You can build strength in your legs by starting to increase the resistance or weight and usually this would result in a need to reduce the repetitions. To get the body used to strength training 3-4 sets of 8-12 repetitions using a moderate weight is recommended for the first three to four weeks.
Once the body has started to adapt to these exercises, progression towards a more advanced and strenuous programme can be started. This usually involves approximately 5 tough repetitions and these would be repeated 3-5 times. When you are lifting heavier weights you are likely to experience more muscle pain and soreness after your activity known as ‘delayed onset muscle soreness’ or ‘DOMS’. Therefore more recovery time is needed between exercise sessions, so usually twice a week is sufficient when strength training with heaver weights. The national physical activity guidelines also recommend adults should aim to achieve “strengthening activities which work all the major muscle groups two days per week.”
Examples of exercises that you can use this approach with in the gym are: Leg Press and Leg Extension Machine, Hamstring Curl Machine, Squats and Deadlifts.