Plyometric Training: Importance for Multi-directional Speed Development and Injury Prevention

Overview

Plyometric is a combination of Greek words that literally means to increase measurement (plio = more; metric = measure), and plyometric training (PT) are methods which involves the stretch shortening cycle (SSC). The SSC is a muscle action that involves the combination a rapid eccentric muscle action (pre-stretch / lengthening) immediately followed by a rapid concentric muscle action (shortening) of the same muscle (Goodwin and Jeffreys, 2016; Potach and Chu, 2016). This mechanism subsequently increases concentric force production, power, and efficiency (compared to isolated concentric only actions) (Turner and Jeffreys, 2010), and the  SSC mechanism is involved in most dynamic movements, such as running, jumping, changing direction, and throwing.

The premise behind PT is to exert rapid and high magnitudes of force over short time intervals. This method was popularised in the “soviet” era known as “shock training” which involved tasks that typically involve some impact (with the ground or object to evoke a greater stretch due to increased kinetic energy from a falling body) such as jumping, bounding and hopping activities (Siff and Verkoshansky, 1993). PT has been shown to be a highly effective method to elicit positive neuromuscular adaptions and subsequently improved performance during a range of dynamic tasks, and is considered an excellent transferable method for improving on-field sporting performance. Additionally, plyometric activities, particular the landing stabilisation component, is also an essential component or reinforcing optimal neuromuscular control and thus prevention of lower limb injuries. In this blog, we will explore the SSC mechanism and the physiological and biomechanical considerations of PT. Additionally, we will discuss and describe the key coaching points for lower-limb plyometric exercises and learn how to design a safe and effective PT programmes.

The Stretch Shortening Cycle Mechanism

The SSC mechanism is mainly composed of two models: 1) Mechanical model; and 2) Neurophysiological model (Potach and Chu, 2016), which is presented in Table 1. In brief, the SSC involves three phases:

1. An eccentric muscle action stretches the agonist (i.e., braking phase) which results in the storage of elastic energy in the musculotendinous / series elastic component (SEC), while the muscle spindle (a proprioceptive organ located in the muscle belly) detects the magnitude (length change) and rate of length change (stretch) to a high load and eccentric muscle action which initiates the stretch reflex. For example, when performing a jump, the countermovement (i.e., lower limb triple flexion), particularly knee and ankle dorsi flexion, targets the agonists (knee extensors and ankle plantar flexors) where they stretched and stimulated.
2. Known as the amortisation phase (potentiation / transition phases), due to the previous eccentric phase, impulses are transmitted from the sensory neuron to the spinal cord and are then transmitted to the motor neuron back to the agonist (part of the stretch reflex) to contract forcefully to prevent any more continued stretching of the agonist. Additionally, there is continued storage of elastic energy in the SEC. The amortisation phase is the duration / transition from phase 1 to 3 (known as electromechanical delay) and this should be rapid (reduced electromechanical delay) to increase the potential force production capabilities. When coaching plyometrics, the objective is to reduce the amortization and encourage a rapid transition from the eccentric to concentric phases to better utilise the SSC (stretch reflex and elastic energy). If this transition is too long, the cross-bridges detach, energy dissipates as heat, and the stretch reflex not fully utilised, with a delay of ~1 second diminishing the benefits of the SSC ~50% (Goodwin and Jeffreys, 2016).
3. Concentric phase is the final phase, also known as the propulsive phase and typically coincides with lower limb triple extension during jumping tasks. The elastic energy is released from the SEC (like a slingshot / spring coil, the greater you draw back, the greater the release) the agonist muscle is stimulated due to the stetch reflex (told to contract more forcefully), and reciprocal inhibition (relaxation) of the antagonist contributes to an increased concentric force production.

To use the jumping example, the SSC results in a greater take-off velocity during the concentric phase, due to the increased propulsive impulse, where lower-limb triple extension occurs, resulting in greater jump height compared to performing a jump without a countermovement. For example, superior performance of athletes performing a countermovement jump (CMJ) (with a pre-stretch) vs a squat jump (SJ) (no pre-stretch) has been observed, with increases of 2–4 cm reported (18–20% to 20–30%) (Turner and Jeffreys, 2010; Goodwin and Jeffreys, 2016).

