Quadriceps concentric and eccentric exercise 2: differences in muscle strength, fatigue and EMG activity in eccentrically-exercised sore and non-sore muscles.

The purpose of this study was to determine whether the previously reported muscle weakness and increases in EMG and EMG/force ratio after eccentric exercise were related to ensuing soreness or simply to the exercise mode. Delayed-onset muscle soreness, maximal voluntary force and the surface electromyogram (EMG) of the quadriceps were studied in 10 healthy male subjects following 20 min of bench-stepping with a constant leading leg. Prior to stepping and at 0, 0.25, 0.50, 0.75, 1, 3, 24 and 48 h afterwards the subjects performed a 30 s isometric maximal voluntary contraction (MVC) of the extensors of each leg during which the isometric force and the root mean square voltage of the surface EMG (rms EMG) were recorded. Muscle soreness was recorded prior to and at 0, 0.75, 3, 24 and 48 h after the stepping task by means of a force probe. No soreness was experienced in the concentrically exercised muscles, but in the eccentrically exercised muscles 5 subjects were not sore and 5 experienced soreness after 24 h, increasing to 48 h. There were no significant differences between sore and non-sore muscles in muscle weakness, fatigue during 30 s, the EMG amplitudes or the EMG/force ratios during peak force and the 30 s MVC (p >0.05). Muscle weakness following eccentric exercise was related to the exercise mode and was independent of subsequent soreness. Both weakness and soreness may be related to muscle damage, but involve different mechanisms.