MUSCLE CONTRACTION
The Energetics
Sources of Energy for Muscle Contraction
- ATP: sufficient for 1-2 seconds of activity (direct energy source)
- Phosphocreatine: 5-8 seconds of activity (secondary energy source)
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ATP Synthesis
- via glycolysis under anaerobic conditions in the sarcoplasm to yield 2-3 ATPs
per glucose molecule
- via oxidative phosphorylation to yield 36 ATP per glucose molecule
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Heat Production During Muscle Contraction
- The muscle converts the free energy from ATP into work and heat.
- Muscle contraction follows the 2nd Law of Thermodynamics. In muscle, the heat produced is lost free
energy and the efficiency of the muscle is the ratio of the work produced to the free energy of ATP:
Efficiency =
(Work Produced) /
(Free Energy of ATP)
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the heat produced during muscle contraction is due to the exothermic
chemical reactions that are vital for maintaining body temperature
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Arnold's muscles are generating heat
as they are contracting
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Two phases of heat are produced during isometric muscle contraction
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The activation heat "appears in a burst soon after activation of muscle and declines before the full force is reached."
- The maintenance heat parallels the force produced.
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More heat, called shortening heat, is generated during isotonic contraction than during isometric contraction.
- Shortening heat is directly proportional to the shortening of the muscle. If the shortening distance increases, the amount of heat produced increases.
The relationship between energy output and chemical breakdown
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The output of heat + work is directly proportional to the breakdown of phophocreatine under contraction.
- Muscle contraction obeys the laws of thermodynamics.
- It transforms mechanical energy to heat energy (first law).
- It does not convert heat into work at constant temperature (second law)
Energy Cost Assessment in Humans
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"Heat generated by humans during rest and muscular activities can be measured by direct and indirect calorimetry.
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1. Direct Calorimetry: This is an airtight, insulated chamber where the
subject is either resting or exercising. Humidified air providing O2 is
constantly supplied while chemical absorbants remove CO2. The heat produced
by the subject is picked up by a stream of cold water flowing at a constant
rate through coiled tubes. The differences in the temperature of H2O entering
and leaving the chanber reflects the subject's heat production.
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2. Indirect Calorimetry: Energy metabolism in the body depends on the
utilization of oxygen. Oxygen consumption is measured and energy equivalent
is determined. One liter of O2-consumption corresponds to 4.8 kcal. Closed-
circuit or open-circuit spirometer measures O2 consumption.
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Muscle Fatigue
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A loss of work-output leading to a reduced performance of a given task
- Causes of Fatigue
- deleterious alterations in the muscle itself
- changes in the neural input to the muscle
- depletion of muscle glycogen
- decrease in blood glucose
- dehydration
- increase in body temperature
- build-up of lactic acid during anaerobic exercise
