Effect of changing water depth on muscle activity and motion kinematics during sit-to-stand motion

Abstract Background Sit-to-stand (STS) movement is a fundamental activity in daily life. Studies have examined the effect of water depth during aquatic exercise; however, no investigation has examined the effect of water depth on STS movements. Research question This study examined whether changing water depth affected muscle activity and motion kinematics during the STS movement. Methods Eleven participants performed the STS movement on the ground (land) and at different water depths (1.0-m [deep] and 0.6-m [shallow]) at a self-determined pace. Lower extremity and trunk muscle activity, lower extremity joint angle, and trunk segment inclination angle were measured. Mean muscle activity during STS movement, initial and final postures, and range of motion of each angle was evaluated. Correlations of a normalised time-series pattern of electromyography and motion kinematics measures between land and shallow (land-shallow) and between land and deep (land-deep) were investigated. Results The rectus femoris was more active in shallow than in deep water STS movement but less than that in land STS. Other muscles showed no significant differences between shallow and land STS movement. Correlation of the time series pattern in the tibialis anterior, rectus femoris, and erector spinae muscles was higher in the land-shallow than the land-deep STS. The trunk showed more forward inclination in the shallow STS than the land but less than the deep. Correlation of the time series pattern in the hip joint and trunk inclination angles was higher in the land-shallow STS than in the land-deep STS. Significance STS exercise in the shallow water depth reduced muscle load for the knee extensor but not for other muscles. The muscle activity pattern showed a higher correlation between the land STS and shallow water than deep. Sitting posture becomes upright when the water depth decreases, but a similar motion pattern could be attained regardless of water depth.