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.

INTRODUCTION
MATERIALS AND METHODS
    1. 연구대상자
    2. 연구설계
    3. 측정 절차
    4. 통계 분석
RESULTS
    1 .연구대상자의 일반적인 특성
    2. 몸통 굽힘 동안 각 그룹 간 척추세움근 근활성도 비교
    3. 완전 몸통 굽힘 시 각 그룹 간 척추세움근 근활성도 비교
    4. 몸통 굽힘 시 각 그룹 간 척추세움근 FRR 비교
DISCUSSION
CONCLUSIONS
REFERENCES

• 김나희(신라대학교 일반대학원 물리치료학과) | Na-hee Kim (Department of Physical Therapy, Graduate School of Silla University)
• 최보람(신라대학교 보건복지대학 물리치료학과) | Bo-ram Choi (Department of Physical Therapy, College of Health and Welfare, Silla University, Busan, Korea) Corresponding Author