Many aspects of human running can be well approximated using a spring-mass model, in which the legs behave like a spring, storing and releasing mechanical energy during contact with the ground. Leg and joint stiffness models have successfully explained many facets of human gait.
Given that leg and joint stiffness are biomechanical constructs that incorporate information about both the motion and the forces encountered by the body, joint and leg stiffness have also been hypothesized to play a role in the development of running-related overuse injury. Previous research has suggested that a nonlinear, U-shaped curve may exist, where “optimal” stiffness levels are associated with the lowest risk of running-related overuse injury. Too much stiffness is thought to increase risk of injuries via increased musculoskeletal loading, particularly on bony structures, while stiffness levels that are too low are thought to allow excessive joint motion. However, this hypothesized relationship between injury risk and stiffness has not been tested.
The nature of any potential stiffness-injury association has important implications for injury prevention and rehabilitation interventions, as many of these interventions target stiffness.
This study aimed to determine the relationship between running-related injury and leg, knee, and ankle stiffness using data from a prospective cohort study of recreational runners.
The authors recruited a sample of recreational runners for a prospective cohort study with a maximum follow-up of 1 year. Volunteers were eligible to participate if they were between the ages of 18 and 65 years, had run at least 16 km per week for at least 2 years, and had no history of injury in the previous 3 months.
Upon enrollment into the study, all participants completed an in-laboratory overground kinematic and kinetic gait assessment. Gait data were captured at 240 Hz using a 9-camera motion capture system synchronized with three 1200-Hz force plates. Each individual was outfitted with retroreflective markers in a bilateral modified Cleveland Clinic lower-body configuration.
Participants completed 2 sets of gait trials: 1 set at a criterion speed of 4.0 m/s and 1 set at the individual’sself-reported preferred training speed. At both speeds, a minimum of 5 successful trials on both legs was collected. Individuals wore a standardized neutral running shoe during the gait collection protocol.
Joint kinematics and kinetics were calculated using Visual3D software. Leg stiffness was calculated using the equations of McMahon and Cheng. Ankle and knee joint stiffness was calculated using the previously published method of Hamill.
Study participants completed a weekly survey that inquired about training volume, nonrunning physical activity, and any running-related pain or injuries that occurred. Running-related injury was defined as any running-related pain that caused a cancellation or reduction in volume or intensity of at least 3 planned running sessions.
Stiffness variables often differ from leg to leg, and, as a result, each leg may be exposed to a different level of injury risk. To account for these differences, the authors recorded injuries at the level of individual legs. Relationships between stiffness and injury were assessed at the level of individual legs (n = 98) using spline terms in Cox proportional hazards models.
Forty-nine participants contributed a total of 2742 person-weeks and 6379.6 hours of running (mean, 27.98 weeks of training and 65.10 hours of total running per leg) to the final analysis. A total of 29 legs across 23 individuals (29.6% of legs; 46.9% of individuals) sustained an injury during follow-up. Median time to injury on either leg was 27 weeks or 53.27 hours of training. The most common location of injury was the foot, but injuries occurred throughout the lower body.
The authors observed a weak and nonsignificant (P = 0.187-0.712) associations between stiffness measures and injury. Relative injury rates tended to be equal across different levels of ankle stiffness and showed weak U-shaped associations for knee and leg stiffness. The greatest degree of nonlinearity in estimated relative injury rates was found for knee stiffness at preferred running speed (P = 0.187 and 0.227 for weeks and hours of training until injury, respectively), with proportionally greater increases in relative injury rates at knee stiffness levels greater than approximately 10 N.m/deg.
The authors concluded that leg, ankle, and knee stiffness were not significantly associated with relative injury rates.
The findings suggest that leg, knee, and ankle stiffness levels observed most commonly in runners may not be clinically relevant factors in the development of running-related injury. The results also indicate that small to moderate changes in stiffness brought about by gait retraining or modification are not likely to lead to unintentional increases in injury risk.
Davis IV JJ et al. (2021) Leg Stiffness, Joint Stiffness, and Running-Related Injury. Evidence From a Prospective Cohort Study. The Orthopaedic Journal of Sports Medicine, 9(5), 23259671211011213
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