Plyometric Training Notes

Plyometric Training

Introduction

Plyometric training (PT) is a type of strength training that primarily involves various forms of jumping.
It is used to improve impulsive qualities of muscular performance, such as speed-strength and reactive-strength.
PT exercises include countermovement jumps, bounding, drop jumps, and depth jumps.
PT is not limited to the lower body; exercises exist for the upper extremities and trunk.
PT can enhance athletic performance components like jumping, sprinting, and change of direction ability.
A basic understanding of the science and evidence behind PT is important for practitioners.
The chapter will cover:
- Mechanisms of plyometric training.
- Physiological and performance adaptations.
- Evidence-based programing and periodization.

Stretch-Shortening Cycle (SSC)

The SSC involves coupling eccentric and concentric muscle actions.
The eccentric action enhances the subsequent concentric action, increasing force, power, and efficiency (SSC potentiation).
SSC consists of:
1. Eccentric phase: active lengthening/stretch of the musculotendinous unit (MTU).
2. Amortization phase: brief time between eccentric and concentric action, involving isometric action.
3. Concentric phase: concentric muscle action.
Plyometric training is associated with the SSC.

Mechanisms of SSC Potentiation

Mechanisms can be mechanical or neurophysiological.
Mechanical Perspective:
- SSC potentiation is attributed to stored elastic energy.
- The MTU behaves like a damped spring.
- During eccentric action, energy is stored in elastic components (SEC and PEC) and utilized in the concentric action.
- The series elastic component (SEC), mainly the tendon, is the primary contributor.
Neurophysiological Mechanisms:
- Involuntary nervous processes, like the stretch reflex, are involved.
- Prestretch initiates a reflex via muscle spindles.
- Muscle spindles detect rapid increase in muscle length, relaying a neural impulse to the spinal cord via type Ia afferent fibers.
- Type Ia afferent fibers synapse with the alpha motor neuron, resulting in reflexive muscle action.
- Pairing voluntary and involuntary actions leads to supramaximal concentric activation of the agonist muscle.
- The eccentric action increases the active state of the muscle, shortening the electromechanical delay.
- This increases the working range of the muscle, generating greater force and impulse during the concentric phase.
- Prestretch may place the muscle in a more optimal region of the length-tension relationship.
SSC potentiation is likely due to a combination of mechanical and neurophysiological properties.
The relative contributions of each mechanism are unknown and may vary between exercises.
Time Interval (Amortization Phase):
- Concentric action must immediately follow the prestretch to utilize stored elastic energy.
- A long interval limits the contribution of reflex action.
Rate and Magnitude of Pre-stretch:
- A large and rapid stretch results in greater SSC potentiation.
- Altering the prestretch (e.g., velocity of descent, height dropped) can manipulate intensity and performance outcomes.

Adaptations to Plyometric Training

Plyometric training elicits neuromuscular adaptations related to enhanced SSC function and athletic performance.
Quantitative Improvements:
- Some evidence shows whole muscle and individual fiber hypertrophy, mainly in untrained individuals.
- Malisoux et al. (2006a) reported increases in fiber diameter of $11\%$ in type I, $10\%$ in type IIa, and $15\%$ in type IIa/IIx fibers.
Qualitative Changes:
- PT alters contractile properties of individual fibers.
- Increases in peak fiber force of $19–35\%$ in type I, $15–25\%$ in type IIa, and $16–57\%$ in type IIa/IIx fibers have been found.
- Increases in maximal shortening velocity of $18\%$ , $29\%$ , and $22\%$ were observed in type I, IIa, and IIa/IIx, respectively (Malisoux et al., 2006a, Malisoux et al., 2006b).
- These alterations occurred alongside improvements in lower-extremity functional performance (vertical jump, leg press, shuttle run).
Muscle Fiber Type Shift:
- Limited evidence suggests a shift in muscle fiber type.
- Studies by Potteiger et al. (1999) and Kyröläinen et al. (2005) found no fiber type transitions after PT alone.
Neural Adaptations:
- Performance improvements may be due to adaptations in the nervous system.
- Proposed adaptations include increased firing rate, motor unit recruitment, reflex excitability, and improved inter-muscular coordination.
- PT may reduce protective inhibitory reflex action from proprioceptors (e.g., Golgi tendon organ).
Muscular Strength:
- PT can increase strength, attributed to neural and muscular adaptations.
- The individual’s training status affects the magnitude of strength adaptations.
- Strength improvements appear greater in lesser-trained individuals.
- Muscular strength is most affected when PT is combined with resistance training.
- PT strongly influences reactive strength and impulsive ability.
- Vissing et al. (2008) reported similar maximal strength improvements between conventional resistance training and PT, but PT had a stronger influence on impulsive abilities.
Complex Training:
- Pairing high-intensity dynamic resistance training exercises with biomechanically similar PT exercises.
Vertical Jump:
- PT improves vertical jump height.
- Markovic (2007) cited mean improvements of approximately $5\%$ in static and depth jumps and up to $9\%$ in countermovement jumps over training periods of $8.6 ± 2.7$ weeks and $8.6 ± 3.4$ weeks, respectively.
Sprinting and Change of Direction (COD):
- PT enhances performance in sprinting and COD.
- Saez-Saez de Villarreal et al. (2012) found that performing 80 high-intensity jumps two times per week over ten weeks improved sprint performance.
- Asadi et al. (2016) concluded that moderate intensity PT improves COD ability over seven weeks.
Neuromuscular Efficiency:
- PT demonstrates adaptations in neuromuscular efficiency, such as improved running economy.
- Improvements in endurance performance occur following PT independent of aerobic fitness improvements.
- This may be due to improved eccentric-concentric coupling and utilization of stored elastic energy.
Upper-Extremity PT:
- Exercises include ballistic push-up variations, medicine ball throws, and depth push-ups.
- Limited research, but some evidence supports its effectiveness.

