Background: Hamstring shortness (HS) contributes to the limit of hip and knee range of motion and can limit the execution of multi-segmental movement exercises such as squatting, thereby altering the muscle activation. The double leg squat (DS) is fundamental exercise for knee injury rehabilitation programs. However, it may aggravate knee joint injury and cause low back pain. The Bulgarian split squat (BSS) is a one-leg squat exercise with greater knee joint stability than DS. Measuring muscle activation during the squat will provide insight into the contribution of hamstring activation and quadriceps and hamstring (Q:H) ratio to the knee in participants with HS. Objects: This study compares hamstring activation and Q:H ratios during squat exercises in individuals with and without HS. Methods: This study classified 30 healthy participants, based on active knee extension, into two groups: with HS and without HS. Surface electromyography was performed to determine muscle activation of the medial hamstring (MH) and biceps femoris of the hamstrings and coactivation of the Q:H. All participants performed DS and BSS. Results: MH activity was significantly higher in the with HS group than in the without HS group during all exercises, and the Q:H ratio was significantly closer to 1.0 during BBS compared to DS. Additionally, the with HS group had a significantly lower Q:H ratio than without HS group. Conclusion: In comparison between exercises, BBS can bring the Q:H ratio closer to 1.0 compared to DS, which may enhance knee stability. However, in between-group comparisons, the increased tension due to the shortening of the hamstrings leads to heightened hamstring activation, resulting in a Q:H ratio that is closer to 1.0 compared to the without HS group. Therefore, relying solely on the Q:H ratio to assess knee stability may be risky.

Background: The flexion-relaxation phenomenon (FRP) refers to a sudden onset of activity in the erector spinae muscles that recedes or fades during full forward flexion of the trunk. Lumbar spine and hip flexion are associated with many daily physical activities that also impact trunk flexion. Shorter hamstring muscles result in a reduction of pelvic mobility that eventually culminates in low back pain (LBP). Many studies have explored the FRP in relation to LBP. However, few studies have investigated the influence of hamstring muscle length on the FRP in relation to the erector spinae muscles during trunk flexion. Objects: This study aimed to investigate the influence of hamstring muscle length on the FRP in relation to the erector spinae muscles during trunk flexion. Methods: Thirty subjects were divided into three groups according to hamstring length measured through an active knee extension test. The 30 participants consisted of 10 subjects who had a popliteal angle of 20˚ or less (Group 1), 10 subjects who had a popliteal angle of 21˚–39˚ (Group 2), and 10 subjects who had a popliteal angle of 40˚ or more (Group 3). A one-way analysis of variance was used to compare the difference in muscle activity of the erector spinae muscles during trunk flexion. Results: The subjects with a shorter hamstring length had significantly higher muscle activity in their erector spinae muscles during trunk flexion and full trunk flexion (p < 0.05). The subjects with a shorter hamstring length also had a significantly higher flexion-relaxation ratio (p < 0.05). Conclusion: The results of this study demonstrate that differences in hamstring muscle length can influence the FRP in relation to the erector spinae muscles. This finding suggests that the shortening of the hamstring might be associated with LBP.