A behavioral and electromyographic study of walking in the chick.

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Abstract
The coordination of locomotor movements is largely a localized property of the spinal cord. Locomotor control in a developmentally accessible animals the chick, was studied. The hindlimb movements and associated hindlimb muscle electromyographic (EMG) activities during walking in the hatched chick were described. Chicks were observed while walking unrestrained or on a treadmill. Hindlimb movements were recorded and joint-angle measurements were made from videotape records. EMG recordings were made from a total of 15 hindlimb muscles; these data were correlated with hindlimb movements by simultaneous videotape recordings in a few animals. The movement of the hindlimb was divided into 3 phases: swing flexion, swing extension, and stance. The swing period begins when the hip, knee and ankle joints are flexed and the foot is picked up (swing flexion). The knee is then extended while the hip and ankle are kept flexed; this movement brings the foot rostral with respect to the body (swing extension). Toward the end of this phase, the ankle is extended and the foot is placed on the ground. The limb then moves caudalward with respect to the body, serving the support and propel the chick (stance phase). During this period, the hip, knee and ankle show complex patterns of join-angle changes. This triphasic pattern of movement during the step is associated with a triphasic pattern of muscle activity. During swing flexion, Srt [sartorius], IFB [iliofibriloris] and TA [tibialis anterior] are active. During swing extnesion, FT [femorotibialis], Srt, aITB [iliotibialis anterior] and ITR [iliotrochantericus] are active. During stance, the extensor muscles MG [medial gastrocnemius], LG [lateral gastrocnemius], pITB [iliotibialis posterior], ADD [adductor], CFX [caudilioflexorius] and ITR are active; the onset of activity in some of these (MG, LG, ADD) precedes the beginning of the stance phase. FT and IFB are active during stance as well, but this activity is distinct from their swing phase activity. The duration of the swing phase and of associated EMG bursts increased little with increasing step cycle duration, while the duration of the stance phase and of associated EMG bursts increased greatly with step cycle duration. No significant differences were observed when free-walking and treadmill-walking were compared. The identification of 3 distinct phases of movement and muscle activity during the step cycle is in accord with published EMG records from stepping and scratching in other tetrapods. The reason that a 3rd phase of movement, i.e., swing extension, is particularly prominent in the chick is discussed.