If you search around, you will find both scientific studies and anecdotal evidence that suggests certain hormones are released during heavy lifting. Testosterone and human growth hormone in particular have been linked to squatting and deadlifting by powerlifters and muscle physiologists alike. I have always just taken their word for it. I suppose testing for free testosterone in the blood is relatively simple procedure but since I personally have never participated in such tests, I've never really put much stock into the science behind this phenomenon.
Ok, deadlifts increase my testosterone. Now what?
You see, there's little I can do to prove this theory and even if I could, how could I measure it? I don't have access to such lab equipment. If I cannot measure it, how is any additional information useful to me?
It's not. I'll just take their word for it - heavy lifitng increases testosterone production.
Personally, I have always been attracted to common sense reasoning. It seems to me, that if I can lift more weight in an exercise (like a deadlift), then I am using more muscle compared to an exercise I can lift less weight in (like a curl). More weight = more muscle. So I get more "bang for my buck" with an exercise that I can lift the most weight.
Common sense analogies have always worked good for me too... Arthur Jones, creator of Nautilus, related compound movements and heavy lifting to throwing a stone in a pond. If you throw a small pebble into a pond you'll get no splash and only slight ripples. If you throw a large rock into the pond you'll get a big splash with larger ripples that radiate further. The same goes for a light exercise versus a heavy one; the heavy exercise radiates further throughout the body. This is known as the "indirect effect".
More importantly, the deadlift is a basic human movement that you use every single day. You pick things up off the floor in real life, so you might as well train yourself to pick up really, really heavy things! I deadlift because having a strong back is a necessity.
Do your deadlifts regardless of hormone production.
An interesting thing happened to me today while I was deadlifting, which is what got me thinking about hormones. After working up to a max single and hitting it for a couple sets, I attempted another rep and failed. I barely got it to knee height and couldn't get it any higher, I had to sit it down. When I stood up, I got a feeling of euphoria. (If you have not experienced exercise induced euphoria, you are missing out!) It felt like I was walking on a cloud. It felt good. I felt good. Even though I had failed the lift, I put out a maximum effort just the same as if I had locked it out.
Similar to the theory of testosterone and HGH being released during exercise, it is widely recognized that endorphins are released through exercise as well. These endorphins are known as the "feel good chemicals" that are prevalent in the brain during exercise, excitement and orgasm. I believe the euphoric feeling I got while deadlifting was a very concentrated endorphin rush. Now we have a hormonal response we can "prove"... kinda.
Okay, so we're not measuring dopamine levels in the brain but nevertheless these "feel good chemicals" is something that you will undoubtedly experience with strenuous exercise. Ever hear of "runners high"? Well, I got high from deadlifting this afternoon and it was more intense than any runners high I've ever experienced!
The point is, there is a certain sensation that comes with heavy lifting. It's undeniable - clinical testing equipment need not apply. Whether the actual cause is testosterone, adrenaline or endorphin; it doesn't matter, you know you're doing something good for your body.
There's a lot theories out there about the benefits of exercise that are either difficult or impossible to prove. Don't waste your time mulling over the "what if's" of these theories because proving the answers will yield nothing to the end result. Just know that it feels good to deadlift, and that deadlifting is good for you! So keep on deadlifting!