Disturbances in Voluntary Recruitment Order of Low and High Frequency Motor Units on Blockades of Proprioceptive Afferent Activity

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Abstract
The voluntary recruitment order of anterior tibial motor units was studied in electromyographic recordings of human muscle using both low and high impedance electrodes. The consequences for the discharge properties of reduced afferent inflow in sustained voluntary contraction were studied, using partial ischemic or anesthesia blockades of the muscle nerve and local cooling of the muscle belly. The recruitment order of certain low and certain high frequency units was reversed. The muscle tension and sense of effort necessary for tonic firing were increased for certain low frequency units but decreased for certain high frequency units. On severe reduction of the afferent inflow, the differences in discharge pattern between low and high frequency units were decreased. The recruitment of low frequency units before high frequency units in sustained voluntary contraction is partly due to proprioceptive afferent activity favoring low frequency units and disfavoring high frequency units. The afferent inflow is involved in restricting certain units to low frequency discharge and others to high frequency discharge.