Introduction: Exercise in water is one of the special treatments that distinguishes it from other methods. Exercise in the water environment allows people to do physical activity and exercise in a state of no pain and it's easier for everyone, especially those with motor problems as they get older.Thus the aim of this study was to investigate 12 weeks of water training on electromyographic activity of trunk and lower limb muscles of middle-aged women during maximal isometric voluntary contraction. Method: Twenty middle-aged women were randomly divided into two groups of 10 subjects each with water training (mean age (43.30 ± 1.64), height (158.11 ± 3.62) and weight (75.53 ±11.09) and Control (mean age (43.50 ± 1.84), height (160.53 ± 6.46) and weight (71.07 ±11.60) were divided. After completing the informed consent form, tests were performed on five muscle areas (lumbar extensors, Biceps femoris, Gastrocnemius, Rectus femoris, and Tibialis anterior) using the BTS Free EMG 300 biomonitor. The experimental group performed 12 weeks of water training three days a week for 70 minutes each session. Paired t-test was used to compare the variables before and after the intervention and independent t-test was used to compare the difference between the experimental group and the control group. Tests were analyzed at the significant level 0.05 using SPSS 21 software. Results: The results showed that the mean normalized EMG increased in the maximal isometric voluntary contraction of the exercise group after 12 weeks (P <0.05). The water training group showed a significant difference in Lumbar extensors, Gastrocnemius and Rectus femoris muscles compared to the control group (P <0.05). Conclusion: According to the findings of the study, it can be said that exercise in water affects muscle function. But in some muscle areas the effect of exercise was better and in some areas less effective. Compared to previous researches, the results of this study showed that the statistical popu
