OBJECTIVE: How much force is needed to stretch skeletal muscle is still unknown. The aim of this study was to develop a device that mechanically stretches rat muscle to compare the force (N) required to stretch the soleus muscle of young and aged rats and the tibio-tarsal angle joint at neutral and stretched positions. METHODS: Twelve female Wistar rats were divided into two groups: a young group (YG, n=6, 311±11 g) of rats 3 months old and an aged group (AG, n=6, 351±43 g) of rats 15 months old. The left soleus muscle was mechanically held in full dorsal flexion and submitted to mechanical passive stretching: 1 bout of 10 repetitions, each repetition lasted 60 seconds with an interval of 45 seconds between repetitions, performed once a day, twice a week, for 1 week. The force required during stretching was measured by a load cell, and the tibio-tarsal angle joint was measured by photometry. RESULTS: The load cell calibration showed excellent reliability, as confirmed by the intraclass correlation coefficient value of 0.93. A decrease in delta force was found in the comparison between YG and AG (0.11±0.03 N vs 0.08±0.02 N, p<0.05, repeated measures ANOVA). There was no difference between the YG and the AG in the tibio-tarsal angle at resting position (87.1±3.8° vs 87.1±3.5°, p=0.35, Kruskal Wallis) and at the end of the stretching protocol (43.9±4.4° vs 42.6±3.4°, p=0.57, Kruskal Wallis). CONCLUSION: The device presented in this study is able to monitor the force necessary to stretch hindlimb rat muscles. Aged rats required less force than young rats to stretch the soleus muscle, and there was no difference regarding the tibio-tarsal angle between the two groups.