It is fairly settled in the scientific community, barring the odd few objections here and there, that regular moderate exercise improves health in the long term, relative to a sedentary lifestyle. When it comes to the details of the dose-response curve for exercise, however, the scientists of the field are still somewhere in the midst of a slow and grand debate that has lasted decades and seems likely to last for decades more. Extracting solid conclusions from human epidemiological data is a challenging endeavor at the best of times. The papers noted below are illustrative of a score or more similar efforts published every year, as researchers add ever more analysis to the existing mountain of thought on exercise and health.
Present evidence is leaning in the direction of a big leap in benefits in the transition from no exercise and minimal physical activity. Benefits increase thereafter up to the point of an hour or so a day, and then may or may not decline with further increases. Clearly there is a point at which too much exertion is harmful, but does that occur prior to the level of exercise undertaken by profession athletes? If so, how to account for their longevity compared to the rest of the population? It may be that people who can become professional athletes are just more robust than everyone else to start with, or alternatively it has to do with social status, wealth, and other confounding factors.
Further, even if there ever comes to be solid agreement on how much exercise is best, what about the different forms of exercise? Are repeated short bursts better or worse than extended effort? Can short term effects be separated from long term effects? Is strength training so important as to be worth sacrificing time on aerobic exercise to undertake it? Is cycling better or worse than rowing? Or swimming? Or moving plants around the garden? An endless series of questions might be posed. Few of them will be definitively answered before we find ourselves in an era in which optimizing the effects of exercise is an amusing hobby and little more, because rejuvenation biotechnologies exist. Their effects on health and life span will far exceed anything that might be produced by finding a way to do a little better than the currently recommended level of exercise.
Homo sapiens are evolutionarily adapted to be very physically active throughout life, and thus habitual physical activity (PA) is essential for well-being and longevity. Never the less, middle-aged and older individuals engaging in excessive strenuous endurance exercise appear to be at increased risk for a variety of adverse cardiovascular effects including atrial fibrillation, myocardial fibrosis, and coronary atherosclerosis. An emerging body of evidence indicates U-shaped or reverse J-shaped curves whereby low doses and moderate doses of PA significantly reduce long-term risks for both total mortality and cardiovascular mortality, however, at very high doses of chronic strenuous exercise much of the protection against early mortality and cardiovascular disease is lost.
The optimal dose, or what we term 'Goldilocks Zone,' of PA may be: at least 150 minutes per week of moderate-intensity aerobic exercise or 75 minutes per week of vigorous-intensity aerobic activity, but not more than four to five cumulative hours per week of vigorous (heart-pounding, sweatproducing) exercise, especially for those over 45 years of age. It is also important to take at least one day per week off from vigorous exercise. There appears to be no concerns about an upper threshold for safety for leisure-time low-to-moderate intensity activities such as walking at a comfortable pace, housework, gardening, etc. After every 30 consecutive minutes spent sitting, stand up and move, ideally walking briskly for about five minutes.
In a study of 36 healthy young adults, the researchers discovered that a single 10-minute period of mild exertion can yield considerable cognitive benefits. Using high-resolution functional magnetic resonance imaging, the team examined subjects' brains shortly after exercise sessions and saw better connectivity between the hippocampal dentate gyrus and cortical areas linked to detailed memory processing. "The hippocampus is critical for the creation of new memories; it's one of the first regions of the brain to deteriorate as we get older - and much more severely in Alzheimer's disease. Improving the function of the hippocampus holds much promise for improving memory in everyday settings."
While prior research has centered on the way exercise promotes the generation of new brain cells in memory regions, this new study demonstrates a more immediate impact: strengthened communication between memory-focused parts of the brain. "We don't discount the possibility that new cells are being born, but that's a process that takes a bit longer to unfold. What we observed is that these 10-minute periods of exercise showed results immediately afterward."
A team of researchers studied eight young adult volunteers as they participated in cycling workouts of varying intensity: (a) moderate intensity consisted of 30 minutes of continuous exercise at 50 percent peak effort; (b) high-intensity interval exercise consisted of five four-minute cycling sessions at 75 percent peak effort, each separated by one minute of rest; (c) sprint cycling consisted of four 30-second sessions at maximum effort, each separated by 4.5 minutes of recovery time.
The research team measured the amount of energy the volunteers spent on each workout and compared mitochondrial changes in the participants' thigh muscles before and after each exercise session. The researchers found that fewer minutes of higher-intensity exercise produced similar mitochondrial responses compared to a longer moderate-intensity activity. "A total of only two minutes of sprint interval exercise was sufficient to elicit similar responses as 30 minutes of continuous moderate-intensity aerobic exercise. This suggests that exercise may be prescribed according to individual preferences while still generating similar signals known to confer beneficial metabolic adaptions."