The Effect of Pushing the Support Surface With the Upper Limbs on Angular Changes and Muscle Activity During the Momentum-Transfer Phase of the Sit-to-Stand.

Introduction The sit-to-stand (STS) transition is a movement in which the body's center of gravity shifts upward from a seated to a standing position while maintaining balance. Previous studies have reported that this movement is significantly affected by the presence or absence of upper limb support. The momentum transfer phase (Phase II) of STS begins at the moment of buttocks lift off and continues until the ankle joint reaches maximum dorsiflexion. This phase is considered critical, as it requires maximal muscle activity to maintain stability during elevation of the center of gravity while kinetic energy is transferred from the trunk to the lower limbs. Therefore, upper limb support during STS may play an especially important role in Phase II. Based on this, the objective of the present study was to clarify the effect of the upper limb pushing-off action on Phase II of the STS movement. Methods Ten healthy adult males (mean age: 25.9 ± 2.7 years; height: 175.5 ± 4.4 cm; weight: 69.2 ± 5.8 kg) with no orthopedic or neurological problems participated. Tests were conducted to examine differences between two conditions: with upper-limb support pushing against the support surface and without upper-limb support, in terms of joint angle changes and root mean square (RMS) of muscle activity during Phase II of the STS. Normality was assessed using the Shapiro-Wilk test; as normality was rejected, comparisons were performed using the Wilcoxon signed-rank test, and effect sizes were calculated. Additionally, correlations between upper-limb pressure and angular changes and RMS during Phase II with upper-limb support were examined. Results In Phase II of the STS, when comparing the presence and absence of upper limb support, the RMS of the vastus lateralis was significantly smaller with support. Conclusions Pressing the support surface with the upper limbs during Phase II of the STS reduced vastus lateralis muscle activity compared to the condition without upper-limb support, while no significant changes were observed in angular movement. Upper-limb pressure increased during Phase II and was directed externally, posteriorly, and downward. These findings indicate a relationship between angular changes, muscle activity, and upper-limb support during Phase II of the STS.