Impact of hip flexion angle on unilateral and bilateral Nordic hamstring exercise torque and high-density electromyography activity

Key points

1. 01Hip flexion to 90 degrees (NHE90) raised knee flexion torque compared to neutral hips (NHE0) over 87% of the bilateral movement and over the entire unilateral movement
2. 02NHE90 showed substantially lower biceps femoris long head and semitendinosus EMG activity than NHE0 despite higher torque, suggesting greater passive contribution at longer muscle lengths
3. 03In neutral-hip NHE, semitendinosus is more active early in the movement but biceps femoris long head dominates near full knee extension
4. 04Unilateral and bilateral versions produced similar EMG activity and torque when relative loads were matched, supporting unilateral use to address limb asymmetry
5. 05Results are exploratory; adaptations to these exercise variations and their injury-prevention impact remain untested

How it was conducted

Design

Acute within-subject repeated-measures biomechanics study

Participants

13 amateur male football and rugby players, age 23 +/- 3 years, all trained 3+ times per week

Exercises compared

Bilateral NHE with neutral hips (NHE0), bilateral NHE with 90-degree hip flexion (NHE90), and both in unilateral mode

Load

Eccentric 1-repetition maximum individually defined for each exercise at 90-15 degrees knee range of motion

Measurements

Knee flexion torque via load-cell device and high-density EMG of biceps femoris long head and semitendinosus, normalised to MVIC

Analysis

Statistical parametric mapping with paired t-tests across the normalised time series; Cohen's dz for effect sizes

What they found

• In bilateral mode, NHE90 produced significantly higher torque than NHE0 from 0-87% of the movement (dz = 2.74 +/- 0.85, p < 0.001)
• In unilateral mode, NHE90 produced significantly higher torque than NHE0 over the entire movement (dz = 3.22 +/- 0.88, p < 0.001)
• BFlh EMG was higher in NHE0 than NHE90 from 36% to end of bilateral movement (dz = 2.18 +/- 1.07, p < 0.001)
• ST EMG was higher in NHE0 than NHE90 at 45-84% and 90-100% of bilateral movement (dz = 1.53 +/- 1.13, p < 0.001 and dz = 1.37 +/- 1.13)
• In bilateral NHE0, ST activity was higher than BFlh at 2-4% of movement (dz = 1.07 +/- 1.07, p = 0.039) but BFlh was higher than ST at 94-98% (dz = 1.11 +/- 1.07, p = 0.030)
• In bilateral NHE90, ST activity was higher than BFlh at 62-69%, 72-80%, and 83-100% of movement (dz = 1.12, 1.09, 1.25; all p < 0.05)
• No significant difference in EMG activity was found between bilateral and unilateral modes for any muscle in either exercise
• Bilateral NHE0 showed higher torque than unilateral NHE0 only in the mid-range (52-78%, dz = 0.95 +/- 0.91, p = 0.005); no torque difference in NHE90

Limitations

• Very small sample (n = 13) increases risk of false-negative findings and may have inflated effect size estimates
• Surface EMG is prone to cross-talk between adjacent hamstring muscles despite high-density array and ultrasound guidance
• Knee angle was measured with a device potentiometer rather than direct joint measurement, introducing some inaccuracy
• Participants were amateur male athletes; findings may not apply to females, elite athletes, or clinical populations

Why it matters

For patients

Athletes wanting to protect their hamstrings during the return-to-sport phase may benefit from knowing that adding hip flexion to Nordic curls changes which part of the muscle works hardest.

For clinicians

Clinicians can use hip position during Nordic hamstring exercise to shift loading toward longer muscle lengths and higher passive torque (NHE90) or toward greater biceps femoris long head activation near knee extension (NHE0); unilateral variants are viable for addressing limb asymmetry.

For readers

This small biomechanics study provides a rationale for varying hip angle and limb mode in Nordic hamstring training, but clinical outcomes such as injury rates and long-term muscle adaptations are still unknown.