Ischemic preconditioning (IPC) is an experimental technique developed in the mid-1980s to enable body tissues of many types to become more resistant to loss of blood flow (ischemia) and the oxygen transported in blood. Originally investigated as a possible method for protecting the muscle tissue of the heart against loss of oxygen during a heart attack, IPC has interested investigators in the fields of sports medicine and kinesiology as an ergogenic aid; more specifically, a way to improve athletic performance in sports requiring speed and power.
IPC is still regarded as experimental rather than standard practice in athletic training to improve an athlete's anaerobic capacity, speed, and power. It is not used to improve such aspects of athletic performance as flexibility, balance, agility, or reaction time. In theory, IPC increases an athlete's performance during anaerobic exercise (high-intensity, short-duration exercise at maximal levels of exertion) and speeds up the body's recovery from this type of exercise. IPC has also been investigated as an aid to endurance training, which is associated with aerobic exercise.
As of 2017, IPC has been studied primarily in universities with sizable physical education departments, such as in Scotland, Brazil, Germany, Canada, Australia, and Japan, as well as the United States. Most studies published since 2010 have recruited fairly small groups of subjects (usually between 15 and 40 persons), who were either recreationally active adults or university students active in team sports. Most athletes recruited for these studies have been sprinters or cyclists, but some studies have focused on competitive swimmers. IPC has not yet been studied in the general population or in children or older adults; the current focus is on IPC as a way for elite athletes (as distinct from people interested simply in general fitness) to gain an edge in major competitions.
IPC in relation to exercise performance takes the form of measuring the athlete's performance during a specific exercise test, most often pedaling a stationary bicycle. The most common test of this type used in IPC research is the Wingate test, a test devised in the 1970s at the Orde Wingate Institute in Israel. The Wingate test requires the athlete to complete a brief warm-up period and then pedal the bicycle as fast as possible for a period of 30 seconds. For the first three seconds the athlete pedals without any resistance; after the first three seconds, an assistant applies resistance to the flywheel of the bicycle. The resistance is calculated on the basis of the athlete's weight in kilograms multiplied by 0.08. The athlete continues pedaling for the remaining 27 seconds, and the assistant records the number of flywheel revolutions for each 5-second period of the test. The results are used to calculate the athlete's anaerobic capacity. The Wingate test is designed to evaluate sprint cyclists and sprint runners, and these are the types of athletes most often recruited for IPC studies.
Experiments investigating the effects of IPC differ in their specific protocols (study designs); no universal protocol for IPC was used by all researchers in all countries as of 2017. In some experiments, the occlusion of the subject's limb is unilateral, involving only one arm or leg, or bilateral, involving both. In addition, researchers distinguish between localized IPC, in which the cuff is applied to the limb that will be used in the exercise test; or remote, in which the cuff is applied to an upper limb when the legs will be tested. The inflation pressure of the cuff also varies somewhat; whereas many studies of IPC involving occlusion of the femoral artery specify a pressure of 220 millimeters of mercury (mmHg), some researchers use a pressure of 200 mmHg. The populations of test subjects also vary; some researchers recruit subjects from college-age athletes, but others recruit recreationally active adults. Some studies recruit both males and females, whereas others select only male subjects.
IPC originated in the mid-1980s as a possible method for protecting the myocardium, the muscular layer of heart tissue, against the damage caused by loss of blood flow. A landmark experiment performed on dogs in 1986 by Charles Murry and colleagues at Duke University Medical Center stimulated interest in IPC as a possible intervention to produce resistance to ischemia in human heart muscle. Murry's team anesthetized the dogs, opened their chests, and occluded the animals' coronary arteries for four periods of five minutes each followed by reperfusion. The final reper-fusion was followed by a 40-minute period of sustained occlusion of the coronary artery. The infarcts (areas of dead or dying tissue) in the heart muscle of the dogs that had been preconditioned were significantly smaller than the infarcts in the hearts of a control group of dogs. The experimenters did not offer an explanation for the protective effects of IPC, and the molecular pathways underlying IPC are not yet fully understood.
Following Murry's experiment, IPC was studied in the 1990s and early 2000s as a possible way to improve patients' chances of recovery following heart surgery. It was not until the early 2010s that IPC was investigated for its possible benefits to athletes training for competition. As of early 2017, about 60 studies had been published in various medical journals on IPC and athletic performance.
The benefits of IPC are unclear as of 2017. Studies are inconsistent as to whether the technique improves athletic performance to a statistically significant degree; which, if any, specific body processes bring about improved performance; and whether there is a distinct subpopulation of highly trained athletes who do not respond to IPC, for reasons that remain unknown.
Precautions for IPC investigations include approval of the proposed study by the university or medical center's institutional review board and securing written informed consent from the research subjects. Most studies of IPC exclude subjects who are regular smokers, have chronic disease conditions, and/or require regular use of medications.
Preparation for IPC in the context of athletic performance involves both general and test-specific preparation. In general, all well-conducted tests of athletic performance meet the following standards:
Specific preparation of subjects for IPC research typically involves familiarization sessions in which the subjects practice completing the Wingate test a week or so prior to the actual experiment. In most cases, the subjects are asked to fast for 4 hours before the test; to avoid alcohol, caffeine, and vigorous exercise for 24 hours before the test; and to sit quietly in the laboratory environment for 5 minutes while their baseline resting blood pressure is recorded. The temperature, humidity, and barometric pressure in the laboratory are kept as steady as possible.
As IPC in the context of exercise is still experimental, the research subjects do not require aftercare other than resting for a few minutes, dressing, and returning to their ordinary activities.
There are few risks involved in IPC investigations, as the basic technique of occlusion and reperfusion with a blood pressure cuff is noninvasive. Subjects are selected on the basis of being young, fit, and healthy; and no drugs or blood doping are involved.
IPC as an ergogenic aid to exercise performance has been researched only since the early 2010s; its efficacy is still debated, with some researchers maintaining that its benefits are modest at best, and others pointing out that some athletes do not respond to IPC at all. IPC seems to be most useful to elite athletes training for the Olympics or similar high-level competitions, as the difference between winning and losing at that level is often a fraction of a second, and the edge that IPC offers may make that difference. The technique is also appealing to some athletes because it does not involve the use of illegal and potentially dangerous drugs or blood doping.
As of early 2017, 20 clinical trials of ischemic pre-conditioning were registered with the National Institutes of Health; however, only 6 of these were clinical trials of IPC in athletes and athletic activity. Most sports medicine specialists think that more research is required to demonstrate the effectiveness of IPC in improving athletic performance; to identify the sports or activities in which IPC is most beneficial; and to elucidate the biochemical processes that underlie IPC.
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American College of Sports Medicine (ACSM), 401 W. Michigan St., Indianapolis, IN, 46202-3233, (317) 637-9200, Fax: (317) 634-7817, http://www.acsm.org .
University of Texas at Austin, Department of Kinesiology and Health Education, 2109 San Jacinto Blvd., Stop D3700, Austin, TX, United States, 78712-1415, (512) 471-1273, Fax: (512) 471-8914, https://education.utexas.edu/departments/kinesiology-health-education .
Rebecca J. Frey, PhD