Some studies have shown that lifting weights has minimal effect on endorphin production. The intensity is there, but it generally takes longer duration workouts. Longer than studied at least. I've never felt it from lifting (or maybe I did in the past, but now I've found a bigger crack rock so lifting just isnt the same?) but I know plenty of people who say they do.
Either way though, I'm addicted.
Cycling does it for me. Give me my bike and I'm happy as can be. Screw up my schedule where I can't ride for a few days and I'll start to feel antsy and on edge sometimes.
I did two hard 35 mile rides Sunday and Monday. Both with some stupid wind and 20+ mph pace the whole way. It's so hard to explain to someone who has never experienced it, but when you're done with the workout and that rush sticks around for hours or days, even something as simple as a deep breath can be the most relaxing, euphoric thing ever.
Endorphins activate the same receptors in the brain as opiates. I can totally understand how people get hooked on painkillers.
Endorphins: Weight trainers miss out on the "runner's high"
When you cycle, swim, run, or carry out any other form of strenuous endurance exercise, your pituitary gland releases substantial quantities of 'endorphins', chemicals which can block sensations of pain and produce overall feelings of euphoria. Endorphins are known to attach to 'receptors' on the outer surfaces of brain cells, acting like chemical keys which fit into locks. If enough endorphin is inserted into the outer 'keyhole' of a nerve cell, that cell is unlikely to convey pain messages to the conscious part of the brain.
Endorphins were discovered almost by accident in the 1970s when scientists were carrying out research on drug addiction. Investigators had wondered for years why the human brain contained receptors for chemicals produced by the poppy plant, and they eventually discovered why: the brain produces its own set of neurochemicals which are actually far more potent than morphine, opium, and heroin but share the same neural receptors with these drugs.
The naturally produced brain chemicals, called the endorphins and enkephalins, are released in times of stress. They can make a mangled accident victim as serene as a Buddhist monk, and they can also make an athlete feel great after an extremely vigorous workout. The latter effect is sometimes referred to as the 'runner's high', and the post-exercise surge in endorphins helps to explain why many exercisers seem to become addicted to their sport. Their workouts become 'fixes' which mask the pain of everyday living, and even injuries or illnesses can't stop the training process because the athlete is relentlessly searching for endorphin-induced mood elevations.
It's been clear for quite a while that strenuous running, cycling, and swimming can stimulate the release of extra endorphin, but the effects of other activities have been uncertain. Recently, researchers from the Department of Health and Sports Science at the University of Richmond in the United States tried to determine whether weightlifting can also heighten endorphin levels.
In the Richmond research, six resistance-trained athletes completed three sets of eight repetitions of isotonic strength training. All exercises were performed at 80% of maximal effort, and blood levels of endorphins were checked before and after the weightlifting.
The results indicated that blood-endorphin levels were not different after weightlifting than before, although there was considerable variability between athletes. The investigation supports previous research which found no significant increases in endorphins following resistance exercise, but a word of caution is in order. Compared to an hour of intense cycling, 24 repetitions of resistance exercise represents fairly meagre exertion, and the response might have been far different if the weight trainers had lifted vigorously for a longer period of time or had carried out rigorous circuit training. However, since the Richmond team found no link between resistance training and boosts in endorphins, we'll have to keep referring to post-workout pleasure as the runner's high, not the strength-trainer's delight.
'Plasma Beta-Endorphin Immunoreactivity-Response to Resistance Exercise.' Journal of Sports Sciences, vol. 11(6), pp. 499-502, December 1993q