Explosive Power

Plyometric exercises–jumping, bounding, hopping, arm pushing, and “catching and throwing” weighted objects such as medicine balls–are movements that involve rapid eccentric (i.e., lengthening) and concentric (i.e., shortening) muscle actions. This type of training therefore enhances explosive muscular performance.

Introduction and Origins of Plyometric Training

According to Russian sports literature, plyometric training had its early roots in the mid-1960s (Radcliffe & Farentinos 1999). In the 1970s other Eastern European countries (e.g., Germany, Bulgaria, Czechoslovakia, Romania) began employing it, calling it “jump training” (Chu 1998). It has been suggested that the dominance of these Eastern European countries in track and field, weightlifting and gymnastics during the 1970s can be partially attributed to this method of training (Chu 1998).

Fred Wilt (1920–1994), a highly respected track and field coach from the United States, first introduced the actual term plyometrics in 1975. The word can be broken into two parts from its Latin roots: plio meaning “more,” and metric meaning “measure,” thus implying measurable increases (Chu 1998).

This review will discuss muscle fiber architecture, the muscle physiology of sports skills, the muscle physiology of plyometrics, research findings on plyometrics, safety precautions for plyometric training, and plyometrics program design. A variety of plyometric exercises are illustrated throughout the article.

Muscle Fiber Architecture: The Microanatomy of Plyometrics

An examination of muscle fiber architecture is necessary before discussing the physiology of sports skills and plyometrics. Muscle fibers are composed of extrafusal and intrafusal components.

The extrafusal components of muscle fibers represent their contractile mechanism. This contractile machinery of muscle is best described at the sarcomere level with the “sliding filament theory” of muscle contraction (Wilmore, Costill & Kenney 2008) (see Figure 1). This theory explains the intricate interaction of the actin and myosin muscle proteins, collectively referred to as myofilaments. With shortening, or concentric, muscle actions, the actin proteins are shifted across the myosin proteins (by small projections on the myosin, referred to as cross-bridges) so that the sarcomere end points (known as Z lines or Z disks) begin to move toward each other. With a lengthening, or eccentric, muscle action, the sarcomere’s Z lines move away from one another as the actin proteins slide across the myosin proteins in the opposite direction.

The intrafusal components of muscle fibers consist of specialized neural stretch receptors referred to as muscle spindles. The muscle spindles are located within the belly of a muscle and are parallel to the extrafusal myofilaments (see Figure 2). As a muscle is stretched, the muscle spindles send a message (through the afferent nerve fiber) to the spinal cord. Then the alpha motor neuron in the spinal cord sends a response (through the efferent nerve fiber) to the extrafusal muscle fibers, directing them to contract, to inhibit the stretch or to govern how much farther the muscle can be stretched. In essence, the muscle spindles are the stretch control regulators within muscle that communicate information directly to the spinal cord about how much stretch and speed of stretch are occurring within the muscle.

The Muscle Physiology of Sports Skills

A brief overview of the physiology of sports skills will help with understanding the physiology of plyometrics. Eccentric muscle actions commonly precede concentric muscle actions in sports movements.

An examination of a basketball player preparing for a lay-up serves as a good illustration. As the player takes the final step toward the basket, the front leg slows the horizontal inertia of the body (absorbing energy with flexion at the hip, knee and ankle) while upholding the body weight. This puts an eccentric load on several muscles of the supporting leg (i.e., gastrocnemius, soleus and quadriceps). In just a few hundredths of a second, a neural message is sent from the eccentrically loaded muscles to the spinal cord, which reflexively sends a signal to the extrafusal muscle fibers (of the supporting leg, under eccentric load) to initiate a powerful concentric muscle contraction (see the reflex loop in Figure 2). This reflex messaging system, known as the stretch reflex (or myotatic reflex), helps the basketball player lift off the ground to complete the lay-up.

Chu (1998) explains that the muscles are responding like a spring. As the front leg is planted on the floor, the muscles coil and absorb energy (like a spring being compressed). The energy is then released (like a spring uncoiling) as the player pushes off the floor. This phenomenon was originally referred to as a stretch-shortening cycle by researchers in Sweden, Italy and the Soviet Union (Chu 1998).

