VARIATIONS IN INTRAMUSCULAR PRESSURE DURING POSTURAL AND PHASIC CONTRACTION OF HUMAN MUSCLE

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Abstract
The validity of the intramuscular pressure method of measuring tonus was detd. on frog and human muscle stimulated by tetanic currents of various strengths. The reliability of the technique was tested on the relaxed biceps. Intramuscular pressure readings were made on the human gastrocnemius stimulated by the gravitational rotatory stresses of postural sway and during voluntary wt.-bearing contraction. In one expt. synchronous observations were made of intramuscular pressure in 2 anti-gravity muscles, the gastrocnemius and quadriceps. Gross changes in tonus were reflected by significant variations in the intramuscular pressure which was lowest in the relaxed muscle, higher during postural contraction and maximal during voluntary effort. It was relatively constant in the relaxed muscle in contrast with the marked rhythmic variability which occurred during postural contraction. The rhythmic variation of intramuscular pressure in the gastrocnemius during relaxed standing was probably due to a shift in the incidence of tension between different anti-gravity groups. The method was not easily applied to stances associated with excessive postural instability.

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