Medial tibial stress syndrome (MTSS) is a common overuse leg injury seen in athletes and can be recalcitrant to management. This cross-sectional study aimed to determine if there are differences in the isotonic endurance of the ankle joint plantar flexor muscles in athletes with MTSS compared to athletes without MTSS. The isotonic endurance of the ankle joint plantar flexors was measured in 30 participants diagnosed with MTSS, and 30 reference participants that were matched to MTSS participants on the basis of age (+/-5 years), gender, BMI (+/-5%) and type of sporting activity. The number of heel-rise repetitions of the participants in each group was compared for differences. There were no significant differences between participants with and without MTSS for age (p=0.34), height (p=0.40) or BMI (p=0.27). The mean number of heel-rise repetitions performed by participants in the MTSS group was significantly less than the reference group (mean 23, S.D. 5.6, versus mean 33, S.D. 8.6; p<0.001). These results suggest that athletes with MTSS have endurance deficits of the ankle joint plantar flexor muscles. Rehabilitation of athletes with MTSS should comprise training designed to enhance endurance of the lower limb musculature, including the ankle joint plantar flexors. It is not known whether a lack of endurance of the ankle joint plantar flexor muscles is the cause or effect of MTSS.