Sport Science
Asymmetry Index
An asymmetry index quantifies the difference in force, power, or performance output between the left and right limbs during a bilateral or unilateral assessment. It is expressed as a percentage, calculated by comparing the weaker or non-dominant side to the stronger or dominant side. In force plate testing, asymmetry indices are commonly derived from countermovement jump, isometric mid-thigh pull, or bilateral squat assessments.
Why coaches track it
Significant inter-limb asymmetry is associated with increased injury risk, particularly for lower extremity injuries in cutting and jumping sports. An athlete who produces substantially more force through one leg during a jump landing, or who loads one side preferentially during a bilateral squat, may be compensating for weakness, pain, or a previous injury. Tracking asymmetry over a training block helps coaches identify whether training is producing balanced development or inadvertently reinforcing existing imbalances.
Interpreting the numbers
Some asymmetry is normal and not cause for concern. Most research suggests that asymmetry indices above 10-15% warrant attention, though the relevant threshold varies by movement, sport, and individual. Directional asymmetry (which side is larger) matters as much as the magnitude — a shift in which side is dominant over time can indicate a developing problem even if the overall asymmetry percentage stays stable. The metric is most useful tracked longitudinally within an individual athlete rather than compared against population norms.
Related terms
Force Plate · CMJ (Countermovement Jump) · Unilateral Training · RSI (Reactive Strength Index)