Intermittent stimulation enhances function of conditioned muscle

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