The influence of hip extensor and lumbar spine extensor strength on lumbar spine loading during a squat lift

Key Points

1. In this study, increased hip and lumbar extensor strength was associated with higher lumbar extensor moments during a squat lift, suggesting stronger individuals utilize these muscles more effectively.
2. Individuals with weaker hip extensors showed increased lumbar paraspinal muscle activation, potentially as a compensatory strategy to manage lifting demands.
3. Contrary to prior assumptions, weaker individuals appeared to modify their movement patterns to reduce lumbar spine loading rather than increasing it.

BACKGROUND & OBJECTIVE

The squat lift movement pattern requires significant engagement from the hip extensors (gluteus maximus and hamstrings) and lumbar extensors (paraspinal muscles), which work synergistically to stabilize and generate force. Weakness in these muscle groups has long been theorized to increase lumbar spine loading, potentially contributing to heightened injury risk and inefficient movement patterns (1-3).

Studies suggest that individuals with weaker hip extensors may experience increased lumbar spine moments during lifting, placing a greater demand on the spinal musculature (4-5). However, the exact relationship between hip and lumbar extensor strength and lumbar spine demand has not been fully quantified. Understanding these relationships is critical for refining rehabilitation protocols, optimizing strength training programs, and mitigating injury risks associated with lifting mechanics.

This study aimed to examine the influence of hip extensor and lumbar spine extensor strength on lumbar spine loading during a squat lift.

Lifting is a fundamental movement in both daily activities and occupational settings, often requiring individuals to handle substantial loads that place mechanical stress on the lumbar spine.

Strengthening the hip and lumbar extensors can enhance force generation during lifting tasks while reducing reliance on compensatory movement patterns that may contribute to injury.

METHODS

Participants: 27 healthy females, aged 18-40 years, participated in the study. Inclusion criteria required participants to be free from lower back or lower extremity pain for at least 12 months. Exclusion criteria included pregnancy, history of spinal or lower extremity surgery, and non-English proficiency.

Instrumentation:

• Hip and lumbar extensor strength was measured using a motor-driven dynamometer.

• Kinematic data was collected via a 16-camera motion-capture system.

• Ground reaction forces were measured using force plates.

• Electromyographic (EMG) activity of lumbar paraspinal muscles was recorded using surface electrodes.

Procedures: Participants performed isometric strength tests for hip and lumbar extensors using a motor-driven dynamometer to determine their maximum torque production. Following strength assessment, participants executed a standardized squat lift while their movements were captured using a motion analysis system. A weighted box equivalent to 40% of their body weight was placed in front of them at a standardized distance. They were instructed to lift the box using a squat-lift technique without additional coaching to ensure natural movement patterns. See Video 1 for an example of this.

VIDEO 1 - SQUAT LIFT TECHNIQUE

Data Analysis:

• Lumbar spine moments were calculated using inverse dynamics equations.

• EMG activity was normalized and analyzed as a percentage of maximal voluntary contraction.

RESULTS

Lumbar Extensor Strength and Lumbar Moments: A significant positive correlation (r = 0.498, p = 0.008) was found between lumbar extensor strength and lumbar extensor moments during the lift. Stronger lumbar extensors generated greater force, likely due to more efficient paraspinal recruitment.

Hip Extensor Strength and Lumbar Moments: Hip extensor strength was positively associated with lumbar extension moments (r = 0.382, p = 0.049). Stronger hip extensors contributed more effectively to force generation, reducing reliance on compensatory mechanisms.

Hip Extensor Strength and Lumbar Paraspinal Activation: A significant negative correlation (ρ = -0.382, p = 0.049) was found between hip extensor strength and lumbar paraspinal muscle activation. Weaker hip extensors showed increased lumbar paraspinal activation, likely as a compensatory stabilization strategy. In contrast, stronger hip extensors distributed forces more efficiently, reducing lumbar stress.

LIMITATIONS

The results of the current study should be interpreted considering several limitations.

• First, the researchers evaluated a small sample of young, healthy, females without a recent history of low back pain. As such, the results cannot be generalized to males, older individuals, or those with low back symptoms.

• Second, the requirement to perform a squat lift may have limited the ability of participants to fully compensate for hip extensor and lumbar extensor weakness. It is possible that allowing for a self-selected lifting strategy (i.e. any variation of a stoop or squat lift) may have resulted in a different outcome.

• Finally, the loads required for lifting were relatively low and higher loads may change the lifting strategy.

CLINICAL IMPLICATIONS

Stronger individuals exhibited greater lumbar extensor moments during lifting rather than reduced spinal loading. This suggests that individuals with higher strength levels actively utilize their hip and lumbar extensors to generate force, while those with weaker extensors appear to modulate their movement strategies to avoid excessive lumbar spine loading. These findings challenge the assumption that weakness in these muscle groups directly increases lumbar stress. Instead, weaker individuals seem to adopt subtle biomechanical adjustments, potentially shifting load to other muscle groups or altering posture to compensate for reduced strength. This aligns with observations in low back pain populations, where movement adaptations emerge to limit lumbar spine stress.

Given the relationship between strength and lumbar loading, progressive resistance training should be a fundamental component of rehabilitation and performance programs. Strengthening the hip and lumbar extensors can enhance force generation during lifting tasks while reducing reliance on compensatory movement patterns that may contribute to injury.

Clinicians should assess movement strategies in individuals with reduced strength, as compensatory mechanisms such as increased reliance on lumbar paraspinal activation can lead to excessive fatigue and heightened injury risk. Identifying these patterns early enables targeted interventions, including strengthening and movement retraining, optimizing load distribution and mitigating lumbar spine stress.

For individuals at risk for low back pain, manual laborers, and athletes who frequently engage in lifting tasks, ensuring adequate strength levels may help reduce injury prevalence and improve long-term musculoskeletal health. Additionally, incorporating realistic lifting scenarios into rehabilitation and strength programs is essential. Training individuals under conditions that mimic their daily activities or occupational demands fosters the development of efficient, strength-based movement strategies.

+STUDY REFERENCE

Patterson C, Lohman E, Asavasopon S, Dudley R, Gharibvand L, Powers C (2022) The influence of hip extensor and lumbar spine extensor strength on lumbar spine loading during a squat lift. J Electromyogr Kinesiol, 62, 102620.

SUPPORTING REFERENCE

1. Puniello, M.S., McGibbon, C.A., Krebs, D.E., 2001. Lifting strategy and stability in strength-impaired elders. Spine (Phila Pa 1976) 26 (7), 731–737.
2. Hwang, S., Kim, Y., Kim, Y., 2009. Lower extremity joint kinetics and lumbar curvature during squat and stoop lifting. BMC Musculoskelet Disord. 10 (5), 1–15.
3. van Dieen, J.H., Hoozemans, M.J.M., Toussaint, H.M., 1999. Stoop or squat: a review of biomechanical studies on lifting technique. Clin. Biomech. (Bristol, Avon) 14 (10), 685–696.
4. Zhu, R., Niu, W.X., Zeng, Z.L., Tong, J.H., Zhen, Z.W., Zhou, S., Cheng, L.M., et al., 2017. The effects of muscle weakness on degenerative spondylolisthesis: A finite element study. Clin. Biomech. (Bristol, Avon) 41, 34-38
5. Hu, B., Ning, X., 2015. The Changes of Trunk Motion Rhythm and Spinal Loading During Trunk Flexion and Extension Motions Caused by Lumbar Muscle Fatigue. Ann. Biomed. Eng. 43 (9), 2112–2119.