Strength training is basically progressive resistance training with compound exercises (and their appropriate variations) such as squats, deadlifts, single leg exercises, push-ups, chin-ups, inverted rows, overhead presses, and other similar movements.
To build strength you must use heavier weights over time, and appropriate repetition (reps) ranges. For example, doing sets of 20+ reps with the same light weights for an extended period of time is not proper strength training. You must use challenging loads for lower rep ranges (primarily 6-12 reps depending on the exercise) to get stronger and achieve results) and add more weight over time.
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