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.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.In-season.A total of 25 male and female elite and subelite athletes from 5 sports.5 × 30-second isometric quadriceps squat exercise using a rigid belt completed over a 4-week period.(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.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.This pragmatic study suggests that a portable isometric squat reduced pain in-season for athletes with PT. This form of treatment may be effective, but clinical trials with a control group are needed to confirm the results.