Low reps with heavyweight tend to increase muscle mass, while high reps with lightweight increase muscle endurance. This doesn’t mean that you have to rely on one method exclusively. On the contrary, alternating between the two may be the best approach for long-term success. Here’s why. Lifting heavy weights builds muscle, but constantly upping the weight exhausts the body. The nervous system must also adjust to the new fibre activation in the muscles. Lifting lighter weights with more reps gives the muscle tissue and nervous system a chance to recover while also building endurance. If you follow the same fitness program over a period of time, you will eventually hit the dreaded “plateau.” When your mind and body have adapted to the routine, it is no longer challenging, and you stop making progress. Changing things gives your body and nervous system the kick in the pants they need to start progressing again. Eventually, you’ll hit a point where you can’t lift any more weight or can’t lift the weight long enough to be effective. This can cause your form to break down, putting you at greater risk for injury. Switching to high rep/low weight for a time will allow you to continue making progress, concentrate on your form, and build up endurance so you can hit those heavy weights again. These workout change-ups should be planned and strategic, such as every other workout, every other week, or a 6-8 week rotation. An unstructured, uneven approach will lead to uneven results. It’s important to remember that many factors go into building muscle mass, apart from the workouts you perform.  Diet, genetics, metabolic rate, hormone levels, body type, and even your muscle fibre composition all contribute.