Plyometric training encompasses any movement or exercise activity that involves a rapid eccentric loading quickly followed by a rapid concentric contraction. Generally speaking, plyometric modalities involve some form of jumping, hopping, or skipping. Due to the range of beneficial adaptations that occur with proper plyometric training, fitness professionals should be familiar with the plyometric exercise technique and program design principles to safely and effectively integrate plyometric training into client programs.

Definition of Plyometric Training

Plyometric training is defined as any rebound activity that take advantage of the neuromuscular and physiological properties of the musculotendon unit. This rebound activity uses an eccentric contraction of the muscle that is rapidly followed by a concentric muscle action to increase force production.¹ The goal of plyometric training should be to increase the potential of the muscle to maximize force production in the shortest time possible.

Benefits of Plyometric Training

Plyometric training has benefits that include improving speed, agility, coordination, running economy, strength and peak power.² Studies show that plyo training has a greater effect on vertical jump height and agility compared to sprinting.³

Rate of Force Production

Rate of force production is the time it takes to produce force in the muscle through a ballistic action.⁴ In many sports and activities there is a need for quick force production to overcome an opponent, obstacle, or weight. Therefore, rate of force development (RFD) is an important variable to understand when considering plyometric training effectiveness and programming. Ballistic training, speed, and weightlifting at appropriate velocities can all improve RFD.⁵

Exercises that enhance the rate of force production and magnitude of force without additional energy requirements rely upon an increased myotatic reflex. The Myotatic reflex is sometimes known as the “knee-jerk” reflex, where a muscle contracts as a response to being stretched. 

Neural Adaptations that Affect RFD

When a movement or reflex is initiated, either the motor cortex or the central nervous system signals the motor unit to activate. At this point the motor neuron discharges, causing the muscle to contract.

Motor units have different sensitivities or thresholds that determine how fast an electrical charge should be sent to create a contraction.⁴ High threshold motor units are sensitive to inputs and have the ability to create rapid muscle contractions. High threshold motor units are more dominant in type II muscle fibers.⁴ 

By utilizing plyometric training, the discharge rate from high threshold motor units and the efficiency of signals from the central nervous system and motor cortex increases to allow for rapid muscle activation and contractions.⁴ The result is an increase in the RFD due to explosive training. Strength training combined with explosive movements can also greatly increase RFD in all ages. ⁴ 

Reactive Strength Index

Reactive Strength Index (RSI) is a fairly accurate way of identifying the effectiveness of a plyometric training protocol. RSI is calculated by taking the jump height divided by the time on the ground or, alternatively, flight time divided by ground contact time.⁶

Ground contact time, defined as the time from the start of a jump to take-off, is an important variable to consider because this takes into account the eccentric, amortization, and concentric phases of a jump.⁷ It has been found that RSI is highly related to eccentric RFD, peak power, jump height, and ground reaction forces.⁷ RSI can be an efficient way to give more information related to multiple variables in plyometric training than jump height alone. If jump height goes up then RSI goes up, if ground contact time decreases then RSI increases. 

The stretch-shortening cycle (SSC) is a concept described by different mechanical and neurophysiological models that explains how an eccentric loading or pre-stretch can lead to a concentric muscle action that has enhanced force production compared to the same movement performed without a pre-stretch.¹

The two widely accepted models used to describe how the stretch-shortening cycle occurs are the biomechanical and neurophysiological model.

Biomechanical Model

In the biomechanical model, the muscle and tendon together are considered the musculotendon unit. When the musculotendon unit is rapidly and forcefully stretched, like in the pre-stretch of a counter movement jump, work is being performed through lengthening of the muscle and tendon. The work performed is then absorbed in the form of elastic energy by the stretched muscle and tendon.⁸ This is called “potentiating the muscle.” The concept is similar to pulling a rubber band and then releasing it. There is elastic energy stored as the rubber band pulls back. When the rubber band is released from its stretched position, a greater force production is achieved. 

Elastic energy releases when a concentric or shortening of the muscle happens following a pre-stretch. The magnitude, rate, and duration of a stretch will determine the amount of elastic energy that can be released to increase the force production of a musculo-tendon unit.⁹ In other words, the larger and faster the braking force occurs, the more powerful and explosive the movement will be.

It is important to note that elastic energy from the muscle and tendon decreases the longer it takes to start a contraction. If a pause lasts more than one second, cross bridging in the muscle cell detaches and potential energy decreases dramatically.¹⁰ When performing a plyometric exercise, it is important to rebound quickly to maximize the force that can be produced.

