Muscles and Heat

1. Muscles and Heat Dr. Maurissa D’Angelo

2. Muscle Heat • Generates heat as contract • Vitally important to maintaining body temperature • Most of heat from skeletal muscle • Release excess heat to maintain temperature

3. Muscles and Heat • Maintenance heat source • Slow liberation of heat via resting muscle • Unrelated to contraction, “background heat” • Initial Heat Liberated during contraction Byproduct of chemistry of contractile process • Activation heat, A: related to excitation-contraction coupling • Shortening heat, ax: related to shortening of muscle • Tension-time heat, f(P,t): related to cross-bridge turnover during time when muscle is maintaining tension • Recover heat – heat generated at end of contraction • Largely aerobic • Related primarily to oxidation of lactate • Liberated during relaxation (if muscle can relax without bearing load) • Degradation of potential energy of lifted load into heat (i.e. no longer bearing load)

4. Initial Heat • Extra heat – i.e. heat that exceeds maintenance heat, liberated immediately following stimulus • Occurs during: • depolarization and repolarization of sarcolemma • Release of activator calcium by sarcoplasmic recticulum • Binding of calcium to troponin C • Initial heat accompanies mechanical activity of activated contractile proteins • Because of heat modeling, Hill equation to predict hyperbolic shape of force-velocity curve, define chemical basis of muscular contraction

6. Isometric Contraction • i.e. muscle isn’t shortening, not performing any external work • Heat • Immediately after stimulation of muscle – activation heat • Brief burst of heat prior to development of significant tension • Following activation heat – slower rate heat production along with development of tension • Small • Ignored for modeling and discussion (in this class) Energy = (Work Produced) / (Free Energy of ATP)

8. Modeling Initial Heat • DE = A + Wi (Activation Heat + Internal Work) • Where DE is the change in external energy • A is the heat of activation • Wi is the internal work which takes place in isometrically contracting muscle • Even though ends of muscle are fixed, small amount of internal work done by contractile elements • Still have slight shape change in muscle, stretching • DE = Q – W • Energy in thermodynamic terms • Q is the activation energy (plus small amount of heat liberated during contraction) • W is the internal work

9. Isotonic Contraction • Muscle shortens while lifting constant load, external work is performed • Additional energy/heat present • Total amount of heat liberated (ax) • a: amount of heat liberated for each cm of shortening (constant for given type of muscle) • x: the distance the muscle shortens • Shortening heat is therefore = a*x

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

11. Refined Equations • Refined precision of measuring heat liberation • Determined a (activation heat) is not constant during isometric contractions at different loads • Found proportional to developed tension • Heat of shortening dependent on load • Mommaerts separated heat of activation into two components, A – true heat of activation and f(P,t) – heat liberation which is a function of tension of muscle (P) and length of time (t) tension is maintained

12. Tension-Time Heat • 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 thetime duration t that the tension is exerted.

13. 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.

14. “Extra Energy” • Extra Energy Liberation = (P + a) xRate of Extra Energy Liberation = (P + a) dx/dt • For an isometrically contracting muscle P = P0and 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.

15. “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

16. “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

17. Measuring Energy Cost via Heat • Measure via direct and indirect calorimetry • Direct – airtight insulated chamber • Person remains dormant or exercises • Humidified air provides oxygen • Chemical absorbents remove carbon dioxide • Heat produced picked up by stream of cold water • Indirect – energy metabolism in body • Oxygen dependent • Measure oxygen consumption, calculate energy equivalent • Measured via spirometer