Alteration of quadriceps muscle activity during functional step tasks after extended sitting session

Background
Prolonged sitting with high knee flexion is a common activity that may affect patellofemoral joint compression and quadriceps length. Exploring the quadriceps activation after sitting may help to explain the mechanism underlying muscle changes and the resulting patellofemoral pain.
Objectives
To examine changes in quadriceps activity after prolonged sitting in cross-legged sitting, side-sitting, and sitting on a chair.
Design
Laboratory observational study.
Method
Thirty healthy women participated and were randomly allocated to three groups of different sitting positions (n = 10/group). Electromyography (EMG) of the vastus medialis (VM), rectus femoris (RF), and vastus lateralis (VL) was performed before and after 15 min after sitting. EMG was conducted during step-up and step-down tests and was reported as %MVIC. The results were analysed using Wilcoxon signed-rank test and Kruskal-Wallis ANOVA.
Results
VM activity and EMG activity during the step-down test after sitting increased significantly in the cross-legged group (p = 0.01). Sitting postures significantly influenced (p = 0.02) muscle activity changes in the VL and VM during the step-up test. Pairwise comparisons revealed significant differences between the cross-legged sitting and sitting on a chair groups.
Conclusions
Prolonged cross-legged sitting and side-sitting caused changes in VM and VL activity during step tasks in healthy young women. After cross-legged sitting, the VM exhibited a significantly higher activity in descending control, and changes in VM and VL activity increased significantly during the step-up task. Increased VM and VL activation possibly controls the patellofemoral joint. Therefore, they may fatigue more easily when many step tasks or squatting exercises are performed.