Muscle and Muscular Exercise

The Elastic Properties and Viscosity of Muscle.

A resting muscle possesses characteristic elastic properties, tension rising more than in proportion to extension. It shows also the phenom enon of "after-extension," continuing to lengthen (or shorten) for some time after being loaded (or released). An active muscle is different in its elastic properties, but shows a phenomenon analo gous to the after-extension of resting muscle, viz., a "viscosity" which largely reduces, at high speeds of shortening, the amount of work which the muscle can perform. This viscosity, which is an essential factor in all muscles, greatest in those which move most slowly, acts in the sense of a "governor" or "automatic brake," preventing the animal from attaining speeds of movement suffi cient to threaten the integrity of its structure.

The Chemistry of Muscle.

Muscle is about 75% water. Its fibres are imbedded in a connective tissue framework; protein forms their chief chemical constituent. Their contents are semi fluid and can be squeezed out as "muscle plasma"; the residue, consisting chiefly of "sarcolemma," connective tissue and nuclei, contains keratin, mucin and nuclein. The ash of muscle forms 1% to 1.5% and consists chiefly of potassium and phosphates, with traces of calcium, magnesium, chlorine and iron. Of nitrogenous substances the most important is creatine : of non-nitrogenous bodies fat glycogen and lactic acid must be mentioned.

Lactic Acid and Glycogen in Muscle.

It has long been known that a muscle in fatigue or rigor becomes acid. The clas sical work of Fletcher and Hopkins (1907) placed the subject of lactic acid in muscle on an exact basis and established :—(i.) That lactic acid is formed during rest without oxygen, during activity or rigor, or after injury; (ii.) that its formation in an injured muscle can be delayed or prevented by sufficient oxygen; (iii.) that once formed it may disappear in the presence of oxygen.

These facts led to the work of A. V. Hill on heat-production during the recovery phase, by which it was shown that lactic acid formed during activity is not removed by oxidation but in some other way, presumably by resynthesis to a precursor. Meyerhof established glycogen (a carbohydrate discovered by Bernard in 1857) as this precursor, and it now seems clear that all, or nearly all, the energy exchanges of active muscle can be de scribed as follows:— A. Initial non-oxidative phase (yielding mechanical energy) Glycogen—> unknown intermediary—>lactic acid—> K-lac tate, the neutralization of the acid being effected by the alkali of the tissue itself : B. Recovery oxidative phase (following activity and lasting for 5 to 8o minutes).

K-lactate—>lactic acid—> intermediary—> glycogen, the en ergy for this endothermic reaction being derived, at any rate in the isolated muscle, from the oxidation of glycogen in amount about 22% of that restored from lactate.

This division of activity into two phases, (A) work without oxygen and (B) recovery with oxygen, is very important, and is found as a general phenomenon in living tissues. The rate of supply of oxygen via the circulation is necessarily limited, and this arrangement allows temporarily for much greater rates of expen diture of energy than would be possible were the muscle dependent on contemporary combustion. The mechanism is essentially that of an accumulator, a charge being stored and capable of release at a high rate for a time without oxidation, a recharge being necessary later, with the expenditure of energy derived from com bustion of food material.

Fatigue appears to be due to the accumulation of lactic acid inside the muscle fibres, and further activity becomes impossible when its concentration reaches about 0.3%. Glycogen is necessary for activity.

Phosphates in Muscle.

The role of phosphates, although not yet clear, is obviously important. The total phosphorus in muscle varies widely but does not exceed 0.25%. Of this a certain vari able fraction is combined with protein and fat-complexes, but probably plays no part in the chemical exchanges of activity. The remainder ("acid-soluble") has been studied under four heads:— A. Inorganic phosphate ; B. Organic phosphate; i. rapidly hydro lysed by acid; ii. hydrolysed slowly by incubation of the muscle in alkali, but not rapidly by acid; iii. unaffected by treatment (i.) or (ii.).

(A) is small in resting muscle, but increases largely in fatigue and rigor; (B) (i.), which has often been mistaken for (A), is a compound of creatine and phosphate ("phosphagen"—Eggleton) which breaks down during stimulation and rigor, is restored in the presence of oxygen, but is not a source of lactic acid; (B) (ii.) is possibly a hexose-diphosphate ; it breaks down during rigor, but may increase during activity; its usual connotation "lactacidogen" is quite unfortunate, since in activity at least it is not the immediate source of lactic acid. (B) (iii.) is probably a chemically stable hexose-monophosphate. These two hexose-phosphates have been isolated from muscle, the diphosphate being identical with that produced by yeast fermentation. Muscles are capable of synthesising hexosephosphates from glycogen and phosphoric acid. These phosphate compounds probably act as intermediaries in the breakdown of glycogen to lactic acid, the monophosphate perhaps being the immediate source of lactic acid in contraction. There is a small but definite rise in blood phosphate following exercise ; this, however, is not comparable in magnitude with that of blood lactic acid under similar conditions.