It is generally accepted that the “intensity” in high-intensity resistance exercise is the key to muscle growth. Intensity is generally described as what percentage of one’s one-rep max (1RM) one trains with on each exercise. All things being equal, the higher the intensity the more effective your workout will be.
University of South Alabama and Baylor University researchers randomly selected trained male subjects to train with high intensity or low intensity while keeping the total volume (sets x reps x weight) the same for both protocols to determine which training method would result in a different outcome when it came to the expression of gene (myosin heavy-chain mRNA) coding for contractile proteins. The high-intensity group did five sets of six reps at 80% of 1RM. The low-intensity group did three sets of 16 reps at 50% of 1RM. Muscle biopsies were performed immediately before training, and 45 minutes, three hours, 24 hours, and 48 hours post-exercise.
Both groups grew significantly during the first six weeks of using the higher-rep/short-rest style of training. After the switch to a more traditional style of strength training using 90% 1RM, only the combi group continued to grow for four more weeks.
In trained subjects, if volume is kept equal, training with high-intensity (80% 1RM) does not lead to more expression of mRNA for contractile proteins than training with low intensity (50% 1RM). Both high intensity and low-intensity training protocols are equally effective.
Compare the training styles of Ronnie Coleman and Kai Greene to see this study in action. Both men built a tremendous amount of muscle but used different methods to do so. Coleman was all about heavy weight. Greene opts to feel the muscle work. In your own training, the trick is to get the most out of the weight you’re using. When you go light, feel the muscle, squeeze the muscle; make each set count. When the weight is heavy, dig deep and drive through every rep; feel the strain in every muscle fiber. - FLEX