Neurophysiological Model

Muscle spindles are located in the intrafusal fibers of the muscle.¹¹ When there is a rapid stretch on the muscle, a deformation within the muscle spindle occurs.⁹ The muscle spindle sends an electrical signal to the spinal cord that is returned to the muscle to generate a reflexive shortening or concentric action. The greater the rate and magnitude of the stretch, the greater the concentric action.^{8, 10} 

Another factor to consider is the golgi tendon organ (GTO). The GTO is in the musculotendinous unit, which plays a protective role in preventing overstretching by contracting the antagonist muscle. For example, the quad is stretched rapidly and the hamstring contracts to limit further rapid stretching or damage to the quad. In plyometric training, the more the GTO is inhibited or turned off, the more muscle fibers can be utilized to generate force and create stiffness or pre-activation of the agonist muscle to allow for a greater force return.⁸ Plyometric training helps to desensitize the GTO.⁹

There is a pre-activation that occurs in the eccentric or stretching phase of a movement. This occurs because as a muscle stretches rapidly, other muscles are activated to slow the stretch and stiffen a limb for action.¹⁰ Time and angle of movement affect how much force pre-activation of the muscle contributes to the movement.

The stretch shortening cycle contributes to force production due to pre-activation of the leg muscles before a jump, a stretch-reflex that occurs with the nerves, a recoil of elastic energy of the musculotendinous unit and the amount of cross bridges in the eccentric phase in the muscles fiber.¹²

There are 3 stages that explain how the stretch shortening cycle is used in explosive movements. These are the eccentric, amortization, and concentric stages.

Eccentric

An eccentric phase of the stretch shortening cycle requires a pre-stretch, also referred to as a braking or eccentric action of an exercise. The eccentric phase begins when force is generated into the ground to prevent a free-fall downward. During this phase, elastic energy is stored into the muscle and tendon as it is stretched. The more eccentric or braking force generated, the greater the rate of concentric force for the jump.⁷

In addition, at this stage there is a reflex created in response to a rapid stretch on the muscle. This reflex is a stimulated muscle spindle that sends information to the spinal cord. Think of jumping, once lowering into the jump, eccentric forces are created to allow for a controlled fall versus a free fall. The faster and more forceful the drop, the more power is created.^9,10

Amortization

After the eccentric phase there is a delay in the signaling from neurons before a concentric action occurs, known as “amortization.” Amortization is an isometric or static contraction that occurs before a concentric contraction of the muscle can occur. For example, in a countermovement jump, this is when the body is as low as possible right before propulsion.

In this stage, pre-activation of the muscle has been created from the eccentric phase to generate stiffness in the muscle and joint.^8,10 This stage and the eccentric phase causes an increase in concentric phase power output. The longer the delay in this stage, the more energy that will be lost towards force production.⁹ Both leg stiffness and neural feedback enhancements are critical to increasing performance.

Concentric

A concentric action of the muscle is a shortening of the muscle. If there is a concentric action immediately following the eccentric action, then an increase in force production occurs to create propulsion or flight. The increased force production at this stage comes from elastic energy released from the musculotendinous unit (biomechanical model) and the reflex action of the muscle spindles (neurophysiological model).⁹

Plyometric training is used progressively to allow for increased excitation of high threshold motor neurons, increased leg stiffness prior to a jump, a decrease in GTO sensitivity, and enhanced feedback from the central nervous system (CNS) to create enhanced performance.^8,9 When programming plyometric exercises, manipulation of different variables allows for overload and increased performance. 

Exercise selection, frequency of training, volume of training, recovery of training and intensity of training are all important considerations.

It has been shown that there is no difference between volume, intensity or a mix of volume and intensity progressions on countermovement jumps (CMJ), squat jumps, sprinting and agility.³ Therefore, if there is progression in volume and intensity using appropriate variable recommendations, performance can be improved.

Exercise Selection and Selection Progression

Plyometric training should start with bilateral and work to unilateral in lower limb exercises as experience increases.¹³ There is a greater demand on the body from single leg jumps that require an adequate amount of strength and biomechanical efficiency.

When considering horizontal plyometric exercises like bounding, adequate technique in vertical jumps should precede horizontal jumps.¹³

In addition to exercise selection, for plyometric training, personal trainers will need to keep in mind that the surface their client is training on is important. A softer surface, like grass, is preferred over surfaces like concrete, while surfaces like trampolines are not practical for exercise. 

Some exercises are ideal for the sport or goal. For instance, horizontal plyometric exercises are more beneficial to improve sprints.¹⁴ For an increase in vertical jump, a combination of exercises like depth jumps, squat jumps and CMJ seem to be better than one single exercise alone.¹⁵

Length of Training

7-12 weeks of plyometric training is recommended to allow for an effective increase in jump performance, sprint performance, strength, and contact time.^16,17

Frequency of Training

Plyometric training should be performed 1-2 times per week to increase performance. It seems that with moderately trained athletes, using high intensity exercises one session per week for seven weeks is enough to induce changes, with two per week being optimal.¹⁶

Plyometric training volume is based on the number of contacts with the ground. These contacts can be counted from multiplying sets by the total number of reps. For example, when performing 1×6 tuck jumps and 2×5 hurdle jumps, the total ground contact would be 6 + 10 = 16 ground contacts.

