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Human muscle spindle discharge during isometric voluntary contractions. Amplitude relations between spindle frequency and torque.

During isometric voluntary contractions, the impulse frequency in muscle spindle afferents was analysed in relation to the contraction intensity in man. Single unit impulses from the finger flexor muscles were recorded with percutaneously inserted tungsten needle electrodes from the median nerve. The spindle frequency increased with the contraction intensity. For very weak contractions the increase was steep but levelled off to attain an average slope of 32.8 ips/Nm at the metacarpo-phalangeal joint for the primaries and 22.8 ips/Nm for the secondaries when the muscles to a single finger were activated. The close relation between spindle frequency and torque suggests parallel modulations of the skeletomotor and the fusimotor outputs during voluntary contractions. The dominating type of fusimotor outflow was of the static type but clear indications of increased dynamic fusimotor outflow were also seen. Quantitative considerations of the spindle frequency as a function of contraction intensity and as a function of muscle length suggest that the muscle spindles and their central connections do not constitute a very powerful mechanism for holding the muscle at constant length when the load varies.