Though it's been in use for at least a century, plyometrics has become trendy among athletes and their coaches in recent years. It is touted for everything from increasing speed and strength to reducing injuries and improving sports performance. But the benefits of plyometrics may not be as great as some proponents claim, especially when used without a well-rounded athletic program. Plyometric exercises done inaccurately can also lead to serious injury. Simply put, plyometrics is any exercise in which the muscle is stretched and loaded before it is contracted. Lower extremity plyometrics often consist of jumping, bounding, and hopping exercises in which the force of the athlete's moving body loads his or her muscles. The most common plyometric exercises are depth jumps, which involve jumping off boxes of various heights to load the muscles on landing, then jumping as high as possible. Many coaches train their athletes in sport-specific plyometric exercises. For instance, a volleyball player may learn to jump as high as possible, then crouch down for a "dig" to punch the ball upward, over the net. "Plyometrics does two things: it trains muscles to utilize stored energy more efficiently, which improves flexibility and range of motion. When proper technique and body alignment are emphasized, plyometrics can also improve the power of the muscles and speed of movement," said Cathy Walsh, program manager of Sportsmetrics at the Cincinnati Sportsmedicine Research and Education Foundation. Sportsmetrics, a program that combines weights, plyometrics, and stretching, has been used to train more than 1500 athletes since 1999. Yet Walsh and other athletic trainers caution that when performed by an untrained athlete with inadequate strength and poor technique, plyometrics can also be harmful. "Not everyone really understands how plyometric exercises should be done," said Dennis Kline, CSCS, strength center manager and assistant track coach at the University of Wisconsin-La Crosse. "The problem is that plyometrics is something of a fad right now," said Rick Ritter, PT, OCS, assistant clinical professor of physical therapy, rehabilitation, and sports medicine at the University of California, San Francisco. "It can be very effective, but it can also be dangerous if the athlete doesn't have the ability to perform the exercises. Anytime you start demanding more of an athlete, the risk of injury goes up." Some of the most popular plyometric exercises, such as depth jumps-in which the athlete drops from a box and then jumps as high as he can-are done from a drop height of up to 42 inches, Ritter said. "That puts a lot of load on the joints, and may lead to injury if the athlete doesn't know how to land," he said. "An overuse injury can occur very quickly when doing aggressive plyometric exercises." In his work treating injuries and conditioning athletes, Dan Gehri, ATC, CSCS, of Affinity Health Systems in Oshkosh, WI, had seen coaches test athletes with plyometric exercises he considered unnecessarily stressful. So he decided to measure the effect of a plyometric countermovement training program-involving jumps in which the knees and hips are flexed from a standing position but no depth component is added-against a more taxing depth jump training program in high school athletes. Gehri tested 28 male and female students randomly placed in one of three groups-a control group and two plyometrics training programs. The two plyometric jump training programs differed in the demands they made on the athlete's body. Gehri wanted to find out whether the more stressful jumps-the depth jumps, a type popular with coaches-conferred any benefit in conditioning. The depth jump group's program had them repetitively jump off the first row of bleacher seats and then jump as high as they could. Another group used repetitive countermovement jumps. Both groups did three sets of eight jumps twice a week for 12 weeks. Testing was performed before and after the 12-week program in the squat jump, countermovement jump, and depth jump. Both plyometrics groups demonstrated improvements in mean vertical jump height for the squat jump (114% in the plyometric group versus 107% in the countermovement group), the countermovement jump (108% versus 105%), and the depth jump (111% versus 109%). The differences between the two groups were not statistically significant. No significant improvements were seen in the control group. "Both types of programs had similar positive training effects," Gehri said. "But a lot of strength coaches insist that high school athletes perform depth jumps, and jump off very high boxes. That puts them at real risk for injuries like Achilles tendinitis or patellar tendinitis. If the results from countermovement and depth jumps are the same, why put that extra stress on the athletes?" The study did, however, prove the effectiveness of plyometrics training in increasing power, Gehri said. All the athletes increased their jump height with a plyometric training program, and thus improved their power and conditioning, Gehri concluded. "With plyometrics training, athletes can harness strength more quickly," he said. "But the exercises need to be age-appropriate in order to prevent overuse injuries." To reduce the chance of injury, athletes should strive to land quietly and bend both the knees and the hips deeply. When landing, the knee should not extend past the toe. "Above all, an athlete shouldn't be asked to perform beyond his limits. Fatigue and bad body mechanics are a setup for injury," Gehri said. Ironically, plyometrics can also be used to lessen the chance of injury-when done correctly-by teaching body control and correct body biomechanics. Tim Hewett, PhD, has developed a program of weights, stretching, and plyometrics to reduce sports injury incidence and improve performance, which he calls Dynamic Neuromuscular Analysis Training. A series of studies, including one published in the November/December 1999 issue of the American Journal of Sports Medicine have shown that his program can reduce the risk of ACL injury in female high school athletes. Hewitt agrees that plyometrics also have performance benefits for athletes-but he cautions that these benefits may not be as great as some programs claim. Elite athletes can often increase their strength and power and improve jump height by six inches or more in a plyometric training program of six-weeks' duration, Hewett said. But while some programs promise that high school and college athletes can develop similar skills, he has found they can add only one to two inches in jump height in a six-week training program. They simply don't have the strength or training necessary for greater gains. "There are a lot of misconceptions about plyometrics," he said. "A lot of people think plyometrics always means depth jumps. But simple countermovement jumps can be just as effective, especially in younger athletes, as long as they're done quickly. The idea is that the time between flexion and extension has to be shortened." Plyometric training may be especially important for reducing injury incidence and improving performance in female athletes, he said. Girls do not experience the same increases in mean strength, neuromuscular performance, and