Technique and experience should be considered when determining the volume needed in a program. More is not better in plyometric training. There seems to be an optimal amount of volume that allows adaptations and performance to occur.²

Session Volume Ground Contacts

Beginner

An optimal amount of ground contacts for beginners is 60 ground contacts per session; however, to generate greater benefits in sprint performance, higher volume is recommended.¹⁸

Intermediate

For more intermediate athletes, programming 80-120 contacts per week has been shown to be effective for increases in performance.^19,20

Advanced

For more advanced athletes it is recommended that 120 or more ground contacts be made per session.¹⁵ It is important to note that more volume does not seem to be beneficial to enhance performance.²

Extraneous volume may have the same benefits as a lower volume of ground contacts, therefore, it is advised to stay between 120-198 ground contacts per session for advanced athletes.^2,15

Rest Period

Research suggests that 240 seconds in between sets and 30 seconds in between repetitions elicits the most beneficial adaptations.¹⁹ However, for practical purposes, as little as one minute between sets and little rest between repetitions has been found to allow sufficient recovery with 3 sets of 10 using 30% of body weight in a loaded countermovement jump.²¹ Age may influence the recovery time of plyometric sets, with adolescents 8 to 14 needing lower rest times between sets of 30-120 seconds.¹⁸

Available training time, volume, intensity and fitness level should all dictate how long recovery between sets should be.

Recovery

Adequate recovery between plyometric sessions is needed based on the intensity, volume, age, and experience of the athlete. High intensity sessions should have at least 48 to 72 hours of rest between sessions and low intensity sessions need at least 24 hours of rest.²⁰

Intensity

Intensity in plyometrics depends upon the amount of ground contact force, the rate of force development needed to execute the movement, and the torque on the joints during the landing.

Intensity should always be specific to the training goal. For example, a basketball player may need to increase vertical height, but a runner might need to increase muscle efficiency at lower intensities to preserve energy. 

High intensity plyometric drills include depth jumps, single leg jumps, and broad jumps. Low intensity plyometrics exercises consist of skipping, double leg bounces, and countermovement jumps (CMJ).³ 

Example of Intensity Progression

Progressions in training intensity of 9-12% are considered appropriate.²³

To increase a depth jump difficulty, the height can increase from 20 cm to 22 cm (a 10% increase). The distance of a single leg jump can go from 3 feet to 3.3 feet (10% increase). Exercises like A-skips could be progressed into power A-skips where height is the focus.

Use of Weighted Jumps

The purpose of using a weighted jump is to increase power development in the lower limbs, specifically, peak power development or the greatest amount of power that can be achieved. Increasing the intensity of jumps using weights requires proficient technique in jumps unloaded before loaded. 

When considering the mode of exercise to use with weighted jumps, greater peak power has been shown to be generated with a hexagonal barbell or trap bar at arm’s length when compared to a barbell.²⁴ 

With a hexagonal barbell, the weight should be between 10-20% of 1-RM box squat (box height set to where legs are parallel to the floor).²⁴

Loaded jumps do take away from the SSC cycle (stretch-shortening cycle) because of longer ground contact times.¹⁵ Therefore it is highly recommended that there is an intermediate to advanced level of efficiency in unloaded plyometric training to maximize the effectiveness of weighted jumps.

Squat Jump

Difficulty: low

Exercise Instructions:

1. Lower hips to right above the knee line while maintaining an upright torso.
2. Explosively extend ankles, knees and hips until legs are off the ground.
3. On landing, contact the ground with toe to heel landing while keeping torso over the hips and knees bent to create a soft landing.

Butt Kick

Difficulty: low

Exercise Instructions:

1. Begin running forward or in place.
2. Relax the ankle and “flick” the heel of each foot back towards the glutes. The knee should be slightly forward of the hips.
3. Landing is based on running mechanics, but usually consists of a toe or mid-foot landing that is capable of “springing the other foot off the ground.”

Multiplanar Jump with Stabilization

Difficulty: low

Exercise Instructions:

1. Stand upright then initiate the movement by bending at the knees and dropping weight rapidly into a ¼ squat position. Knees and toes face the same direction and knees bend at the same time the hips drop.
2. Rapidly extend hips, knees ankles to triple extension until feet leave the ground. Look at 90 degrees from your starting position and rotate body into a new direction while in midair.
3. Decelerate body on landing with hips, knees and toes facing the same direction and hold in the squat landing position for 1-3 seconds.

