Classification of motor units in flexor carpi radialis muscle of the cat

Motor units in the cat flexor carpi radialis (FCR) muscle, one of two primary wrist flexors, were classified into three groups: slow twitch, fatigue resistant (S); fast twitch, fatigue resistant (FR); and fast twitch, fatigue sensitive (FF). Classification was based on 1) the ratio of the tension-time area produced by a train of stimuli delivered at 40 pps and the maximum tension-time area (A40/Amax), and 2) the cumulative force index (CFI), calculated from a series of trains (40 pps) delivered intermittently for a period of 4 min. The CFI is defined as the ratio between the force accumulated in the last 2 min of stimulation to the first 2 min of stimulation. Motor units with values for A40/Amax greater than 0.50 were classified as type S units, and less than 0.50 as type F. A40/Amax is essentially equivalent to the "sag" profile of an unfused tetanus in its ability to separate units into "slow" and "fast" contracting units. In general, units with area ratios less than 0.50 had twitch contraction times less than 25 ms, whereas units with area ratios greater than 0.50 had contraction times greater than 25 ms. Separation of type F units into two groups was based on the CFI, with ratios less than or equal to 0.75 corresponding to type FF units and greater than 0.75 as type FR units. Type S units also had CFIs greater than 0.75. Based on this classification scheme, 40.4% of FCR motor units were type S, 37.5% type FR, and 22.1% type FF. The a priori assumption of three motor-unit types based on the distributions of A40/Amax and CFI was evaluated by cluster analysis. The analysis supported the assumption of three primary groups of motor units. Furthermore, when cluster formation proceeded to the point where only three clusters remained in the analysis, each of these clusters consisted exclusively of one type of unit (i.e., S, FR, or FF). The validity of the classification scheme was further tested by stepwise discriminant analysis. Units classified as types S, FR, and FF were predicted to be classified with 100% accuracy. All units had a high probability (a posteriori) of having group membership in their originally classified group (P greater than 0.99 for 129 units; P greater than 0.90 for 7 units). The duration of potentiated twitch contractions for type FR and FF units was found to be less than reported for most populations of hindlimb units.(ABSTRACT TRUNCATED AT 400 WORDS)