Energy dynamics of skeletal muscle during load moving contractions

The mechanical energy (potential+kinetic energy) and force variations associated with a muscle contracting from rest against a gravitational load were studied in nine cat muscles of the hind limb. Acceleration and deceleration phases were observed, and associated with muscular force variations of up to 25% of the mass weight in fast twitch muscles at low loads. Variations of 10% or less were associated with slow twitch muscles. It was observed that optimal loads which maximize a muscle´s mechanical energy output were close to the middle of the muscle´s isometric force range. It was concluded that the kinetic energy used by a load moving muscle is only a small percentage of that expected from the length-force relationship, and this was observed more at low loads. This confirms that the efficiency of kinetic energy conversion is very low at small loads, gradually improving as the load increases, substantiating early experiments with heat evolution (Hill, 1938), and metabolic energy (Curtin & Woledge, 1991).