Digit flexor muscles in the cat: Their action and motor units

The relation between muscle action and the mechanical properties of motor units has been explored in the main digit flexors of the cat hind limb: plantaris (PL); flexor digitorum brevis (FDB); flexor hallucis longus (FHL); and, flexor digitorum longus (FDL). General observations on muscle action revealed that PL is an ankle extensor as well as a digit flexor. PL and FHL were shown to be the major force contributors to digit flexion with FDL playing a lesser but still significant role. The mechanical properties of PL, FHL and FDB motor units were studied by noting twitch and tetanic tensions produced by electrical stimulation of single alpha axons, functionally isolated from the ventral root filaments. These data were compared to similar data reported by Olson and Swett (1966) for flexor digitorum longus (FDL). Our sample (114 PL, 60 FDB and 124 FHL units) disclosed that PL, FDB and FHL have units of uniformly fast contraction times (means 22, 27 and 27 msec respectively). PL units developed the most tetanic tension (3 to 160, mean 62 gm‐wt) followed by FHL (2 to 87, mean 31 gm‐wt) with FDB units producing very little tension (1 to 20, mean 6 gm‐wt). Swett and Olson's FDL sample (108 units) showed tensions ranging from 0.3 to 100 gm‐wt (mean 10 gm‐wt). A division of labor among the four muscles is proposed. The large PL units are advantageous for forceful phasic inputs to the digits during the locomotion and in keeping with PL's additional role as an ankle exstensor. The low output forces of FDB units are optimal for discrete input to the digits during subtle adjustments of posture. We propose that the larger fast contracting units of FHL are used primarily for forceful digit flexions required in locomotion and for phasic protrusion of the claws while the predominately small and slow contracting units of FDL are used for sustained claw protrusion.