A25 Biomechanical Imbalance

Topic

This athlete's musculoskeletal system exhibits a biomechanical imbalance whenever a chronic asymmetry—whether between two distinct muscle groups, between the two sides of the body, or between two different movement patterns—alters the final distribution of loads within that system. Consequently, this imbalance may manifest as a strength or power asymmetry between the athlete's legs, an imbalance between antagonist muscle groups within a single leg, or the dominance of one joint over another during a specific movement pattern.

This strength or power asymmetry between the legs differs from the bilateral strength deficit—defined by another model in this framework as a neural coordination inhibition affecting both legs simultaneously—because it compares the individual capacity of each leg against the other, rather than comparing the combined output of both legs against individual performance. Similarly, a stiffness asymmetry between the athlete's left and right legs represents another form of this biomechanical imbalance, specifically regarding leg stiffness—a concept also addressed by another model in this framework concerning a collapsed spring-mass system.

This biomechanical imbalance shifts the athlete's final joint load toward the stronger or stiffer side of the body, thereby overloading that side; this mirrors the repetitive overload described in another model, where tissue sustains an accumulated load exceeding its repair capacity. The more pronounced this bilateral imbalance becomes, the greater the resulting overload on the athlete's stronger or stiffer side. The imbalance between an athlete's quadriceps and hamstrings is the most widely studied biomechanical imbalance of all, precisely because of its direct link to the risk of anterior cruciate ligament (ACL) injury—a risk established by another model in this series. The quadriceps generates a forward-pushing force on the tibia that the ACL must resist, whereas the hamstrings counteract this forward force on the tibia. If the ratio of quadriceps strength to hamstring strength is lower, the counterbalancing effect of the hamstrings against the load on the ACL becomes even more insufficient, thereby increasing the risk of the non-contact injury described in another model of this series.

Consequently, this model—which began by addressing asymmetries between muscle groups, body sides, or movement patterns—uses a final symmetry index to quantify any of these specific asymmetries in the athlete. This approach is similar in spirit to the ratio of produced force to expected force used in another model in this series to quantify muscle strength loss. An asymmetry in ground reaction force between consecutive steps indicates a bilateral imbalance during the athlete's run. As the athlete's final symmetry index deviates further from the balanced value, the biomechanical imbalance between the two sides of the body becomes more pronounced, carrying the associated risks of overload and injury addressed by this comprehensive model.

ID:307