Influence of muscle temperature on the contractile properties of the quadriceps muscle in humans with spinal cord injury

Low muscle temperature in paralysed muscles of individuals with spinal cord injury may affect the contractile properties of these muscles. The present study was therefore undertaken to assess the effects of increased muscle t emperature on the isometric contractile properties of electrically stimulat ed paralysed quadriceps muscles. When muscle temperature at a depth of 3 cm was increased from similar to 32 degrees C to similar to 36 degrees C by u ltra-short-wave application, the half-relaxation time shortened and low-fre quency force responses became less fused, but the maximal rate of increase in force remained unchanged. Heating had no effect upon either force declin e or slowing of relaxation during fatiguing contractions. The force-frequen cy relationship of the paralysed quadriceps muscle was shifted to the right after the muscle was heated. Despite this shift, however, the relationship still resembled that in muscles of non-paralysed individuals, probably due to the unexplained high twitch forces. These results indicate that reduced muscle temperature in spinal-cord-injured individuals may lead to an under estimation of the changes in contractile properties in terms of relaxation rate or the degree of fusion with low-frequency stimulation. In addition, t he force-frequency relationship of paralysed muscles does not accurately re flect the magnitude of these changes, even when the muscle is heated, and s hould therefore be treated with caution.