The Muscle Physiology of Plyometrics

Plyometric exercises incorporate this stretch-shortening cycle. This type of training attempts to capture the stored energy during the eccentric loading phase of a movement and very quickly utilizes this energy required during the concentric phase of the movement. The phase in movement when the muscles are rapidly stretched (under some type of load, as when supporting the body and/or slowing down a movement) is referred to as the amortization phase (Chu 1998). Thus, the amortization phase is when the target muscles are absorbing energy. Böhm and colleagues (2006) have shown that the amortization phase provides 32% of the total energy required in the push-off, or concentric, phase of the movement. Radcliffe and Farentinos (1999) add that if the amortization phase lasts too long, the energy being absorbed is lost, as the muscle dissipates it as heat.

Böhm et al. (2006) have shown that faster eccentric loading phases (and thus shorter amortization phases) result in stronger concentric muscle contractions. Mariani, Maton & Bouisset (1980) have demonstrated that there are two major ways to increase force production in muscle: (1) increase the speed at which motor units are recruited and (2) increase the number of motor units that are activated during a given contraction—essentially what plyometric training accomplishes.

Research Findings on Plyometrics

Chu (1998) notes that athletic performance has its own developmental time course and that a 6-week (sometimes up to 6-month) training program may not always yield statistically significant results, even if the program produces meaningful real-life changes for athletes. An athlete’s skill level, age, history of injury, motivation and fitness level factor into the results attained. Realizing these limitations and challenges, there is still very convincing research that plyometric training can improve upper-body and lower-body explosiveness, agility, strength and flexibility in girls, boys, women and men.

In a 6-week study, Faigenbaum et al. (2007) compared the effects of plyometric training and resistance exercise (13 boys) with the effects of static stretching and resistance training (14 boys). The boys (aged 12–15) were volunteers who were active participants in baseball and/or American football. Power, acceleration, speed and agility were tested using the vertical jump, long jump, seated medicine ball toss, 10-yard sprint and pro agility shuttle run. Lower-back and hamstring flexibility were also assessed. Subjects trained on Tuesdays and Thursdays for 6 weeks. All subjects completed the same resistance training program, but one group did additional plyometric training (25 minutes), while the other group did additional static stretching (25 minutes). All boys attended 100% of the training sessions, and no injuries occurred during the 6 weeks. Results indicated that the plyometrics group showed significantly greater improvements in vertical jump, long jump, medicine ball toss, pro agility shuttle run and flexibility, whereas the stretching group improved significantly only in medicine ball toss and flexibility.

Fatouros et al. (2000) compared the effects of plyometric training, weight training and combination training (plyometrics followed by weight training) on vertical jump performance and leg strength. Forty-one men were randomly assigned to one of four groups: plyometric training (n = 11), weight training (n = 10), plyometrics plus weight training (n = 10) and a control group (n = 10). Vertical jump, power and maximal leg strength were measured before and after 12 weeks of training. Subjects in each training group exercised 3 days per week, whereas control subjects did not participate in any training activity. Results showed that all training groups improved significantly in all tested variables. However, the improvements produced by combination training (plyometrics followed by weight training) were significantly greater than those produced by either type of training alone. This study provides support for combining plyometric drills with weight training to improve measures of explosive power. Interestingly, despite the fact that the combination group performed plyometric and resistance training exercises on the same day, performance did not suffer.

Rubley et al. (2011) examined the effect of plyometric training on vertical jump and kicking distance in teenage female soccer players (average age = 13.5). The training lasted 14 weeks. A control group of six athletes did soccer training only (three soccer practices plus two to three games per week), while the experimental group (10 subjects) did the same amount of soccer training plus a once-weekly plyometric training session of jumps, hops, skips footwork and sprint drills. After 14 weeks, kicking distance and vertical jump were significantly greater in the experimental group. The authors concluded that coaches should consider incorporating plyometric exercises into training programs for adolescent female soccer players, as they are likely to see improvements in lower-body power, vertical jump and kicking distance as a result.

Miller et al. (2006) examined the effects of 6 weeks of plyometric training on agility. Twenty-eight subjects were divided into two groups: an experimental group completed a 6-week plyometric training program, while a control group did not perform any plyometric training techniques. All subjects continued with their normal physical activities of daily living. Subjects participated in a T-test agility test, the Illinois Agility Test and a force plate test for ground reaction times both before and after the 6 weeks. On the agility tests, the plyometric training group had quicker postintervention times than the control group. The results show that plyometric training can be an effective way to improve some agility parameters.

