Intermittent stimulation enhances function of conditioned muscle.

Skeletal muscle is highly adaptable in that its metabolic and contractile characteristics are largely regulated by its pattern of use. It is known that muscle phenotype can be manipulated via chronic electrical stimulation to enhance fatigue resistance at the expense of contractile power. Type 2A fibers are fatigue resistant, powerful, and considered most desirable for cardiac assist purposes. We have found that 12-wk of intermittent-burst stimulation produces a high percentage of 2A fibers and increases fatigue resistance and power in rabbit latissimus dorsi muscle. Fixed-load endurance tests were used to quantify fatigue resistance among normal and trained muscle groups. Control muscles were found to fatigue completely within 10-20 min. Muscles stimulated continuously for 6 wk retained 35% (71.5 +/- 19.5 g. cm) of their initial stroke work at 40 min. Muscles stimulated 12 h/day for 12 wk had the highest initial stroke work (449.7 +/- 92.4 g. cm) and the highest remaining stroke work (234.7 +/- 50.1 g. cm) at 40 min. Results suggest that employing regular resting periods during conditioning preserves strength in fatigue-resistant muscle.