This is the second installment of my answer to Mindy's question: Why do kids bunny hop?
1 Variability, learning to balance in prone kneeling and early crawling
2 What abilities are needed for reciprocal crawling?
3 Infants who bunny hop need to acquire more variable patterns of movement
For me any assessment of an infant's task performance always starts with a review of the basic postural and movement abilities needed for performing the motor task.
So in this case I asked the question: what postural and movement abilities are needed for reciprocal crawling? And to be quite sure that I understood the task, I reviewed my video clips of a typical infant crawling and took a look at what the research literature says about the kinematics of crawling.
Postural and movement abilities needed for reciprocal crawling.
1 Mature reciprocal crawling involves a swing and a stance phase for each extremity, and is characterized by almost synchronous movements between the ipsilateral arm and the contralateral leg.
2 Crawling is initiated by shifting weight onto one hand and the contralateral knee (stance phase) to allow the contralateral hand and knee to be lifted up and moved forwards (swing phase).
The shoulder and hip of the swing limbs are flexed, moving the knee and hand forwards.
At the same time the shoulder and hip of the stance limbs are extended as the COM is shifted forwards in readiness for transferring the weight onto the swing limbs.
During the swing phase the pelvis is laterally tilted on the side of the swing LL.
For details of the kinematics of crawling see: Righetti, L., Nylén, A., Rosander, K., & Ijspeert, A. J. (2015). Kinematic and Gait Similarities between Crawling Human Infants and Other Quadruped Mammals. Frontiers in Neurology, 6, 17. http://doi.org/10.3389/fneur.2015.00017
The following clips illustrate the complex coordination of the limbs when crawling. They also illustrate how the infants' pattern of movement is influenced by the task goal, such as crawling towards and reaching for a toy.
While the coordinated movements of the limbs are the most obvious aspect of crawling, there are also complex adjustments in configuration of the limbs to dynamically maintain the center of mass (COM) within the base of support (BOS) to maintain balance. The slow motion part of each clip highlights these shifts in weight bearing.
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