The right combination of training parameters will spark an unparalleled release of testosterone, growth hormone and insulin-like growth factors. Research shows that, from beginners to experienced lifters and athletes, the anabolic response as a result of strength training plays a major role in increasing strength and muscle mass Hakkinen, West. Serum testosterone levels are increased the most by lifting higher loads that range from 85% to 95% of the one repetition maximum Kraemer.
The cortisol spike as a result of strength training is an indication of the metabolic cost of the exercise. The more demanding your workout, the greater the release of cortisol. Cortisol is a catabolic hormone, but research shows that acute spikes of cortisol can be good for you. Some studies even found a positive correlation between cortisol release and muscle growth. Chronic elevated cortisol levels though, like in an over-training state, cancel out testosterone.
“It’s true that long-term chronic elevations in cortisol are associated with muscle breakdown / immune shutdown / other bad stuff, but emphasis here on LONG-TERM (= high cort for many hours/days/weeks, at levels high enough to affect the “low-affinity” type of cortisol receptor). Short-term elevations in cortisol (a few hrs or less) have very different effects. What you really need to focus on (according to the recent animal literature) is not the peaks, but what your “baseline” level of cortisol is throughout the rest of the day. There’s actually a lot of evidence now that occasional spikes in cortisol are probably good for you. Having a good strong peak of cortisol now and then, especially in response to a “positive” stressor like exercise or excitement, seems to recalibrate your whole stress-hormone system (technically: it downregulates cort-receptor density) so that later, cortisol ends up being lower at other times of day and does not elevate too much in response to other stressors through the day” Redditor and cortisol researcher 99trumpets.