Multiplanar Box Jump-Down with Stabilization

Difficulty: medium

Exercise Instructions:

1. Stand on top of a box. 
2. Step one foot off the box and drop without jumping.
3. Toes, knees and hips face the same direction upon landing with knees aligned with hips and slightly bent over the shoe laces (not faced inward). 
4. Hold the landing squat position for 1-3s.

Lunge Jump

Difficulty: medium (no leg switch in air), high (switching legs midair)

Exercise Instructions:

1. Begin with one foot forward, and one foot back. Width between feet should mirror an athletic stance. Rear foot will be far enough back to allow the rear leg knee to drop underneath the hip. Rear foot heel will be off the ground and bottom of toes will be pushed into the ground. Rear foot will have 20% of body weight. Front foot will have 80% of the weight and will have toes facing forward and heel flat on the ground.
2. Bend at both knees and use hands to rapidly extend knees, ankles, and hip into the air. For increased intensity switch legs in mid-air. The same foot that was in the front starting should be in the rear.
3. Absorb the impact of landing by bending the knees as the body touches the ground. A toe to heel landing of the front foot should occur and the same time the “balls of the feet” landing on the rear foot happen.

Tuck Jump

Difficulty: medium

Exercise Instructions:

1. Initiate the movement by bending at the knees and dropping weight into a ¼ squat.
2. Rapidly extend ankles, knees, and hips into triple extension until feet leave the ground. As the body reaches maximal height tuck the knees into the chest.
3. As the body prepares for landing, “unwind” the legs from the chest rapidly and create a “soft landing” by creating a toe to heel contact with the ground and bending the knees to “absorb” impact.

Repeat Box Jumps

Difficulty: high

Exercise Instructions:

1. Initiate movement by bending from the knees on the ground in front of the box and rapidly lowering the body into a ¼ squat.
2. Immediately extend the ankles, knees, and hips into triple extension until propulsion off the ground is created. Create soft landing contact with the box and do not stand up completely.
3. Use a rapid, spring-like contact with the box to quickly bring feet back off the box back to the ground where the athlete started. Repeat the desired repetition box jumps as quickly as possible with no rest.

Power Step-Up

Difficulty: high

Exercise Instructions:

1. Initiate the movement by bouncing or hopping on one foot while simultaneously flexing the contra-lateral hip (opposite side of the body from flexed hip) to 90 degrees. The opposite arm chops up simultaneously with hip flexion.
2. Drive flexed hip down and underneath the hips until both feet bounce off the ground at the same time. Simultaneously both arms extend toward the floor.
3. Forcefully bounce off the ground for height with the opposite leg while rapidly driving the contra-lateral leg into hip flexion at 90 degrees. The opposite arm chops up simultaneously with hip flexion. Repeat the second step.

Skaters

Difficulty: high

Exercise Instructions:

1. Initiate the movement by pushing off the outside leg while simultaneously bending the inside leg to prepare to jump.
2. Swing the outside leg behind the front leg while preventing rotation of the hip and keeping shoulders over the hips.
3. Land softly with the front foot in contact with the ground, toes forward, knees aligned with the toes, and “belly-button” forward. There should be no rotation in the hips.
4. Swing the rear leg behind and forward while simultaneously pushing off the ground. Landing mechanics are the same as above.

Single Leg Power Step Up

Difficulty: high

Exercise Instructions:

1. One foot will be on a box and the other foot will be underneath the hips on the ground and flat.
2. Using the foot that’s on the box (not the rear foot), forcefully and rapidly extend the hips and ankle until standing upright. Hips, knees, and ankles should be extended, and rear foot should be off the ground.
3. Upon landing, absorb the impact by bending the back leg and lower rear foot back to the ground to the starting position.

Depth Jump

Difficulty: high

Exercise Instructions:

1. Start standing on a box between 4 and 20 inches. Reach one foot out in front and drop onto the ground. Do not jump off the box.
2. Bend the knees to absorb the landing on the ground, and spring back up as quickly as possible into a jump with the goal of achieving maximal height.

Ballistic Push Up

Difficulty: high

Exercise Instructions:

1. Initiate the movement by starting into a plank with palms flat on the ground, elbows extended, knees extended, glutes squeezed, and abs flexed.
2. Drop chest to the ground rapidly and then execute a forceful, powerful push-up with the goal of both hands coming off the ground.
3. Allow the upper body to be absorbed into the ground to allow for rapid execution of another ballistic push-up.

Plyometric training encompasses any rebound activity that take advantage of the neuromuscular and physiological properties of the musculotendon unit such a rebounding, jumping, hopping. Plyometric training has benefits that include improving speed, agility, coordination, running economy, strength and peak power.

It involves the stretch-shortening cycle, and includes eccentric, amortization, and concentric phases. Intensity, volume, frequency, and exercise selection are all based around the level of trainee.

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