Muscle Booster takes information from your device's activity manager to track your steps, speed, and whether the device is stationary or moving (walking/running). This means you need to have your phone with you while walking in order for your steps to be recorded correctly.
Also, please check whether you’ve enabled access to physical activity data on your phone. To do this, please go to Settings > Privacy > Permission Manager > Physical Activity > Muscle Booster > Allow.