Exercise and stimulants

Stimulants, such as caffeine, ephedrine/Ma Huang, amphetamine, methamphetamine, and cocaine, are commonly believed to grant users increased physical strength and endurance, and resistance to pain. These substances, however, may cause serious health problems.

A study of amphetamine showed "significant increases in knee extension strength". This effect is mediated by norepinephrine increasing the calcium uptake and therefore greater force production in the muscle. This mechanism is the same activated by naturally released epinephrine (adrenaline) and norepinephrine during periods of stress.

Some stimulants, such as methamphetamine and cocaine, produce varying degrees of euphoric sensations, mediated by an increase in extracelluar dopamine levels in the ventral striatum. Together with norepinephrine, these stimulants increase pain tolerance, which allows the user to continue using muscles that would ordinarily be sending crippling messages of pain - the muscle may not be stronger, but working them to failure becomes easier. The nervous system effects also cause an increased ability to concentrate, and a focus on continuing a workout, exercise, or feat of strength past usual thresholds of pain becomes far easier as well. In addition, the stimulant effects increase metabolism and motor activity, which makes more energy available to muscles and permit greater endurance and longer workouts. Taken together, these effects can easily cause complications, some life-threatening. It is also important to keep in mind that studies have proven weightlifting shows better results with rests between sets, so regardless of pharmaceutical fatigue suppression this should be observed. 

Health risks
An important risk associated with stimulants is that decreased pain feedback allows users to easily push the muscle past the point of capable regrowth to the point of serious damage, possibly resulting in torn muscles.

More seriously, the euphoric effects of most stimulants can block adequate pain feedback in conditions that place high stress on the whole body - hyperthermia, the condition that causes heatstroke, is a prominent example of this. The increase in metabolism due to the stimulant's effects increases the amount of heat the body produces, possibly aggravating the condition. The euphoric effects of the drug often then mask warning signs of organ failure until serious complications, or even death, occurs. Recent examples of players who have died from complications resulting from hyperthermia include Steve Bechler, a baseball pitcher for the Baltimore Orioles, and Korey Stringer, an American football tackle for the Minnesota Vikings. Both had used ephedrine before their deaths; however, generally these players also had other conditions such as high blood pressure, indicative of a normally high level of CNS functioning, were training in hot weather, and even without stimulant thermogenesis would have had a body temperature in the hyperthermic range, making the role of ephredrine in their deaths unclear. Later, a published pathological review of all reported ephedrine deaths only found one death related to ephedrine use alone. 

Stimulants also place greater strain on the heart and circulatory system. Most increase heart rate and blood pressure to some degree - for example, the combination of ephedrine and caffeine (two components of the ECA stack, a popular polypharmaceutical "cocktail" for weight loss) was found to increase these two variables. The increased exertion by the heart and increased pressure on the blood vessels means that risks of circulatory system damage due to overexertion are increased while using stimulants. In particular, trying to lift or move large amounts of weight (for example, in high-weight and low-repetitition strength training) is a highly risky activity, due to the fact that this particular type of exercise causes momentary but extreme spikes in blood pressure, and can cause potentially life-threatening complications (for example, aneurysm of the walls connected to the aorta). Weightlifters who lift exceptionally heavy weights are at increased risk for aneurysm even without stimulants, and the use of stimulants has been linked to many weightlifting deaths due to complications of this kind. However, a moderate dose of stimulants only effects the heart rate and blood pressure by a few percent at best, and these effects are quickly attenuated through beta receptor downregulation, showing no difference from controls in various studies. The short plasma half-life of most stimulants can be utilized to avoid their effects while weightlifting until a tolerance develops to their effects, or the dose can be gradually increased to avoid sudden changes in sympathetic nervous system output.

regulation
The use of many stimulants during training or competition is highly regulated, and many are banned outright in most sports, though regulations vary. The use of stimulants during training or competition is considered by many experts (including sports physicians) to be very dangerous; however, evidence surrounding these claims remain disputed. Those who make the decision to use stimulants during training activities must be aware of the above considerations to avoid serious injury or death, and most importantly must be aware of the contraindicators for stimulant use such as other drugs (especially cocaine), genetic heart defects, and high blood pressure. If an individual does choose to use a stimulant of any kind during a training or exercise routine, a tendency to avoid working muscles, or the body as a whole, extremely close to failure, especially in very strenuous conditions, may tend to avoid overtaxing the muscular, circulatory, or temperature-regulation systems of the body due to the causes described above. However, this does not completely mitigate the risks involved, and must ultimately consider if they indeed offer any benefit at all aside from weight loss as most sports ban, and test for them.