Virtually all competitive athletes train by taking a hard workout on one day, feeling muscle soreness on the next, and then recovering at a reduced intensity for as many days as it takes for the soreness to go away. Then they take their next intense workout. Intense workouts cause muscle damage, as evidenced by bleeding into the muscles themselves and disruption of the fibers and Z bands that hold muscle fibers together. Significant increases in muscle strength and size come only with workouts intense enough to break down muscles. When muscles heal they become stronger and larger. The faster you move on your hard days, the faster you can move in competition. However, continuing intense exercise when muscles feel sore causes injuries and an overtraining syndrome that can takes weeks and months for recovery.
Most athletes in endurance and strength sports exercise on their recovery days and do not plan to take days off. However, they work at a markedly reduced intensity to put minimal pressure on their muscles. If you develop pain anywhere that gets worse as you continue exercising, you are supposed to stop for that day.
Active recoveries on easy days at low intensity make muscles tougher and more fibrous so the athlete's muscles can withstand harder hard days. Almost all top runners, cyclists and weight lifters do huge volumes or work, and most of it is on their less intense recovery days. The stresses of intense workouts are extreme; the recoveries take a tremendous amount of time and are done at low pressure on the muscles. Top endurance runners run more than 100 miles/week, cyclists do more than 300 miles per week and weight lifters spend hours each day in the gym.
Research data comparing active and passive recovery are scant. I am amazed at how few quality studies are available to answer this question. New training methods are developed by athletes and coaches. Then when these athletes win competitions, scientists do studies to show why the new training methods are more effective. A recent report from The University of Western Australia shows that runners recover faster by taking a relaxed swimming workout 10 hours after high intensity interval running, rather than just resting (International Journal of Sports Medicine, January 2010). However, in another study, runners recovered strength and power faster aftr a marathon by resting for five days compared to those who ran slowly (Journal of Applied Physiology, December 1984).
Active recovery should be of limited intensity that does not interfere with the healing process. German researchers showed a one-hour recovery ride is more effective than a three-hour ride for recovery from 13 days of intense bicycle training. Those who rode for 3 hours on their four recovery days had much lower maximal heart rates and maximal lactic acid blood levels, lower power output and slower 30 minute time trials, showing that they were unable to exert themselves as intensely (Scandinavian Journal of Medicine & Science in Sports, June 2009).
What else can you do to recover faster? Athletes in intense training recover faster by getting off their feet immediately after they finish their hard workouts and not even walking until it's time for their next day's recovery workout. Eating a high-carbohydrate meal within one hour of intense workouts hastens recovery (Journal of Sports Sciences, January 2004). Adding protein to that meal hastens recovery even more (Sports Science Exchange, 87:15, 2002). Adding salt and drinking lots of fluids are also necessary for a faster recovery (Journal of Sports Sciences, January 2004). So within one hour after your intense workouts, eat fruit, vegetables, cereals and grains (for carbohydrates), seafood or corn and beans (for protein), add salt to replace what you have lost, and drink plenty of fluids.
IF YOU EXERCISE ONLY FOR FITNESS: Recent research shows that intense exercise is more effective than casual exercise in preventing cancers, heart attacks and premature death. However, you should not exercise intensely more often than every other day. The hard-easy principle applies to all exercisers, even if your hard days are far less intense than those of competitive athletes. Intense exercise can cause heart attacks in people with blocked arteries, so you should check with your doctor before you increase the intensity of your exercise program.