1. 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/A(max)), 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. 2. Motor units with values for A40/A(max) >0.50 were classified as type S units, and <0.50 as type F. A40/A(max) 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 < 0.50 had twitch contraction times < 25 ms, whereas units with area ratios > 0.50 had contraction times > 25 ms. Separation of type F units into two groups was based on the CFI, with ratios ≤ 0.75 corresponding to type FF units and > 0.75 as type FR units. Type S units also had CFIs > 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. 3. The a priori assumption of three motor-unit types based on the distributions of A40/A(max) 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). 4. 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 > 0.99 for 129 units; P > 0.90 for 7 units). 5. 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. As a consequence of this, type F units, due to their very brief contraction (mean = 19.4 ms) and half-relaxation (mean = 16.4 ms) times, produced on average 12% of their maximum tetanic tension when stimulated at 40 pps. In contrast, type S units had significantly longer mean twitch contraction (42.3 ms) and half-relaxation (41.2 ms) times than type F units. Furthermore, type S units produced 80% of their maximum tetanic tension when activated at 40 pps. 6. As in cat hindlimb muscles, the mean maximum tetanic tension of each unit type was found to be scaled as follows: FF > FR > S. Moreover, the mean maximum tetanic tension for each unit type was found to be very similar to that reported for units of cat medial gastrocnemius (10, 13, 24), a much larger muscle.
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