Structural muscle damage and muscle strength after incremental number of isometric and forced lengthening contractions.

Exercise-induced muscle damage is characterized by histological changes, like Z-line streaming, inflammatory response and decreased muscle function reflected in a prolonged decline in maximal isometric muscle strength after eccentric work. It is assumed that force decrement is mainly related to the amount of structural damage. However, the relationship between number of eccentric contractions, magnitude of structural damage and force decrement is not very well documented. Therefore we studied the effect of an increasing number of both isometric and eccentric (forced lengthening) contractions on histological parameters of muscle damage and maximal isometric force in an experimental in situ rat model. Tibialis anterior muscles of male Wistar rats were subjected to an increasing number of either isometric or eccentric contractions and were examined for histological markers of muscle damage. The present study shows that muscle damage increases progressively with the number of forced lengthening contractions. Maximal isometric torque was found to decline after both types of exercise. However, the decline after forced lengthening exercise was more pronounced. Only a weak relationship between percentage of histological muscle damage and isometric torque after forced lengthening contractions was found. The findings of the present study suggest that the decline in muscle force after eccentric exercise may partly be attributed to other factors than structural damage.