Forms of forward quadrupedal locomotion. I. A comparison of posture, hindlimb kinematics, and motor patterns for normal and crouched walking.

1. Posture, hindlimb kinematics, and activity patterns of selected hindlimb muscles were compared for normal and crouched treadmill walking (0.5-0.6 m/s) for eight cats. To elicit crouched walking in which the trunk and head were lowered, cats were encouraged to walk under a light-weight Plexiglas ceiling suspended 17-20 cm above the treadmill belt. Kinematic data were obtained from high-speed ciné film, and electromyograms (EMGs)-synchronized with the kinematic records-were taken from 11 hindlimb muscles. 2. The postures for the two forms of walking were distinctly different. During crouched walking, each cat lowered its entire body keeping its trunk horizontal to the treadmill belt. Also the head was lowered, with the top of the head in line with the dorsal surface of the trunk. Hip height, used as a measure for hindlimb crouch, was reduced by 30%, from an average height of 23 cm to an average height of 16 cm above the belt during the entire step cycle. 3. Average cycle periods (766 +/- 30 ms, mean +/- SD) and percentage of time devoted to swing (30%) and stance (70%) were similar for normal and crouched walking. The profiles of the hindlimb kinematics were also similar for the hip, knee, ankle, and metatarsophalangeal (MTP) joints during the step cycle, but the timing of some of the motion reversal, as well as the ranges of motion during various phases, were different at some joints for the two forms of walking. 4. During the swing phase, the transition between the flexion and extension (F-E1 reversal) occurred later in the normalized swing phase at the hip, knee, and ankle joints, and the range of flexion was increased at each joint. With greater flexion at these joints, the anatomic axis of the hindlimb (measured from hip joint to toe) was decreased and the hind paw advanced in the narrow space between the abdomen and treadmill belt. At contact, the position of the paw was less anterior to the perpendicular reference line (hip joint marker to belt) and all joints were more flexed for crouched than normal walking. 5. Throughout the stance phase, the knee and ankle joints remained significantly more flexed by 41-45 deg during crouched than normal walking. Although the hip and MTP joints started in a more flexed position at paw contact, both joints extended more during stance for crouched than normal walking, and at the time of peak extension (just before paw lift-off), the degree of extension at the hip and MTP joints was similar for both forms of walking. 6. Muscle patterns for crouched and normal walking were similar with some exceptions. The burst durations for three primary flexor muscles, the semitendinosus (knee flexor), extensor digitorum longus (EDL, ankle flexor), and flexor digitorum longus (digit flexor) were longer for crouched than normal walking, and this was consistent with the increased range and duration of flexion during the swing phase of crouched walking. Also, two muscles that normally showed mainly swing-related activity during normal walking, the EDL and the extensor digitorum brevis, had distinct stance-related bursts that occurred after midstance during crouched walking. 7. Crouched walking requires a postural change that typically occurs when cats stalk prey and when cats walk up and down sleep slopes. Postural set during walking appears to be determined by brain stem and diencephalic centers, and the postural orientation of the cat may require adjustments in the motor program provided by spinal centers for the cat to walk. The role of posture and locomotion and the adjustments in hindlimb kinematics and EMG activity patterns have been studied for forward and backward walking in the cat and now for crouched walking on the treadmill. These data will assist us in understanding the role of posture, especially crouched posture, during other walking behaviors.