as fast as they can. They experience incomplete recoveries in hypoxic conditions. The exercise to rest time ratio is less than 1:4, which means for every 30 second all out sprint, there is less than 120 seconds of rest. When comparing RSH and repeated sprints in normoxia (RSN), studies show that RSH improved time to fatigue and power output. RSH and RSN groups were tested before and after a 4-week training period. Both groups initially completed 9–10 all-out sprints before total exhaustion. After the 4 week training period, the RSH group was able to complete 13 all out sprints before exhaustion and the RSN group only completed 9. Possible physiological advantages from RSH include compensatory vasodilation and regeneration of phosphocreatine (PCr). The body's tissues have the ability to sense hypoxia and induce vasodilation. The higher blood flow helps the skeletal muscles maximize oxygen delivery. A greater level of PCr resynthesis augments the muscles power production during the initial stages of high-intensity exercise. RSH is still a relatively new training method and is not fully understood. Artificial altitude Altitude simulation systems have enabled protocols that do not suffer from the tension between better altitude physiology and more intense workouts. Such simulated altitude systems can be utilized closer to competition if necessary. In Finland, a scientist named Heikki Rusko has designed a "high-altitude house." The air inside the house, which is situated at sea level, is at normal pressure but modified to have a low concentration of oxygen, about 15.3% (below the 20.9% at sea level), which is roughly equivalent to the amount of oxygen available at the high altitudes often used for altitude training due to the reduced partial pressure of oxygen at altitude. Athletes live and sleep inside the house, but perform their training outside (at normal oxygen concentrations at 20.9%). Rusko's results show improvements of EPO and red-cell levels. Artificial altitude can also be used for hypoxic exercise, where athletes train in an altitude simulator which mimics the conditions a high altitude environment. Athletes are able to perform high intensity training at lower velocities and thus produce less stress on the musculoskeletal system. This is beneficial to an athlete who had a musculoskeletal injury and is unable to apply large amounts of stress during exercise which would normally be needed to generate high intensity cardiovascular training. Hypoxia exposure for the time of exercise alone is not sufficient to induce