Inter-joint coordination variability during a sit-to-stand fatiguing protocol.

A repetitive sit-to-stand (STS) task is often used as a fatiguing protocol. Although post-fatigue reductions in muscle strength have been frequently used to indicate fatigue, little is known about changes in body movement during the fatiguing process. This study examined changes in variability of hip-knee and knee-ankle coordination during the STS fatiguing course in 15 young (age: 26.7 ± 5.8 years; 9 females, 6 males) and 15 older adults (age: 69.3 ± 5.7 years; 9 females, 6 males). Participants were asked to perform repetitive STS movements until exhaustion or for 30 min at a self-selected pace. Motion data from 3 consecutive STS cycles were extracted from every minute during the entire fatiguing course and time normalized to STS duration and dummy coded as five stages: 0-20, 20-40, 40-60, 60-80, and 80-100% of the course. Outcome variables were hip-knee and knee-ankle inter-joint coordination variabilities. A 2*5 mixed-effect ANOVA was used to examine changes in variability during different stages of the course in young and older adults. No Age × Time interactions were found in either hip-knee or knee-ankle coordination variability. The hip-knee coordinative variability significantly increased along the STS fatiguing course regardless of age, and the knee-ankle variability from standing to sitting was also higher at the ending, when compared to the beginning, of the fatiguing protocol. The impact of fatigue during repetitive STS protocol is not only limited to a force production decline but also manifested as increased coordinative variability, which could be considered as a fatigue indicator.