Background: Studies have shown that participation in sports during adolescence has several positive aspects, both physical and mentally. Today there is more focus on competition than before and more adolescents choose to specialize in one sport. High sport specialization can cause a higher risk of injuries. Knees are one of the most common sites for adolescence to experience pain in and girls are more often exposed to this problem, compared to boys.

Aim: The aim of the study was to longitudinally investigate the frequency of reported knee pain in 7^th grade and 9^th grade in a secondary school with a sports profile stratified for sex, difference in frequency of knee pain between 7^th and 9^th grade and whether knee pain affected performance in countermovement jump with arm swing.

Methods: To compare frequency of knee pain between 7^th and 9^th grade, the test subjects (n=42, 17 girls and 25 boys) that answered the questionnaire about knee pain both years were included. The questionnaire had six options for frequency, where those who reported “never” or “rarely” were grouped as “no knee pain”. Those who reported “monthly”, “weekly”, “more than once a week” or “almost daily” were grouped as “knee pain”. Jump height was measured by countermovement jump with arm swing. The tests were performed twice, in May 2014 when the test subjects were in 7^th grade (age: mean (SD) 14±0.2) and in May 2016 in 9^th grade (age: 16±0.2).

Results: In 7^th grade, 74% reported knee pain (girls 82 %, boys 68%). In 9^th grade, 36% reported knee pain (girls 36%, boys 35%). The difference between the frequency of knee pain in 7^th and 9^th grade was significant, with a lower frequency in 9^th grade. In 7^th grade the jump height for those with knee pain was 32.9±7.5 cm, and for those with no knee pain 33.8±5.6 cm. In 9^th grade the jump height for those with knee pain was 33.5±7.2 cm, and for those with no knee pain 36.5±10.1 cm.

Conclusion: The frequency of knee pain decreased significant from 7^th to 9^th grade. There was no significant difference regarding jump height in countermovement jump with arm swing between the group that reported monthly to almost daily knee pain, and the group that reported that they experienced knee pain rarely or never. The result of this study therefore indicates that self-reported knee pain does not affect jumping performance.