Inter-limb asymmetry in function or performance is an external measure used in sports science and medicine assessing readiness. How asymmetries propagate and affect internal joint loading is, however, poorly understood. This study presents the first investigation of asymmetries in ground reaction forces and knee joint reaction forces during a countermovement jump. Eleven uninjured male participants performed three countermovement jumps. Forces and motion were analysed in the Anybody Modelling System. Total resultant knee joint reaction force (KJRF[R]) was six times higher than the total resultant ground reaction force (GRF[R]) and the magnitude of the interlimb asymmetry was 6.6 times higher for KJRF[R] than GRF[R] indicating a significantly (p < 0.05) higher load in the joint structures of the dominant knee than the non-dominant knee. KJRF[AP] reached 1743 ± 556(877–2910) N in the posterior direction at a knee angle of 105°, representing a load carried solely by the passive structures of the knee together with the articular contact surfaces. Even a small asymmetry determined through external measures such as GRF, therefore had a high impact on joint loading. Previously used thresholds (10 % deficit) for return to sport may therefore be questioned, and lower thresholds may be relevant to prevent injuries in the dominant limb.