Plyometrics can be considered to have to met three key criteria (Goodwin and Jeffreys, 2016):

1. They should contain an SSC component
2. They should be subject to a high rate of stretch loading
3. They should have a brief amortisation phase

Thus, plyometrics can be classified as fast and slow SSC activities (Schmidtbleicher, 1992).

Fast SSC activities:

• Short time periods < 0.25 seconds (smaller angular displacements, shorter time for force production thus impulse)
• Typically, repeated jumps, drop jumps are classed as fast SSC.

Slow SSC activities:

• Longer time > 0.25 second (larger angular displacements, greater time for force production thus impulse)
• Typically jumps initiated from static positions such as horizontal broad jump or vertical CMJ are classed as slow SSC.

It should be noted that when intensity increases for an exercise, the amortisation phase can be increased to a point where typically fast SSC activities take on slow SSC amortisation periods (influenced by athlete physical capacity, fatigue, etc.)

Plyometric training exercise selection

Plyometric training is a versatile and creative method to achieve specificity and the transfer of training, particularly the direction of movements, force-vector specificity, muscle actions, and contraction velocities and durations. Consequently, a plethora of different exercises are available to the practitioner to improve upper-limb and lower-limb force-time characteristics and neuromuscular function. Some examples are presented in Figure 1 below, but note this is not an exhaustive list. Plyometric exercises can be a single or repeated efforts, can be performed bilaterally or unilaterally, and can be easily performed in the field or the gym environment, generally involving minimal equipment.

Upper-limb examples include:

• Med-ball throws and catches
• Resistance (barbell)
• Body weight (push ups, supine rows)

Essentially, they involve pushing / pulling / throwing movements.

Lower-limb examples include:

(Bilateral and unilateral – impact plyometrics)

• Jumps in place
• Standing jumps
• Multiple hops and jumps
• Bounds / skips
• Box drills
• Depth jumps

The above exercises can be:

• Vertically orientated (emphasising vertical displacement)
• Horizontally orientated (emphasising horizontal displacement)
• Multidirectional / multiplanar to increase sport specificity in sports which involve multiplanar movements (for example, cutting sports)
• Manipulate angle (Single / repeated efforts)

Introducing plyometric training to the athlete

As most lower-limb plyometrics involve jump-landing activities, there is an inherent risk of injury (knee and ankle) due to the potentially large mechanical loads associated with landing during foot contact (Padua et al., 2018). Thus, technical mastery and displaying safe “landing mechanics” to reduce the relative risk of injury is considered integral when introducing lower-limb PT for the athlete (Turner and Jeffreys, 2010; Herrington and Comfort, 2013; Goodwin and Jeffreys, 2016). A summary of the optimal landing mechanics is presented below in Figure 2 based on Herrington and Comfort (2013):

1. Relatively upright torso that does not lean or sway in any direction on landing (no lateral trunk flexion or rotation – trunk dominance)
2. Arms athletic position – or on pelvis
3. Pelvis should stay level in the frontal (no lateral tilt) and sagittal plane (no excessive anterior tilt)
4. Hip(s) should flex greater than 45° – no hip adduction / internal rotation
5. Strong frontal plane knee alignment (avoid dynamic valgus – ligament dominance)
6. Adequate knee flexion (generally 60° with active touch-down and pre-activation) – avoiding quad dominance/stiff landing 
7. Toe-to-heel landing (reduce impact GRF, increase impulse (time), surface area, stability) – stable (no pronation/supination) – “credit card under heel”
8. Symmetrical landing (leg dominance)

Externally focused verbal cueing can be a highly effective method to reduce impact GRFs and torques while improving neuromuscular control and movement quality (Herrington and Comfort, 2013; Dos’Santos et al., 2019), such as “land like a ninja” / “cushion the landing” / “make minimal noise”. Equally from a performance aspect, reinforcing strong frontal plane lower-limb alignment during the propulsive phase, and encouraging a rapid transition from braking to propulsion to utilise the SSC is integral (Goodwin and Jeffreys, 2016). Researchers have shown that verbal cueing can affect the displacement, contact times, vertical jump height, force and velocity characteristics associated with jumping based plyometrics (Khuu et al., 2015; Sánchez-Sixto et al., 2021). As most dynamic movements are time constrained, athletes should be encouraged to perform PT exercises as quick as possible. Thus, when coaching PT exercises for performance purposes, external cues should be used, such as:

