RELATIONSHIP OF REDUCED HAMSTRING FLEXIBILITY WITH LUMBOPELVIC RHYTHM AND MUSCLE ACTIVATION: A CROSS-SECTIONAL BIOMECHANICAL ANALYSIS

Key Findings (narrative form) The study examined how hamstring flexibility influences the way the lumbar spine and pelvis share movement during forward bending and how this affects muscle activity.

1. Flexibility and lumbar contribution When participants had tighter hamstrings, their lower back took on a larger share of the bending motion. In the full forward-bend task this relationship was strong, while in the partial bend it was still clearly evident. In other words, limited hamstring length forces the spine to bend more to reach the same position.
2. Meaningful flexibility thresholds

Participants were divided into three straight-leg-raise (SLR) groups:

• ≤ 60 ° (short hamstrings)
• 61-79 ° (moderate flexibility)
• ≥ 80 ° (good flexibility) Those in the ≤ 60 ° group showed a significantly higher lumbar contribution in both bending tasks than their more flexible peers. Once SLR exceeded roughly 60 °, additional gains in flexibility produced only modest further improvement in spine-pelvis balance, suggesting that 60 ° is a clinically important threshold.

3. When the differences appear The greatest gap between flexibility groups occurred during the first half of the bend-particularly as participants began to lean forward. As they returned to upright, the differences narrowed. This indicates that early-phase movement is the critical moment when tight hamstrings shift load onto the lumbar spine.
4. Impact on muscle activity Better hamstring flexibility was linked to a more even distribution of work between the lumbar extensor muscles and the hamstrings themselves. Participants with looser hamstrings did not have to activate their spinal muscles as forcefully, whereas gluteus maximus activity remained low in all groups because the tasks were unloaded.

Practical Take-Aways

• Hamstrings shorter than about 60 ° on the SLR "lock" the pelvis and make the lower back bend excessively, increasing spinal strain.
• Improving hamstring length shifts motion back toward the pelvis, reducing demand on lumbar joints and muscles.
• Even small everyday bends-such as reaching for an object on a chair-follow the same pattern, so stretching benefits daily life, not just sports performance.
• Patients and families can adopt simple hamstring-stretch routines; clinicians should consider targeted flexibility training whenever SLR is 60 ° or below before progressing to heavy lifting or core-stability programs.

Limitations The study involved healthy young adults and measured only unloaded forward bending. Outcomes may differ in older individuals, manual laborers, or tasks that involve twisting or weight. Long-term research is needed to confirm that stretching actually prevents low-back pain.

Bottom Line Flexible hamstrings let the pelvis and lower back "share the job." If your hamstrings are tight, your spine must work harder, which may invite discomfort or injury over time. A regular stretching program that brings SLR above roughly 60 ° can restore a healthier, more balanced bending pattern and help protect the lower back.