The most commonly used AAS in medicine are testosterone and its various esters (but most commonly testosterone undecanoate , testosterone enanthate , testosterone cypionate , and testosterone propionate ),  nandrolone esters (most commonly nandrolone decanoate and nandrolone phenylpropionate ), stanozolol , and metandienone (methandrostenolone).  Others also available and used commonly but to a lesser extent include methyltestosterone , oxandrolone , mesterolone , and oxymetholone , as well as drostanolone propionate , metenolone (methylandrostenolone), and fluoxymesterone .  Dihydrotestosterone (DHT; androstanolone, stanolone) and its esters are also notable, although they are not widely used in medicine.  Boldenone undecylenate and trenbolone acetate are used in veterinary medicine . 
It can also bring stubbornness. "The [older] guys can be set in their ways, thinking the way they've always trained is the right way," says Vincent, "so new stuff takes a bit of adapting." According to Luke Zocchi, who trains the relatively spry Chris Hemsworth, 33, "the younger guys are into more [diverse] training using gymnastics, calisthenics, rings and battle ropes, which give you that athletic look as opposed to looking like a body builder." Netflix's Iron Fist star Finn Jones, 29, agrees. "I didn't want to be this hulk-like slab of muscle. I wanted to be toned and healthy, but still agile and nimble."
A fat Superman would never fly. A pudgy Spiderman can't swing. And an actor who can't get jacked on deadline doesn't have a shot at being a leading man in today's Hollywood. Given the choice between acting chops and physique, producers and directors will often choose the better body. Today studios make bigger bets on fewer movies, aiming for blockbusters that are more expensive and complex than ever to make and whose trailers and posters rely on a ripped leading man. An out-of-shape actor can force a director to recast roles, reshoot scenes, or use CGI effects, often at great expense. Once he is signed on for a role and a production schedule is set, the actor is expected to do whatever he has to to get in the shape required of his character. Fitness budgets are baked into most contracts; studios typically pay for trainers, nutritionists, and even home-delivered meals. Some studios make a point to hire their own trainers so they can control the outcome.