• “Jump as fast and as high/far as possible”
• “When jumping, imagine the surface is hot lava”
• “Hang time”
• “Levitate”
• “Bounce like a trampoline”
• “Imagine you are springy / stiff like a pogo stick”

Consequently, when introducing PT for novice athletes, the following sequence presented in Figure 3 should be adopted (Suchomel et al., 2019; Goodwin and Jeffreys. 2016). Briefly, squat patterns are reinforced before progressing to landing patterns, then progressing to static and slow SSC jumps (possibly on to a box to reduce mechanical loading), before increasing intensity with repeated hopping, drop jumps and continuous jumping with higher experienced athletes. A similar sequencing is then also adopted for unilateral/ horizontal / multiplanar lower-limb plyometrics.

Effects of plyometric training on performance and injury risk

1. Short-term PT, for instance, 2-3 sessions a week for 4-16 weeks using a combination of bilateral and unilateral lower-limb drills improved vertical jump height, sprint and COD performances in team sport players (Slimani et al., 2016).
2. Traditional, assisted, and resisted PT methods are effective modalities for augmenting VJ performance in healthy adults (Makuruk et al., 2020).
3. 7 weeks (with 2 sessions/wk) using moderate intensity ~100 jumps per training session with a 72-h rest interval tends to improve COD ability (Asadi et al., 2016).
4. Combination of vertical and horizontally orientated exercises (DJs, VJs, and SLJs) is better than 1 form of exercise potentially due to improved specificity (force-vector specificity)
5. Improved sprinting performance following ~10 weeks PT have been reported; minimum of 15 sessions; and high-intensity programs with >80 combined jumps per session (De Villarreal et al., 2013)
6. Combination of different types of vertical and horizontal jumps
7. Increased running economy (Decreased 3km time), tendon stiffness and jump performance after 6-weeks PT (Spurrs et al., 2003).
8. Energy conservationand enhanced propulsive forces (Increased stride length at same/reduced metabolic cost) (Turner and Jeffreys, 2010)
9. Jump-landing and plyometric training programs can reduce injury risk through improvements in neuromuscular control (Herrington and Comfort, 2013; Padua et al., 2018; Webster and Hewett, 2018), reductions in injury rates and injury risk factors, particular those associated with non-contact anterior cruciate ligament (ACL) injuries such as reduced knee valgus angles, GRFs, torques, and improved hamstring:quadriceps muscle activation.
10. Easily integrated into warm-ups (gym & field / court)

Neuromuscular and musculoskeletal adaptations from PT

A plethora of neuromuscular and musculoskeletal adaptations can be obtained following chronic exposure to PT training (Goodwin and Jeffreys, 2016; Booth and Ohr, 2016; Sole, 2017):

• Increased tendon stiffness and cross-sectional area
  • Increased elastic energy storage and transfer of force, reduced energy dissipation
• Greater capacity to tolerate greater stretch loads
• Fast twitch fibre hypertrophy due to EIMD
• Bone loading stimulus
• Reduced electromechanical delay (EMD) (shorter amortization phase / reduced contact time)
  • Strong correlation (r=0.77, p=0.014) between increase tendon stiffness and reduced EMD
• Reduced neuromuscular inhibition
• Reciprocal inhibition of antagonist
  • Reduces co-contraction, allowing greater acceleration and force production from agnostic
  • Reduced golgi tendon organ (GTO) activity – inhibition of GTO – (protective organ) antagonist to increase force
• Increased muscle activation and pre-activation
  • Increases activate state, muscle stiffness, and spindle sensitivity, force generation
• Neural adaptations
  •  MU recruitment, rate coding, reflex excitability, inter-muscular coordination
• Increased fibre force and contraction velocities
• Strength adaptations (eccentric, reactive, and dynamic)

PT considerations

When designing a PT programme for an athlete, several factors must be considered to optimise the training response and mitigate potential risk of injury.

Needs analysis

What are the key movement characteristics associated with sport? Will enhancing SSC function enhance sporting performance? This process will help inform exercise selection. Many sports are inherently “plyometric” such as basketball, volleyball, and netball. In these situations, targeting high load/force low velocity characteristics and tissue conditioning may be of greater importance (Goodwin and Jeffreys, 2016).

