It has long been believed that unleashing hormones through exercise helps to build muscle or speed up muscle growth. However, according to a recent study by McMaster University, hormones activated by exercise do not directly affect how much muscle you can pack on.

While testosterone and growth hormone do play a vital part in body development and puberty and have been linked with muscle growth, they were only assumed to play a part in muscle growth because of the increased levels of each of them that are created after a strength training session.

So what matters in muscle growth? It is the sensitivity of your own muscles to signals from your hormones. This sensitivity is a genetic factor, which explains why everyone puts on muscle at different rates.

The aim of McMaster’s study was to “determine whether resistance exercise-induced elevations in endogenous hormones enhance strength and hypertrophy with training.” It spanned ten years and measured post-workout hormonal levels and the resulting muscle gains after weeks and months of training. They concluded that there was no correlation between hormonal levels and the rate of muscle growth.

During the study, nine different hormones were measured, both in the blood and in the muscles. No link was found between any of the individual hormones, combined hormones and the amount of muscle that was gained as a result of the increased levels, post-exercise.

So, what is the factor that allows some people to gain muscle more quickly than others? One explanation involves androgen receptors. They are a type of protein located inside muscle cells that can detect the existence of our hormones like testosterone and react by turning muscle-building genes on or off. The study participants who had the most of these receptors gained the most muscle.

The doctoral student and leader of the study, Robert Morton, said that hormones such as testosterone can be referred to “…as keys, and their receptors as locks. Healthy young men have plenty of keys, so the number of locks matters more.”

Before this study, it was believed that by performing compound exercises such as squats and deadlifts that activated more muscles, a hormone boost would result. This boost was supposed to create more muscle growth. Now it has been determined that it is not the hormone levels, but the number of receptors in the subject that decides how much muscle is grown, performing compound movements is theorized now to turn on more of these receptors rather than benefitting directly from a boost in hormones.