Small-Strain Lengthening Contractions Protect Skeletal Muscle from Subsequent Acute Large-Strain Injury in Rats

Unaccustomed mechanical stresses to skeletal muscle, particularly during maximal lengthening ("eccentric") contractions often result in injury. However, a subsequent bout of the same activity performed days or even weeks after an initial bout results in significantly less damage. This phenomenon has been termed the "repeated bout effect" and its underlying mechanisms are still unclear. While a bout of eccentric exercise confers protection against subsequent bouts of the same exercise, the protective effect of eccentric exercise (repeated small-strain contractions) compared to an acute muscle strain (several large-strain contractions) on a subsequent acute muscle strain is unknown. PURPOSE: To compare the protective effect of two different injury protocols performed on the ankle dorsiflexors in the rat, 4 weeks after an initial injury. METHODS: The dorsiflexors muscles in one leg of adult (12-14 wk), male Sprague-Dawley rats (N = 4) were subjected to 10 repetitions of large-strain lengthening contractions (90°-180° of plantarflexion) at 900°/s. In a second group of rats (N = 4), the dorsiflexors of one leg were exposed to 150 repetitions of small-strain lengthening contractions (70°-110° of plantarflexion at 900°/s). Both groups were exposed to the large strain injury after complete recovery (28 d) from the initial injury. RESULTS: Both protocols induced a similar (∼50%) loss of isometric torque compared to pre-injury torque following the first bout of contractions. The group exposed to large-strain contractions lost 50 ± 6 % of pre-injury torque when exposed to a second bout of large-strain contractions. The group exposed to small-strain contractions lost 27 ± 6 % of pre-injury torque when subsequently exposed to large-strain contractions. CONCLUSIONS: Our data indicate that eccentric exercise in the form of repeated small-strain contractions offers a protective effect against an acute muscle strain from several large-strain contractions. Our results also suggest that an acute muscle strain from large-strain contractions does not offer a protective effect against a subsequent muscle injury from large strain contractions. Supported by the Muscular Dystrophy association (MDA, grant 4278) and NIH-NIAMS (K01AR053235) to RML and grants from the MDA, Jain Foundation, and the NIH (RO1AR055928) to RJB