Training status

Is this athlete a novice with limited to no experience with PT methods? A simpler PT programme with lower intensity exercises is warranted in this scenario compared to an experienced athlete with extensive history with PT.

Strength

Does the athlete have the physical capacity to tolerate the loads associated with PT activities? NSCA (2016) PT recommendations state that athletes should be able to back squat ≥ 1.5 x BM before commencing PT, while Goodwin and Jeffreys (2016) have suggested that athletes should be able to back squat 60% of 1RM for 5 repetitions within a 5 second period before commencing PT. While stronger athletes should be better able to respond to PT (Suchomel et al., 2016; Cormie et al. 2010), most athletes will perform plyometric activities in their sport such as running, jumping, changing direction, and they will encounter high mechanical loads; thus, it would be non-sensical to completely omit PT until the NSCA (2016) threshold is achieved. Additionally, athletes must become accustomed to plyometric activities, with relatively weaker individuals generally performing low-to-moderate PT activities as part of a periodised training programme, while simultaneously developing their strength capacity before progressing to more complex and higher intensity tasks.

Technique / neuromuscular control

As stated above, landing mechanics and technique is most likely the fundamental and most important consideration when designing a PT plan. An athlete with poor mechanics should not progress to highly complex and intensity plyometrics where the mechanical loads are likely to be amplified with poor technique and/or neuromuscular control (trunk, ligament, quad, leg dominance). For example, if an athlete cannot display a technically sound bilateral squat pattern, they have not earned the right to undertake repeated jump PT exercises.

Physical capacities (testing)

Strength diagnostic testing, in particular vertical jump testing, can be used to identify strengths and weaknesses and inform key training priorities. For example, the eccentric utilisation ratio can help inform training priorities (ratio of CMJ to SJ height should be ~1.1) (Goodwin and Jeffreys, 2016). For instance, an athlete with a greater squat jump height relative to CMJ height would indicate a deficiency in SSC function, and this athlete would therefore warrant training which focuses on SSC development via PT activities. Consideration of vertical jump heights, reactive strength index, stretch-load tolerance and explosive strength deficits are also additional metrics used to inform training (Goodwin and Jeffreys, 2016; Sheppard et al., 2016).

Time of season and training goal

PT should be an all-round yearly feature of a periodised training programme but will be emphasised and prioritised at certain phases of the season (i.e., emphasis modelling) (Sole, 2017). For example, low intensity PT training activities will generally be performed during preparation (pre-season phases), whereas higher-intensity PT, and greater emphasis on PT, will coincide with power and peaking mesocycles where optimising performance levels and the transfer of training is essential.

Exercise-induced muscle damage

It should be noted that PT activities can induce high levels of exercise-induced muscle damage (EIMD) and tissue damage due to high mechanical loads and eccentric muscle actions associated with this method (Goodwin and Jeffreys, 2016; Potach and Chu, 2016). As such, the monitoring, sequencing, and progression of PT is an integral process for optimising performance, fatigue management and mitigating injury risk; thus, a conservative approach between volume and intensity is necessary.

Surfaces

Different surfaces will have different frictional properties and hardness which can increase impact GRFs, mechanical loads, compliance, and ground contact times (Goodwin and Jeffreys, 2016). Generally, for increased transfer, performing the PT on a sports-specific surface with sports specific footwear would be optimal. However, to reduce impact GRFs and mechanical loading, PT may take place on soft matting, sand, or aquatic environments (Goodwin and Jeffreys, 2016). This is particularly pertinent for athletes rehabilitating and returning to play from injury.

Contraindications

Practitioners needs to be aware of specific conditions such as previous injuries (particularly lower-limb knee and ankle), the ageing athlete, and health conditions (particularly bone) when designing PT programmes. Specific attention should be placed for the growing adolescent athlete who may develop overuse injuries and growth-related pain associated with high exposure to mechanical load and demanding tasks.

Plyometric training programming guidelines

Presented below are PT programming guidelines for developing SSC efficiency, impulsive, power characteristics and neuromuscular performance of athletes. These guidelines are adapted from NSCA (2016); Sole (2017); Goodwin and Jeffreys (2016).

