Muscle Generated Heat Primary sources of liberated heat energy from skeletal muscle due to chemical processes: Maintenance (Resting) - Slowly liberated background heat, unrelated to muscle contraction. Recovery - Heat generated at the end of muscle contraction, related mainly to chemical reactions associated with energy production. Initial - Liberated immediately after stimulation and throughout muscle contraction (tension).

Initial Heat Isometric Contractions - Activation Heat Constant Length - No External Work Isotonic Contractions - Shortening Heat Constant Force (Load) - External Work Tension-Time Heat

Initial Heat - Isometric Two components: Activation - Brief burst, immediately after stimulation. Slower rate of heat production associated with the development of increasing tension. DE = A + Wi (Activation Heat + Internal Work) DE = Q - W

Initial Heat - Isotonic In addition to external work (muscle shortening while lifting a constant load), additional heat is liberated due to the shortening process itself. Note: This so called shortening heat is function of the shortening distance, but independent of the load. Although the amount of heat production is independent of the load, the rate of heat production decreases as the load increases.

Isotonic - Shortening Heat Shortening Heat = ax a is muscle specific (units of force) x is the shortening distance DE = A + We + ax A is Activation Heat We is External Work ax is Shortening Heat DE = Q + Px = Heat (A + ax) + Work (Px)

Tension-Time Heat Note: Early work of Hill (1930’s) was refined in the 1960’s with the advent of more precise measurements of heat. Activation Heat was found to not be a constant for isometric contractions at various loads, but rather is proportional to the developed tension. Shortening Heat however is a function of the load.

Tension-Time Heat continued Isometric DE = A + Wi + f(P, t) Isotonic DE = A + We + ax + f(P, t) Where f(p, t) represents the heat liberated as a function of both the muscle tension P and the time duration t that the tension is exerted.

Hill’s Equation “Characteristic Equation of Muscle” “Extra Energy” Let muscle lift a load P through a distance x. The energy generated as work W = Px The energy generated as Shortening Heat = ax Activation Heat A is omitted (not related to contraction) f(P, t) omitted for simplicity Extra Energy = Px + ax = (P + a) x Represents the total amount of extra energy liberated by a muscle contracting under isotonic conditions.

“Extra Energy” Extra Energy Liberation = (P + a) x Rate of Extra Energy Liberation = (P + a) dx/dt For an isometrically contracting muscle P = P0 and the Rate = 0 since there is no Work (x = 0) nor is there any Shortening Heat (isometric). For an unloaded freely shortening muscle (P = 0) the rate of energy release is a maximum.

“Extra Energy” - continued There is a direct linear proportionality for Rate of Extra Energy Released and the Difference between Max Load (P0) and the Actual Load (P), i.e. E » (P0 - P) That is to say, the smaller the load, the greater the rate of energy released. (P + a) v = (P0 - P) b

“Extra Energy” - continued (P + a) v = (P0 - P) b P v + a v = P0 b - P b P v + a v + P b + a b = P0 b + a b (P + a) v + (P + a) b = (P0 + a) b

Hill’s Equation (P + a) v + (P + a) b = (P0 + a) b