Exercise to reduce tendon pain: A comparison of isometric and isotonic muscle contractions and effects on pain, cortical inhibition and muscle strength

Introduction: Patellar tendinopathy affects the ability to jump and land due to pain and associated corticospinal changes to motor patterning. There is a need for interventions that reduce pain immediately, enabling participation in sport yet do not negatively impact on muscle fatigue, which may affect performance. The purpose of this study was to compare an acute bout of either isometric or isotonic muscle contractions on patellar tendon pain and function and maximal voluntary isometric leg extension. Methods: This was a cross over design study with baseline testing and two intervention arms. The baseline testing consisted of single leg decline squat (score out of ten), transcranial magnetic stimulation measures to assess corticospinal excitation and inhibition (modified stimulus response curve and short intra-cortical inhibition) and maximal compound wave (stimulation of the femoral nerve directly.) Surface EMG was used on the rectus femorismuscle. The intervention sessions consisted of the baseline testing protocol, followed by one of the intervention types then repeated the same measures. Four male athletes with patellar tendinopathy who participated in volleyball three times per week completed all three sessions one week apart with the order of intervention randomized by sealed envelope allocation. The isometric muscle contraction protocol consisted of 5 × 45 s at 70% of their maximal voluntary isometric contraction. The isotonic protocol consisted of 4 × 8 repetitions (3 s concentric phase and 4 s eccentric phase) at 100% of their 8 repetition maximum. The protocols were matched for time under load and both sessions allowed two minutes rest between sets. Data were analysed using SPSS using a univariate time-point analysis. Results: All participants at baseline demonstrated high levels of inhibition ratio (36 ± 0.10%) compared with normal data for the quadriceps muscle (reported 50–70%). At baseline single leg decline squat pain was mean 7.5/10, range 7–8. Both isometric and isotonic muscle contractions reduced patellar tendon pain immediately regardless of the order of intervention (mean 0.5/10 range 0–1, mean 0.75/10, range 1–3/10 respectively). However, isometric muscle contractions resulted in sustained pain relief for up to thirty minutes post intervention, whereas the isotonic intervention demonstrated only immediate pain reduction. Cortical inhibition was reduced following isometric contractions and isotonic contractions regardless of the order of intervention. Maximal voluntary isometric contraction was maintained or improved following isometric muscle contraction intervention, whereas this was reduced following the isotonic muscle contraction protocol. Discussion: Isometric muscle contractions reduce tendon pain immediately and for at least 30 min post intervention and do not cause a reduction is maximal isometric voluntary contraction. The clinical implications are that isometric muscle contractions may be used to reduce pain prior to sport without resulting in muscle fatigue that may impact on performance. Whilst isotonic (heavy slow resistance) has been shown to be effective for tendon rehabilitation, this may not be appropriate immediately prior to activity due to the effects on muscle fatigue. Furthermore, it appears the pain reduction is short lasting (<30 min).