Frequency:    1 to 3 sessions per week (depending on sport and time of year), with 48-72 hours recovery between sessions. Two sessions per week appears to be a sufficient training stimulus for sprint and change of direction speed (Asadi et al., 2016; De Villareal et al., 2013).

Volume:         Number of foot contacts (each time a foot, or the feet together, contact the surface) or, alternatively, the horizontal distance per session (note athletes of different statures and jump abilities may achieve different number of foot contacts)

• Beginner = 80 – 100 contacts
• Intermediate = 100 – 120 contacts
• Advanced = 120 – 140 contacts

Recovery:       2 to 3 minutes rest between sets and 2-4 days between sessions (depending on intensity) / 1:10 ratio

• Submaximal and neuromuscular control sessions require less rest

• Clustering techniques are also effective for preserving intensity and technique

Intensity: All of the below can be manipulated to modulate plyometric intensity.

• Less points of contact = greater intensity (bilateral vs. unilateral)

• Greater speed (incoming velocity) = greater intensity

• Greater height of fall (impact) = greater landing forces (> acceleration due to gravity)
• Greater body weight (or added weight, vests) = greater intensity

Progression:  Progressive overload via intensity, complexity or volume, but technique and quality of the movement is imperative to reduce injury risk (Herrington et al., 2018). Training can be micro-dosed whereby the volume or foot contacts can be divided over smaller and more frequent sessions.

It should be noted there should be an inverse relationship between volume and plyometric intensity, and careful monitoring and periodisation of PT activities is central to improve performance and mitigate injury risk (Sole, 2017; Goodwin and Jeffreys, 2016). For example, even for an advanced athlete with good landing mechanics and strength capacity, prescribing 140 drop jumps from a 60cm height will be considered high and potentially excessive, and practitioners should be conscious of the of acute / chronic mechanical loading when prescribing PT.

It is worth noting that most athletes or coaches will not dedicate entire sessions to PT. Instead, practitioners will generally use PT exercises in a combination approach as part of strength and power RT session, or will micro-dose plyometric volumes across the micro-cycle. Additionally, advanced training methods such as strength-power potentiating complexes or contrast / complex sets will use PT exercises prior or following a high-load, low-velocity movement for acute performance enhancement. Alternatively, an effective time to implement PT is part of a field-based warm up for technical / tactical sessions or multidirectional speed sessions (if applicable). However, there is evidence that 5-7 sets of 6 reps is likely beneficial and effective when performed in conjunction with other exercises (strength / high-load power) based on previous work (Kijowski et al., 2015; Cormie et al., 2010) as part of a mixed-methods approach for developing power.

Figure 5 Illustrates how PT should be integrated into the periodisation model. As stated previously, PT should be an all-round yearly feature of a periodised training programme but will be emphasised with greater priority at certain phases of the season (Figure 5) (i.e., premise of emphasis modelling) (Sole, 2017). For example, low intensity PT training activities will generally be performed during preparation (pre-season phases) with a lower training priority, whereas higher-intensity PT, and greater emphasis on PT, will coincide with power and peaking mesocycles where optimising performance levels and the transfer of training is essential. PT will be emphasised when fatigue is low, and quality is high which is typically associated with peaking / tapering phases (Sole, 2017); however, PT will generally be most effective once a foundation of strength has been developed (Suchomel et al., 2016), and combines well with a phase potentiation model. But PT is also a critical round feature for injury mitigation, neuromuscular control, general preparation, and tissue conditioning for the potentially intense / damaging nature of sport (Harper and Kiely, 2018; Herrington et al., 2018).

Sole (2017) provides some excellent example PT sessions below for the general preparation, strength, and explosive phases, and we provide an example plyometric training session for multidirectional speed (MDS) development for a soccer player.

Summary

Plyometric training is a simple, yet highly effective training modality to enhance athletic performance and mitigate injury risk in athletes. This method can be easily integrated as isolated training units, or incorporated into resistance / field-based technical, tactical, speed / MDS sessions to develop a range of neuromuscular and mechanobiological qualities. In addition to focusing on the proposed aspect, equal attention should be placed on the braking and stabilisation components of these plyometric jump-landing actions to improve movement quality, neuromuscular control and to mitigate lower-limb injury risk.

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