Practical Application of Plyometric Training

Integrating PT enhances jumping, sprinting, and change of direction ability.
Effective implementation requires knowledge of programing, progression, variation, and overload.
Programing must consider the athlete’s needs, training history, and the broader training process.
Mode and Specificity of Plyometric Training
- The mode of PT exercise should be selected based on the demands of the sport/position and the athlete’s needs.
- PT can be divided into lower-extremity, upper-extremity, and trunk exercises.
Specificity of training is critical, with the most specific exercises resulting in the greatest transfer.
- Practitioners should consider the magnitude of forces, rates of force development, velocity, acceleration, and temporal characteristics.
- Overly specific training can lead to deleterious stimuli if not specific to the required adaptive response.
- Nagahara et al. (2014) found different relationships between jumping tasks and acceleration across each phase of a 60-meter sprint.
  - The static jump was most strongly related to the early acceleration phase, whereas the ankle jump was most strongly related to maximum velocity sprinting.
- The kinetic and kinematic characteristics of exercise must align with the movements being enhanced.
- For high jump takeoff (≈ $175$ ms ground contact time), a drop jump is more appropriate than a countermovement jump.
  *Instructions and Coaching Cues:
- Instructions influence performance outcomes.
- Talpey et al. (2016) reported that instructing participants to "minimize ground contact time" increased peak force, mean acceleration, and propulsive impulse during the depth jump.
- Proper instructions impact the stimulus and adaptation of the exercise.
Frequency and Recovery
- Training frequency depends on the training phase, competition schedule, and sport practice time.
- Two sessions per week appear sufficient based on meta-analyses.
- Between 48 and 72 hours is suggested for recovery between sessions.
- Recovery time varies based on volume, intensity, and individual athlete factors.
- Tracking volume and intensity of training and sport practice is recommended.
- Intraset and interset recovery intervals are important.
- Work to rest ratios range from 1:5 to 1:10 depending on the exercise.
Volume and Intensity
- Volume quantifies work performed.
- Quantification methods include total distance covered or number of ground contacts.
- Experts suggest 80–100, 100–120, and 120–140 contacts per session for beginners, intermediate, and advanced athletes, respectively.
- Exercise intensity is the primary factor determining volume.
- Intensity is related to the rate of work performed and the mechanical demands.
- Intensity guidelines include the speed of movement, points of contact, amplitude of the movement, and body weight.
- Technology such as force platforms can quantify kinetics (peak force, rate of force development) and impulse.
- Intensity can be manipulated by altering jump height, box height, external load, or speed of movement.
- Changing the exercise is the simplest method of manipulating intensity.
- Caution is needed when increasing intensity to avoid altering movement mechanics.
  *Progression
- Progression and variation are key to effective implementation and to avoid monotony.
- A traditional approach is to progress from general low-intensity to specific high-intensity exercises.
- Progression is important for teaching effective mechanics.
- Turner and Jeffreys (2010) suggest focusing on jumping, landing, and load absorption mechanics.
  *Safety
- PT is considered safe for most individuals, including adolescents, if implemented appropriately.
- Learning proper landing mechanics is essential.
- Some suggest a relative strength level equivalent to a back squat of $1.5 \times$ bodyweight before lower-extremity PT.
  *Periodization of Plyometric Training
- Periodization involves planned distribution and variation of training stimuli.
- Effective periodization integrates all training modalities.
- PT may be best implemented in training phases where overall training volume is low and movement quality is emphasized.
- Low-intensity PT can be integrated into warm-ups.
- Emphasizing strength and maximal strength through heavy resistance training (> $85\%$ 1RM) prior to PT optimizes effectiveness.
- Adaptations to the MTU following heavy resistance should optimize stiffness and force generation.
- Increasing muscle strength and connective tissue should reduce injury likelihood.
Strength training prior to PT may maximize the overall training process. * If impulsive ability is the target, PT should be preceded by strength-focused training.
- Optimizing adaptation to PT can be achieved through a specific sequence of complementary training phases.
- This model is based on phase potentiation.

Conclusions

Plyometric training is a form of explosive resistance training comprised of various jumping and throwing exercises.
As nearly all plyometric training exercises are considered to involve a stretch-shortening cycle, knowledge of the underlying mechanisms of this muscle function will aid the practitioner in effectively implementing plyometric training.
A large body of empirical evidence exists supporting the effectiveness of plyometric training in enhancing a variety of elements of athletic performance.
Practitioners considering adding plyometric exercises into their athlete’s training should not only consider the basic principles of training but also possess an understanding of how this training modality can be integrated into the training process as a whole.