Strain Smart: Ranking Single-Leg ACL Loads

Abstract

Introduction & Purpose

ACL injury rehabilitation progressions typically advance from controlled bilateral to high‑velocity, unilateral, multiplanar tasks, yet load prescription remains predominantly qualitative. Currently, ACL loading is primarily assessed via surrogate measures (e.g., knee joint moments). These approximations can miss ligament-level mechanics. Given that ACL loading is mediated by ligament strain, model-based ACL strain estimates provide a more mechanistic basis for progression (Bates et al., 2020). However, direct comparisons of ACL strain across commonly used single‑leg assessment tasks are lacking. We therefore quantified ACL strain during the single‑leg squat (SLS), single‑leg counter-movement jump (SLCMJ), and single‑leg forward jump (SLJF), and tested the hypothesis that SLJF elicits the highest peak strain, followed by SLCMJ and SLS.

Methods

Eleven active participants (5 male, 6 female participants, 1.73 ± 0.08 m, 71.4 ± 9.9 kg, 28  ± 3 years old) performed SLS, SLCMJ, and SLJF with a marker-based 3D motion capture system. Marker coordinates and ground reaction forces were processed in OpenSim. ACL strains were estimated using musculoskeletal simulations incorporating a multi-body knee model with 14 ligament bundles (including ACL) combined with the COMAK framework (Smith et al., 2016). Peak ACL strain (%) for the complete squatting movement and for landing phase (ground contact) till deepest center of mass position were extracted. Differences across exercises were tested with a repeated-measures ANOVA (α = 0.05) with adjusted post hoc pairwise comparisons. Ethical approval and informed consent were obtained.

Results

Tasks had a significant, large effect on ACL strain. Post hoc comparisons revealed significant differences (p < 0.017) for SLCMJ > SLS (p = 0.003) and SLJF > SLS (p < 0.001), but not for SLJF > SLCMJ (p = 0.035) (Figure 1).

Discussion

SLJF, especially horizontal landings, produces the highest ACL strain, followed by vertical SLCMJ and SLS.

Our rank ordering aligns with studies that contrasted vertical and horizontal single leg jumps using joint work, which show higher demands on the knee joint in horizontal versus vertical jumps (Kotsifaki et al., 2021).

Some limitations should be acknowledged. The musculoskeletal model and COMAK framework were not yet validated for sport specific dynamic movements, only for gait simulations. However, our simulation results are within the same range as in-vivo and cadaver studies (Bates et al., 2020).

Conclusion

Strain-based exercise classification enables more precise dosing of ACL-relevant mechanical load than purely kinematic or uniplanar moment metrics, informing progression and return-to-sport decisions. This study can be taken as start point to establish evidence-based rehabilitation programs based on ACL loading rather than surrogate measures or expert opinions.

References

Bates, et al. (2020). Timing of Strain Response of the ACL and MCL Relative to Impulse Delivery During Simulated Landings Leading up to ACL Failure. J Appl Biomech, 36(3), 148–155. https://doi.org/10.1123/jab.2019-0308

Kotsifaki, et al. (2021). Vertical and Horizontal Hop Performance: Contributions of the Hip, Knee, and Ankle. Sports Health, 13(2), 128–135. https://doi.org/10.1177/1941738120976363

Smith, et al. (2016). The Influence of Component Alignment and Ligament Properties on Tibiofemoral Contact Forces in Total Knee Replacement. J Biomech Eng, 138(2), 021017. https://doi.org/10.1115/1.4032464