderate intensity is mainly due to the number of cross-bridges 6,33,34) . However, push-in meters have indicated that the increase in muscle hardness is blunted at contractions with higher intensity 35) . As muscle fibers fuse at contractions of higher intensity, muscle force is increased by an increase in firing rate 36) .…”

“…As muscle fibers fuse at contractions of higher intensity, muscle force is increased by an increase in firing rate 36) . Morisada et al 35) concluded, since muscle hardness during tetanic nerve stimulation was closely correlated with the M-wave amplitude, that the contractile activity (interactions of cytoskeletal filaments accompanied by neuromuscular transmission) and muscle membrane excitability represent mechanisms likely related to the development of muscle hardness. Therefore, currently, the conclusions based on SWE-derived data contrast the long-held hypothesis regarding motor-unit recruitment strategies.…”

Association of muscle hardness with muscle tension dynamics: a physiological property

“…It is also not clear whether the change in muscle hardness is linear with muscle tension, i.e., muscle stiVness. Several studies have focused on the relationship between muscle hardness and muscle activation for convenient muscle force estimation using transverse measurements (e.g., Leonard et al 2003Leonard et al , 2004Gennisson et al 2005;Morisada et al 2006). In brief, these studies showed that muscle hardness is signiWcantly correlated with muscle tensile force.…”

Muscle Shear Moduli Changes and Frequency of Alternate Muscle Activity of Plantar Flexor Synergists Induced by Prolonged Low-Level Contraction

“…Resting muscles become stiffer under several conditions, including those involving cramps and damage (Fischer, 1987 ; Murayama et al, 2000 ). Similarly, muscle fatigue has been considered as one of possible causes for the increment of muscle stiffness (Morisada et al, 2006 ; Descarreaux et al, 2010 ). It is known that a resting tension develops when muscles fail to fully relax during fatigue (Gong et al, 2000 , 2003 ).…”