Jumper's knee paradox--jumping ability is a risk factor for developing jumper's knee: a 5-year prospective study.

BACKGROUND The 'jumper's knee paradox', where symptomatic athletes appear to perform better in a counter movement jump (CMJ) compared to asymptomatic controls in previous case-control studies is not fully understood. AIM The aim was to examine the relationship between jumping ability and change of jumping ability as potential risk factors for developing jumper's knee. METHODS A 5-year prospective cohort study among elite volleyball players, aged 16-18. Jump tests were done on a portable force plate at the time of inclusion and semiannually. Jumper's knee was diagnosed based on a standardised clinical examination. RESULTS All 150 students (68 males and 82 females) were included and 28 developed jumper's knee (22 males and 6 females). At the time of inclusion, male athletes who went on to develop jumper's knee had significantly better results in CMJ (38.0±5.8 cm) compared to asymptomatic males (34.6±5.5 cm, p=0.03), while no difference was detected in standing jump (SJ: jumper's knee: 30.3±7.4 cm, asymptomatic: 28.1±6.1 cm, p=0.23). In a multivariate logistic regression analysis corrected for gender and previous volleyball training, the OR was 2.09 (1.03-4.25) per cm difference in CMJ at the time of inclusion. Our results did not reveal any significant differences in the change in jumping ability between the groups, although both groups improved their jump performance. CONCLUSIONS Volleyball players with a natural ability for jumping high are at an increased risk for developing jumper's knee.