Hamstrings Stiffness and Landing Biomechanics Linked to Anterior Cruciate Ligament Loading

Context:

Greater hamstrings stiffness is associated with less anterior tibial translation during controlled perturbations. However, it is unclear how hamstrings stiffness influences anterior cruciate ligament (ACL) loading mechanisms during dynamic tasks.

Objective:

To evaluate the influence of hamstrings stiffness on landing biomechanics related to ACL injury.

Design:

Cross-sectional study.

Setting:

Research laboratory.

Patients or Other Participants:

A total of 36 healthy, physically active volunteers (18 men, 18 women; age = 23 ± 3 years, height = 1.8 ± 0.1 m, mass = 73.1 ± 16.6 kg).

Intervention(s):

Hamstrings stiffness was quantified via the damped oscillatory technique. Three-dimensional lower extremity kinematics and kinetics were captured during a double-legged jump-landing task via a 3-dimensional motion-capture system interfaced with a force plate. Landing biomechanics were compared between groups displaying high and low hamstrings stiffness via independent-samples t tests.

Main Outcome Measure(s):

Hamstrings stiffness was normalized to body mass (N/m·kg⁻¹). Peak knee-flexion and -valgus angles, vertical and posterior ground reaction forces, anterior tibial shear force, internal knee-extension and -varus moments, and knee-flexion angles at the instants of each peak kinetic variable were identified during the landing task. Forces were normalized to body weight, whereas moments were normalized to the product of weight and height.

Results:

Internal knee-varus moment was 3.6 times smaller in the high-stiffness group (t₂₂ = 2.221, P = .02). A trend in the data also indicated that peak anterior tibial shear force was 1.1 times smaller in the high-stiffness group (t₂₂ = 1.537, P = .07). The high-stiffness group also demonstrated greater knee flexion at the instants of peak anterior tibial shear force and internal knee-extension and -varus moments (t₂₂ range = 1.729–2.224, P < .05).

Conclusions:

Greater hamstrings stiffness was associated with landing biomechanics consistent with less ACL loading and injury risk. Musculotendinous stiffness is a modifiable characteristic; thus exercises that enhance hamstrings stiffness may be important additions to ACL injury-prevention programs.Key Words: viscoelastic, musculotendinous, valgus, anterior tibial shear force

Key Points

• Individuals with greater hamstrings stiffness displayed more favorable landing biomechanics for anterior cruciate ligament (ACL) loading and injury risk than individuals with less hamstrings stiffness as evidenced by smaller frontal-plane knee moments and a more-flexed knee at the instants of critical biomechanical knee events.
• Greater hamstrings stiffness was associated with smaller anterior tibial shear forces.
• A high level of hamstrings stiffness may limit ACL injury risk by limiting frontal- and sagittal-plane ACL-loading mechanisms.

Anterior cruciate ligament (ACL) injury commonly occurs during landing,¹ and researchers have suggested that a landing biomechanics profile consisting of large ground reaction forces, anterior tibial shear force, knee-valgus angle, and external knee-flexion and -valgus moments increases ACL loading.^2–4 A more-extended knee during landing exacerbates this profile, whereas a more-flexed knee decreases these variables,^4–6 likely limiting ACL loading and injury risk. For example, Blackburn and Padua⁵ demonstrated that increasing knee-flexion angle during landing reduced ground reaction forces. Similarly, Pollard et al⁶ categorized participants into high- and low-flexion groups based on performance of a landing task and reported smaller knee-valgus angles and moments in the high-flexion group.Stiffness quantifies the resistance of the musculotendinous unit to lengthening, and hamstrings stiffness may have important implications for ACL loading and injury risk. Greater hamstrings stiffness is associated with greater function in ACL-deficient individuals.⁷ During controlled perturbations, healthy individuals with greater hamstrings stiffness also display less anterior tibial translation, which is an arthrokinematic motion that directly loads the ACL.⁸ Given that anterior tibial translation results from anterior tibial shear force, greater hamstrings stiffness seemingly would resist anterior tibial shear force during landing more effectively than less hamstrings stiffness. Greater hamstrings stiffness also is correlated with less hamstrings flexibility.⁹ This heightened resistance to knee extension may lead to a more flexed knee during landing, producing more favorable landing biomechanics for ACL loading and injury risk. This notion is supported by Boden et al,¹ who reported that participants with ACL injuries displayed greater hamstrings flexibility than an uninjured cohort, suggesting that “above-average” hamstrings flexibility and, therefore, less hamstrings stiffness may increase ACL injury risk.Musculotendinous stiffness is a modifiable neuromuscular property^10,11 that could be targeted in ACL injury-prevention programs. Whereas greater hamstrings stiffness appears to limit ACL loading during controlled perturbations,⁸ it is unclear how hamstrings stiffness influences biomechanical ACL-loading mechanisms during dynamic tasks in which ACL injury commonly occurs. Therefore, the purpose of our investigation was to evaluate the influence of hamstrings musculotendinous stiffness on lower extremity kinematics and kinetics during landing. We hypothesized that individuals with greater hamstrings stiffness would display greater knee flexion during landing, resulting in smaller peak ground reaction forces, anterior tibial shear forces, internal knee-extension and -varus moments (ie, the internal/muscular responses to external moments), and knee-valgus angles. We also hypothesized that individuals with greater hamstrings stiffness would display greater knee-flexion angles at the instants of peak kinetics.