Effects of trunk anterior tilt and knee joint flexion angle changes on muscle activity in the lower limb muscles

[Purpose] We examined the effects of trunk anterior tilt angle (TA) and knee flexion angle (KA) on lower limb muscle activity. [Participants and Methods] Twenty-eight healthy male participants (age, 24.7 ± 4.7 years) performed nine standing tasks with different TA and KA. The participants were instructed to remain still during each task. The nine standing tasks were randomly performed while measurements of muscle activity were obtained for seven muscles: gluteus maximus (GMAX), medial hamstrings (MH), lateral hamstrings (LH), rectus femoris (RF), vastus lateralis (VL), medial gastrocnemius (MG), and soleus (SOL). The activities of these muscles were normalized using isometric grade 3 of the manual muscle testing (isoMMT3). The intra-rater reliability for the mean values of the muscle activities measured with the isoMMT3 (intra-class correlation coefficient with 95% confidence interval) was confirmed using equation ICC (1,3). [Results] GMAX, MH, LH, RF, and MG were affected by both TA and KA, whereas VL was affected by KA, and SOL was affected by TA. [Conclusion] Our findings may facilitate a better understanding of the changes in muscle activity of the lower limb muscles due to differences in TA and KA.