In a study that compared the effects of two push-up training programs, Vossen et al. (2000) measured the effects of dynamic (i.e., normal descending/ascending) push-ups versus plyometric push-ups on two measures: (1) two-handed medicine ball put (a push from the chest) from a seated position and (2) maximum two-handed chest press in a seated position. Thirty-five healthy females (aged 17) completed 18 training sessions over a 6-week period, with training time and repetitions matched for the dynamic push-up (n = 17) and plyometric push-up (n = 18) groups. Dynamic push-ups were completed from the knees, using a 2-second-up, 2-second-down cadence. Plyometric push-ups were also completed from the knees, with subjects falling forward onto their hands and then explosively pushing themselves upward to the starting position. The plyometric push-up group improved significantly more than the dynamic push-up group on the medicine ball put and also improved more on the seated chest press.

Safety Precautions for Plyometric Training

Any type of exercise involves some risk. However, the explosive nature of plyometrics suggests that personal trainers need to be very thoughtful and progressive when introducing it and using it with athletes or other clients (Kutz 2003). Modifying the activity appropriately for the individual is paramount. Since plyometric training involves hopping, jumping, bounding and other explosive exercises, it is essential to teach proper landing and rebounding mechanics.

Before initiating any plyometric exercises, the client or athlete should complete some force-absorbing drills (such as land-and-hold drills) to learn landing mechanics and balancing after a jump. According to Allerheiligen & Rogers (1995), spring-loaded floors are ideal landing surfaces for plyometric exercises, as are shock-absorbing mats. These authors state that basketball floors and artificial turfs should be used with caution.

Age, experience, past injuries and health status are prime considerations for personal trainers to keep in mind when incorporating plyometric training (Radcliffe & Farentinos 1999). Swanik et al. (2002) explored the effect of plyometric shoulder training on the shoulder muscle rotators in 24 college female swimmers. The 6-week intervention resulted in significant improvements in peak torque, amortization time and torque decrement. In addition, the authors concluded that significant neuromuscular benefits might be attained if such training were implemented earlier in shoulder rehabilitation programs. This study presents a scientific rationale for including appropriate plyometric training in a shoulder rehabilitation and injury prevention program.

Myer et al. (2006) examined the effects of plyometric versus dynamic stabilization and balance training on power, balance and landing force in female athletes. Nineteen high-school female athletes participated in training three times a week for 7 weeks. The results of this study suggest that both plyometric and balance training are effective at increasing measures of neuromuscular power and control and can potentially reduce the risk for anterior cruciate ligament injuries.

For key safety guidelines from the National Strength and Conditioning Association’s position statement on plyometric exercises, see the sidebar “Safety Precautions.”

Is Plyometrics Right for Your Clients?

Seven reasons to incorporate plyometrics into your own clients’ training are shown in Figure 3. For guidelines to consider when creating a program, see the sidebar “Plyometrics Program Design—From Theory to Application.” Convincing research suggests that adding this unique training regime may bring people closer to reaching their athletic potential. Jump into it!

Tuck Jump. Rapidly drop to quarter-squat position and then explode upward, driving knees to chest. Upon landing, rest 3–4 seconds and repeat.

Weighted Vertical Jump. With engaged core and dumbbells held next to sides, quickly lower to quarter-squat position and then explode upward. Upon landing, rest 3–4 seconds and repeat.

Depth Jump. Drop from elevated height to floor surface, and upon landing, immediately thrust arms upward, while powerfully extending body off floor.

Twist Toss. While holding medicine ball with slightly bent arms, twist torso in direction opposite to intended toss. Quickly stop preliminary
action and powerfully twist in opposite direction and maximally toss medicine ball. Repeat on other side.

Plyometric Push-Up. Lower rapidly to “down” position of push-up, then forcefully explode vertically with arms, shoulders and feet, landing with feet and arms away from midline of body. Reposition body to starting position and repeat.

Alternate Leg Bounding. From walk or run, push off back leg and lift front knee forward and upward as far and as high as possible before landing. Upon landing with front leg, continue with alternate leg bound. Progressively increase number of consecutive alternate leg bounds.