The development of quadrupedal locomotion in the kitten

The development of bipedal treadmill locomotion and overground locomotion has previously been studied in the kitten; the development of quadrupedal treadmill locomotion has not. We evaluated and compared all three forms of locomotion in the normal kitten and present quantitative data comparing the development of quadrupedal treadmill and overground locomotion. Overground locomotion was studied from the day of birth to 5 months of age and quadrupedal treadmill locomotion was studied in the same animals from 9 weeks to 5 months of age. Treadmill locomotion was initiated postweaning, since it could not be reliably elicited without a food reward. Three locomotor characteristics (weight support, balance, and coordination between the forelimbs and the hindlimbs) were evaluated quantitatively. Kittens first consistently demonstrated overground steps with the ventral surface of their bodies supported above the walking surface throughout the entire step cycle during the second and third postnatal weeks. By 4 weeks of age, overground locomotion consistently showed full weight support and midline positioning of the hindquarters. Coordination between the forelimbs and the hindlimbs developed differently in the two forms of quadrupedal locomotion evaluated. During overground locomotion, the kittens initially used a single pattern in which only one limb was in swing at any time. As the kittens' weight support and trunk control improved, additional swing phase coordination patterns emerged and these patterns were correlated with the animals' ability to change speeds during locomotion. The consistency with which a dominant interlimb swing phase pattern was used at a particular speed increased with age and, by 6 weeks, the frequency of each speed-related dominant pattern approached 100% during overground locomotion. At 6 weeks, interlimb coordination also was evident in the nearly consistent interlimb phase interval present between the forelimb's initiation of the first extension subphase and the ipsilateral hindlimb's initiation of the flexion phase. The consistent patterns appeared to be fostered by maturation of weight support and balance. In contrast, the interlimb phase interval was inconsistent during quadrupedal treadmill locomotion until 20 weeks of age. Moreover, the interlimb swing phase patterns used during quadrupedal treadmill locomotion differed from those used during overground locomotion. The differences in the developmental time course and patterns of interlimb coordination between overground and quadrupedal treadmill locomotion suggest that different mechanisms regulate the control of interlimb coordination during these two different forms of quadrupedal locomotion.