OBJECTIVE:To investigate the effectiveness of an isometric squat exercise using a portable belt, on patellar tendon pain and function, in athletes during their competitive season. DESIGN:Case series with no requirements to change any aspect of games or training. The object of this pragmatic study was to investigate this intervention in addition to "usual management." A control or sham intervention was considered unacceptable to teams. SETTING:In-season. PARTICIPANTS:A total of 25 male and female elite and subelite athletes from 5 sports. INTERVENTION:5 × 30-second isometric quadriceps squat exercise using a rigid belt completed over a 4-week period. MAIN OUTCOME MEASURES:(1) single-leg decline squat (SLDS)-a pain provocation test for the patellar tendon (numerical rating score of pain between 0 and 10), (2) VISA-P questionnaire assessing patellar tendon pain and function, and (3) self-reported adherence with completing the exercise over a 4-week period. RESULTS:Baseline SLDS pain was high for these in-season athletes, median 7.5/10 (range 3.5-9) and was significantly reduced over the 4-week intervention (P < 0.001, ES r = 0.580, median change 3.5). VISA-P scores improved after intervention (P < 0.001, ES r = 0.568, mean change 12.2 ± 8.9, percentage mean change 18.8%, where minimum clinical important difference of relative change for VISA-P is 15.4%-27%). Adherence was high; athletes reported completing the exercise 5 times per week. CONCLUSIONS:This pragmatic study suggests that a portable isometric squat reduced pain in-season for athletes with patellar tendinopathy (PT). This form of treatment may be effective, but clinical trials with a control group are needed to confirm the results.