power after puberty that boys do, which makes girls less likely to have good joint control when jumping-a fact that may explain the gender gap in ACL injury risk, Hewett said. Hewett has performed a study, yet to be presented, which shows that girls do not experience neuromuscular performance spurts following puberty as boys do. In an analysis of a study that tested a high school group in Oakland involving 35 girls and 35 boys (both pre- and postpubescent), which was part of the Adolescent Growth Study from the University of California system, he found that there was a significant correlation between chronological age and standing long jump and vertical jump in boys (p = 0.71 and 0.53, respectively) but not in girls (p = -0.22 and -0.11). Dynamic neuromuscular training may significantly reduce this gender gap in performance, as well as knee injury incidence in female adolescent athletes, his abstract concludes. Hewett and other athletic trainers emphasize that plyometrics confer the most benefit when used in a complete athletic training program, including strength training. "Plyometrics can help an athlete run faster, jump higher, and throw better, but only when used with a well-rounded athletic training program," Ritter said. Although there are no studies to support this position, plyometrics should make up only a small part of the training program, he said, to enhance performance and decrease injury. The University of Wisconsin's Kline has found that when his athletes increase their muscle mass by several pounds with weight training, they also increase their power and jump distance-and the ability to complete plyometric exercises successfully. Plyometrics may also be more valuable in some sports than in others. "Plyometrics carries the greatest benefit in anaerobic sports where power is part of the equation," said Donald A. Chu, PhD, PT, ATC, CSCS, director of athletic training and rehabilitation at Stanford University. Chu is the developer of the Plyoball, a type of medicine ball frequently used in plyometrics, and the author of Plyometric Exercises with the Medicine Ball (Bittersweet, 1987). Chu consistently uses plyometrics with the U.S. synchronized swim team and Olympic skeleton team he trains. Though it is often used to help athletes who need short bursts of power, plyometrics can also help the long-distance endurance athlete, he said. "It improves endurance cycling in triathlons and can help long-distance runners kick, or surge, at the end of a race," he said. Though it's not often touted as a remedy for fatigue, researchers have also found that plyometrics can delay the onset of exhaustion during exercise-even more than weight training-as seen in a February 2001 study of 25 untrained college women between 18 and 35, published in the Journal of Strength and Conditioning Research. "We found that explosiveness is even more important than strength in preventing fatigue," said lead researcher Erin McLaughlin-Hall, EdD, an assistant professor of physical education and health at California State University at Stanislaus. "And certainly preventing fatigue is an important element in improving performance in athletes." The study subjects were divided into three groups: a plyometric group, a conventional weight-training group, and a control group. Both the plyometric group and the weight-training group met three days a week for 10 weeks. The plyometric group performed a series of box jumps from boxes of 6-, 10-, and 14-inch heights. The weight-training group lifted weights for three sets of six to 10 repetitions. The control group received no training. The athletes' ability to do repeated maximal vertical jumps for 30 seconds with their hands on their hips was tested using a Kistler force plate before and after the 10-week training programs. The force plate was programmed to sample at a rate of 100 Hz and for a 30-second sampling time during which data were collected. For the purpose of the study, onset of fatigue was defined as 80% of the participant's maximum vertical velocity. A participant who began performing the repeated vertical jumps at 100% of his or her vertical velocity would stay in the air for a specific amount of time. When the subject began to tire, the air time would decrease, indicating that the participant was fatiguing and was unable to stay in the air as long as at the beginning of the test. When the participant reached a point at which air time was 80% of the original air time, this point was identified as the onset of fatigue. The results revealed that not only did the plyometric group prolong their original fatigue threshold by 3.85 seconds, but the weight-training group fatigued 0.55 seconds faster after training. The control group prolonged their original fatigue threshold by 0.36 seconds. The researchers concluded that highly specific plyometric training-designed to target the same muscle groups used in an athlete's sport, and the speed at which he or she plays-could increase endurance, speed, and athletic performance as a result of delayed fatigue, McLaughlin-Hall said. Plyometric jump training may also improve bone mass, leg strength, and medial/lateral balance in adolescent girls, according to a study published in the June 2000 issue of Medicine and Science in Sports and Exercise. Substantial data support high-magnitude loading as the most effective type of bone-building activity, according to lead researcher Christine Snow, PhD, professor of exercise and sports science and director of the bone research laboratory at Oregon State University in Corvallis. Athletes whose skeletons are subjected to forces of high intensity, such as gymnasts, display significantly higher bone mass at the hip and spine compared with athletes who participate in activities associated with lower skeletal forces. Twenty-five exercisers trained in weighted vests for 30 to 45 minutes, three times a week for nine months, performing various exercises, including squats, lunges, and calf raises. They also performed plyometric exercises including hopping, jumping, bounding, and box depth jumps. A control group of 28 girls maintained their usual activities. The results showed that though there were no significant differences between the two groups in overall bone mass, only the exercisers improved bone mineral content of the greater trochanter. The exercise group also experienced an increase in knee extensor strength of 7.4% and medial/lateral balance of 28.6%, while the control group showed no such changes. Yet there was no statistical difference between the exercise and control groups in overall body composition, strength, power, or stability. The reason for the lack of difference in those measures may be explained by the fact that there were more athletes in the control group than in the exercise group, Snow said. The researchers concluded that plyometric training conducted over a longer period of time may increase peak bone mass. Because the exercise group experienced an increase in knee extensor strength, the study suggested to the researchers that plyometrics can result in increased leg power and strength, a benefit that should transfer to sports training, Snow said. Source: BioMechanics
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