Energetics of Muscle Contraction
When a muscle contracts against a load, it performs work. This means that energy is transferred from themuscle to the external load to lift an object to a greater height or to overcome resistance to movement.
In mathematical terms, work is defined by the following equation:
W = L x D
in which W is the work output, L is the load, and D is the distance of movement against the load. The energy required to perform the work is derived from the chemical reactions in the muscle cells during contrac-tion, as described in the following sections.
We have already seen that muscle contraction depends on energy supplied by ATP. Most of this energy is required to actuate the walk-along mechanism by which the cross-bridges pull the actin filaments, but small amounts are required for (1) pumping calcium ions from the sarcoplasm into the sarcoplasmic retic-ulum after the contraction is over, and (2) pumping sodium and potassium ions through the muscle fiber membrane to maintain appropriate ionic environment for propagation of muscle fiber action potentials.
The concentration of ATP in the muscle fiber, about 4 millimolar, is sufficient to maintain full contraction for only 1 to 2 seconds at most.The ATP is split to form ADP, which transfers energy from the ATP molecule to the contracting machinery of the muscle fiber. Then, the ADP is rephosphory-lated to form new ATP within another fraction of a second, which allows the muscle to continue its con-traction. There are several sources of the energy for this rephosphorylation.
The first source of energy that is used to reconsti-tute the ATP is the substance phosphocreatine, which carries a high-energy phosphate bond similar to the bonds of ATP. The high-energy phosphate bond of phosphocreatine has a slightly higher amount of free energy than that of each ATP bond. Therefore, phospho-creatine is instantly cleaved, and its released energy causes bonding of a new phosphate ion to ADP to reconstitute the ATP. However, the total amount of phosphocreatine in the muscle fiber is also very little— only about five times as great as the ATP. Therefore, the combined energy of both the stored ATP and the phosphocreatine in the muscle is capable of causing maximal muscle contraction for only 5 to 8 seconds.
The second important source of energy, which is used to reconstitute both ATP and phosphocreatine, is “glycolysis” of glycogen previously stored in the muscle cells. Rapid enzymatic breakdown of the glyco-gen to pyruvic acid and lactic acid liberates energy that is used to convert ADP to ATP; the ATP can then be used directly to energize additional muscle contraction and also to re-form the stores of phosphocreatine.
The importance of this glycolysis mechanism is twofold. First, the glycolytic reactions can occur even in the absence of oxygen, so that muscle contraction can be sustained for many seconds and sometimes up to more than a minute, even when oxygen delivery from the blood is not available. Second, the rate of for-mation of ATP by the glycolytic process is about 2.5 times as rapid as ATP formation in response to cellu-lar foodstuffs reacting with oxygen. However, so many end products of glycolysis accumulate in the muscle cells that glycolysis also loses its capability to sustain maximum muscle contraction after about 1 minute.
The third and final source of energy is oxidativemetabolism. This means combining oxygen with theend products of glycolysis and with various other cel-lular foodstuffs to liberate ATP. More than 95 per cent of all energy used by the muscles for sustained, long-term contraction is derived from this source. The foodstuffs that are consumed are carbohydrates, fats, and protein. For extremely long-term maximal muscle activity—over a period of many hours—by far the greatest proportion of energy comes from fats, but for periods of 2 to 4 hours, as much as one half of the energy can come from stored carbohydrates.
Efficiency of Muscle Contraction. The efficiency of anengine or a motor is calculated as the percentage of energy input that is converted into work instead of heat. The percentage of the input energy to muscle (the chemical energy in nutrients) that can be converted into work, even under the best conditions, is less than 25 per cent, with the remainder becoming heat. The reason for this low efficiency is that about one half of the energy in foodstuffs is lost during the formation of ATP, and even then, only 40 to 45 per cent of the energy in the ATP itself can later be converted into work.
Maximum efficiency can be realized only when the muscle contracts at a moderate velocity. If the muscle contracts slowly or without any movement, small amounts of maintenance heat are released during con-traction, even though little or no work is performed, thereby decreasing the conversion efficiency to as little as zero. Conversely, if contraction is too rapid, large pro-portions of the energy are used to overcome viscous friction within the muscle itself, and this, too, reduces the efficiency of contraction. Ordinarily, maximum effi-ciency is developed when the velocity of contraction is about 30 per cent of maximum.