Author(s): OGA, R., NAKAGAWA, K., CHEN, Y.C., NITA, Y., TAMAKI, H., Institution: NATIONAL INSTITUTE OF FITNESS AND SPORTS IN KANOYA, Country: JAPAN, Abstract-ID: 1000

INTRODUCTION:
Eccentric training has been shown to lead to more muscle hypertrophy than concentric training, while also producing more muscle damage. The magnitude of muscle damage and hypertrophy is influenced by muscle length and the intensity, number, velocity, and range of lengthening of the eccentric contractions (ECC). The objectives of the present study were to characterize the histomorphological appearance and quantify the contractility of skeletal muscle followed by small- versus large-range ECC interventions.
METHODS:
Nineteen male Fischer 344 rats were assigned to three groups: age-matched control group (Control), small-range ECC group (SR-ECC; 135–140 degrees), and large-range ECC group (LR-ECC; 80–180 degrees). For the ECC groups, the left tibialis anterior (TA) muscle was subjected to eccentric exercise bouts of direct muscle electrical stimulation (ES) consisting of 80 forced ECCs twice a week for four weeks. We measured the twitch and tetanic contraction forces in the TA evoked by direct ES immediately before muscle sampling. We also performed immunohistochemical staining against Dystrophin, Pax7 and Ki67 of frozen transverse sections of the TA, as well as Evans blue and picrosirius red staining for histopathological observations.
RESULTS:
The number of damaged muscle fibers identified using Evans blue dye was greater in the LR-ECC than in the SR-ECC group. The myofiber cross-sectional area (CSA) distribution was somewhat larger in the SR-ECC group than in the Control group, although the difference was not significant. Concurrently, the number of double-positive (Pax7+/Ki67+) cells per Pax7+ cells increased in both ECC groups, but no collagen area expansion occurred in the SR-ECC group. In addition, TA muscle force profiles were smaller in the SR-ECC group than in the Control and LR-ECC groups.
CONCLUSION:
These data suggest that a small-range ECC intervention induces muscle fiber hypertrophy and satellite cell proliferation without triggering myofibrosis. Notably, this intervention also altered the muscle contraction profiles.