The influence of hip extensor and lumbar spine extensor strength on lumbar spine loading

Key points

1. 01Less available hip flexion was associated with greater peak lumbar flexion in females (r = -0.407, p = 0.035) but not in males (r = -0.341, p = 0.120)
2. 02Lower lumbar extensor strength was associated with greater peak lumbar flexion in females (r = -0.398, p = 0.040) but not in males (r = -0.310, p = 0.161)
3. 03Available hip flexion strongly predicted actual hip flexion used during the squat lift in both sexes combined (r = 0.774, p < 0.001)
4. 04Males had significantly greater lumbar extensor strength than females (1.9 vs 1.6 Nm/kg, p = 0.003)
5. 05Females used significantly more lumbar flexion during the squat lift than males (11.3 vs 6.9 degrees, p = 0.049)

How it was conducted

Design

Cross-sectional controlled laboratory study

Participants

50 healthy adults aged 18-40 years (27 females, 23 males), no history of low back or lower extremity pain in the prior 12 months

Strength measurement

Isometric lumbar extension torque via motor-driven dynamometer (PrimusRS), normalized to body mass

Kinematics

16-camera 3D motion capture at 100 Hz; peak lumbar and hip flexion quantified during squat lift descent phase

Hip flexion mobility

Seated maximal hip flexion measured with 3D motion capture, average of left and right over 3 trials

Lift task

Squat lift of a box weighing 10% body weight at self-selected speed; stance width and box height normalized to individual anthropometrics

What they found

• Combined correlation between available hip flexion and peak lumbar flexion: r = -0.404, p = 0.004
• Female-only correlation between available hip flexion and peak lumbar flexion: r = -0.407, p = 0.035
• Male-only correlation between available hip flexion and peak lumbar flexion: r = -0.341, p = 0.120 (not significant)
• Combined correlation between available hip flexion and peak hip flexion during squat lift: r = 0.774, p < 0.001
• Female correlation between available hip flexion and peak hip flexion: r = 0.844, p < 0.001
• Male correlation between available hip flexion and peak hip flexion: r = 0.614, p = 0.002
• Combined correlation between lumbar extensor strength and peak lumbar flexion: r = -0.419, p = 0.003
• Female-only correlation between lumbar extensor strength and peak lumbar flexion: r = -0.398, p = 0.040
• Male-only correlation between lumbar extensor strength and peak lumbar flexion: r = -0.310, p = 0.161 (not significant)
• Mean peak lumbar flexion: females 11.3 +/- 7.7 degrees, males 6.9 +/- 7.8 degrees (p = 0.049)
• Mean lumbar extensor strength: females 1.6 +/- 0.35 Nm/kg, males 1.9 +/- 0.3 Nm/kg (p = 0.003)
• Mean available hip flexion ROM: females 109.4 +/- 8.9 degrees, males 111.9 +/- 6.5 degrees (p = 0.248, not significant)
• Between-trial ICC: peak lumbar flexion 0.97 (95% CI 0.95-0.98), peak hip flexion 0.99 (95% CI 0.98-0.99), lumbar extensor strength 0.98 (95% CI 0.97-0.99)

Limitations

• Only young healthy adults without low back pain were studied, limiting generalizability to clinical populations or older individuals
• The highly controlled laboratory lift at low loads (10% body weight) may not reflect real-world lifting conditions in work or gym settings
• Movement speed was self-selected and not standardized, which could have influenced spinal flexion patterns
• Cross-sectional design prevents establishing cause-and-effect relationships between the measured variables

Why it matters

For patients

Women who tend to round their lower back when picking things up from the floor may benefit from exercises to improve hip flexibility and lumbar extensor strength.

For clinicians

When assessing female patients who exhibit excessive lumbar flexion during lifting, screening for hip flexion mobility and lumbar extensor strength deficits is warranted, as both were independently associated with greater spinal flexion in this study.

For readers

This study provides controlled kinematic evidence that hip and lumbar impairments contribute differently to lifting mechanics in men and women, supporting sex-specific movement assessment and rehabilitation strategies.