| Functioning in daily life as an independent human being involves having the ability to stand up from a seated position, yet little information is available on the sit-to-stand (STS) movement in children. The primary purpose of the present study was to assess the mechanics of the STS movement in young children. A secondary purpose was to compare the biomechanics of the STS movement among children of different ages. The subjects were 12 children who fell into one of three age groups: 12–18 months (N = S), 24–36 months (N = 3), and 48–60 months (N = 4). The subjects were videotaped as they stood up from a seat adjusted to each subject's knee height. Data on forces at the buttocks and at the feet were collected simultaneously from a force platform mounted on the stool and a force platform located beneath the feet. The kinematic and kinetic variables obtained or derived from the analysis of the video and force records included movement times; joint ranges of motion; peak hip flexion, hip extension, and knee extension velocities; velocity of the whole-body center of gravity; forces at the buttocks and feet; and peak hip and knee joint moments. The results of the study indicated that the overall pattern of the STS movement is well-developed even in the youngest children and is similar to that seen in adults. These similarities include: (a) the overall joint motion pattern, in which hip flexion and ankle dorsiflexion characterize the early part of the movement while extension of the hip, knee, and ankle (i.e., plantar flexion) occur in the latter part of the movement; and (b) the pattern of forces at the buttocks and the feet, which included three types of forces: a posteriorly-directed, propulsive horizontal force at the buttocks that initiates the movement; a vertical, propulsive force at the feet; and an anteriorly-directed, braking horizontal force at the feet which acts to check the forward momentum of the body. Overall, the STS movement in the children was characterized by fast movement times (c. 1.2 s), relatively fast joint angular velocities, hip joint moments that were greater in magnitude than knee joint moments, and the center of gravity being located behind the base of support at seat-off. There was a trend toward increasing joint ranges of motion, velocities, forces, and moments with increasing age, with the oldest children having results quite similar to adults. These results suggest that the oldest children employed a strategy of transferring momentum from the upper body to the lower body during the movement as seen in younger adults. While the younger children exhibited many of the characteristics of this strategy, they also used elements of a strategy in which the goal of upper body movements is to bring the center of gravity close to the base of support at the time of seat-off, probably due to a relative lack of balance and postural control as compared to the older children and adults. While this study provides much-needed normative data on able-bodied children, additional research needs to be done on how the mechanics of the STS movement in children are affected by elements of seat design, and on the mechanics of the STS movement in children with disabilities. |
