Acromegaly patients, who suffer from natural growth hormone levels of up to 100 times higher than normal, have lower stamina towards physical activity than people with regular levels.  When the patients are treated and their growth hormone levels decrease, their stamina improves.  This knowledge is part of the evidence behind the new belief that athletes who use supplemental HGH to raise their levels far above average could actually decrease their exercise tolerance, and thus hurt their athletic performance.  Further backing was provided in a study done by the Danish Institute of Sports Medicine. They found cyclists of good health and endurance “were unable to complete accustomed cycling tasks after administration of exogenous hGH” and concluded that HGH can inhibit recuperation from exercise.  Participants have also been found to have lower stamina after HGH treatment along with higher rates of fatigue. 
"Every man desires to live long," wrote Jonathan Swift, "but no man would be old." He was right, but the fountain of youth has proved illusory. And while more study is needed, GH does not appear to be either safe or effective for young athletes or healthy older men. But that doesn't mean you have to sit back and let Father Time peck away at you. Instead, use the time-tested combination of diet and exercise. Aim for a moderate protein intake of about .36 grams per pound of body weight; even big men don't need more than 65 grams (about 2 ounces) a day, though athletes and men recovering from illnesses or surgery might do well with about 20% more. Plan a balanced exercise regimen; aim for at least 30 minutes of moderate exercise, such as walking, a day, and be sure to add strength training two to three times a week to build muscle mass and strength. You'll reduce your risk of many chronic illnesses, enhance your vigor and enjoyment of life, and — it's true — slow the tick of the clock.
Of course, it’s the increasing volume of success stories that is driving the demand for HGH through the roof. Justin Hull played college football in Connecticut and tore his ACL in his last year of eligibility. In 2008, he had surgery to repair his right knee and received a doctor’s prescription to inject himself with HGH after the surgery. Afterward, his body healed fast and Justin found the increased energy and endurance that typically accompanies a boost in growth hormone levels. After injecting himself with 2cc of HGH each day over a six-month period and working out twice a day, Justin says he bulked up from 215 to 235 pounds and saw increases in areas he wasn’t specifically seeking improvement in.