Effects of pH and temperature on force and stiffness of skeletal muscle fibers during contraction and relaxation in relation to musculoskeletal disorders.

Effects of low pH (6.0, 6.5) were studied at three temperatures (5, 15, 25 degrees C) on the isometric force and stiffness of glycerinated muscle fibers dissected from soleus (type I) and psoas (type IIX) muscles of the Japanese white rabbit. It was observed that the maximum force and stiffness declined as pH decreased, the extent of which was diminished by an increase in temperature in both muscle types. The pH-induced changes in force were greater than those in the stiffness for both muscle types: at 5 degrees C, the psoas showed greater change in stiffness than the soleus. As the pH of the contracting solution decreased, the time to peak stiffness decreased and the time to relaxation increased in both muscle types. At pH 6.0, the latter was significantly longer than the former. The force-stiffness curves indicated that, when pH decreased, force increased as fast as stiffness during contraction and force declined faster than stiffness during relaxation. The results suggested that one of the causes of muscle injury during hard muscular work could be the longer relaxation time in an acidic solution.