Summary

The peak ACL strain rate increases during more demanding activities, while the maximum ACL elongation remains unchanged, which suggests the ACL strain rate, and not simply the kinematics, may be an important factor related to an athlete’s susceptibility to injury during high demand activities.

Abstract

Purpose

ACL stress and failure load depend upon the strain rate. No studies have reported the ACL strain rate during different activities, which may be important for understanding ACL injury risk. The purpose of this study was to compare the peak rate of ACL elongation (i.e. the “strain rate”) during the initial impact when ACL injury is believed to occur (within 40ms after initial contact) during walking, jogging, fast running and a drop jump using dynamic biplane radiography. Based on the impact forces during each activity, it was hypothesized that the peak elongation rate would be greater during fast running and drop jump than during jogging and walking.

Methods

Data from ten young healthy adults (5 male/5 female, 27±4 years) and 19 healthy collegiate athletes (11 male/ 8 female, 20±1 years) were pooled from two previous studies. Participant’s knees were imaged during walking (1.4m/s), jogging (2.6m/s), fast running (5.0m/s) and double-legged drop jump (60cm platform). Synchronized biplane radiographs were collected at 100 images/second during walking and 150 images/second for the other activities. Subject-specific MRI-based bone models of the femur and tibia were co-registered to CT-based bone models, providing a 3D transformation between the MRI and CT models. The ACL insertion points, digitized from the MRI scans, were transformed to the CT bone using the MRI to CT transformation. The ACL length was calculated as the distance between the femoral and tibial insertion points. A validated tracking technique was used to match digitally reconstructed radiographs, created from the subject-specific CT-based bones, to the biplane images with submillimeter accuracy. ACL elongation was calculated each frame of tracked motion as the ratio of dynamic ACL length divided by resting ACL length during MRI. The peak rate of ACL elongation within the first 40ms after contact, and maximum elongation during stance, were calculated and compared among activities using a Kruskal-Wallis test and a Steel-Dwass post-hoc test, with significance set at p < 0.05.

Results

The peak rate of ACL elongation was 0.07±0.06 (range 0.01 to 0.20) %/ms for walking, 0.14±0.10 (range 0.01 to 0.37) %/ms for jogging, 0.24±0.15 (range 0.04 to 0.69) %/ms for fast running, and 0.15±0.32 (range -0.10 to 1.2) %/ms for drop jump. The peak rate of ACL elongation was greater during fast running compared to walking (p = 0.005). The maximum ACL elongation during stance was 0.3±2.9%, 1.7±3.4%, 1.1±4.0, and 1.8±3.5 during walking, jogging, fast running, and drop jump, respectively, with no differences among activities.

Conclusion

The peak ACL elongation rate increases during more demanding activities while the maximum ACL elongation remains unchanged. This suggests the ACL strain rate, and not simply the kinematics, may be an important factor related to an athlete’s susceptibility to injury during high demand activities. These findings highlight the potential significance of ACL strain rate in injury risk, injury prevention screening, and rehabilitation protocols.