The use of plyometrics as a training method has been viable for a number of decades. Even so, a huge question mark hangs over whether coaches truly understand the protocol behind such training in order to provide their athletes with safe plyometric sessions. Before we get ahead of ourselves, does anyone know what exactly plyometrics are?
Plyometrics aim to train the muscle to reach maximum force in a short a time as possible (Kutz, 2003). The ability to combine velocity and force is what we often refer to as power, thus highlighting the attempt of plyometric training to improve or impact power. It’s very difficult to think of a sport where power isn’t important; it can help you run faster, throw farther and jump higher amongst an almost infinite list of improved abilities! Don’t get too excited though… Plyometrics isn’t a method of training for our everyday Average Joe! Ebben & Blackard (1997) stated that individuals needed to possess functional strength in order to compete, or otherwise perform a ‘preparatory cycle’ before training plyometrically. Simply put, athletes need to be well conditioned! A guideline was put forward by Chu (1998) to suggest athletes should be able to perform 5 squat repetitions at 60% of their bodyweight. He continued to highlight the need for thought when working with very heavy athletes and younger athletes. To make it easier for us all, the following presumes that those we are working with are suitably conditioned adults!
A variety of studies have highlighted the use of plyometrics to improve strength and power measures (Myer et al., 2006; Adams et al., 1992; Swanik et al., 2002), providing evidence to the benefits of training in such a way. Let’s be fair though, if we don’t know the physiology and principles behind plyometrics, it doesn’t matter how many studies have proved the benefits!
Listen up… time for a science lesson!
Now, what’s coming up might seem complicated, but if you truly understand it, your athletes need never look back (literally if they’re runners!). We’ve already discussed that the aim of plyometric training is to produce maximum force over the shortest time possible. That’s all well and good, but how do we do this?
Have you ever heard of the stretch shortening cycle (SSC)? Yes? No? I’m going to explain it anyway! The stretch shortening cycle is a natural muscle function which occurs due to the combination of eccentric and concentric muscle contractions (Komi, 2000). During many activities, an eccentric muscle action is quickly followed by a concentric muscle action. Plyometrics aim to utilise this to the highest level. Kutz (2003) presented three stages to the stretch shortening cycle (Figure 1.) which help to explain it in further detail:
1. Eccentric Phase (stretch) – Preloading or stretching of the muscle occurs (generally at ground contact) which in turn prompts proprioceptors to signal that a stretch is evident in the muscle. This will ultimately cause the muscle to contract later in the cycle.
2. Amortization Phase– Essentially the phase between landing and jumping again, the longer the amortization phase, the more stored elastic energy is lost. This highlights the importance for a plyometric movement to be classified to require an amortization period of under 0.2 seconds as referred to by Thomas, French & Hayes (2009).
3. Concentric Phase (shortening) – Stored elastic energy is combined with both the voluntary and involuntary (the protective mechanisms brought about by proprioceptive signalling) contractions in order to provide force for the subsequent movement.
Figure 1. The Stretch Shortening Cycle in the gastrocnemius. Adapted from Komi (2000).
So, what’s the point? you might be asking. Well, importantly, there isn’t just one point, there are a number. Let’s think about what we are trying to achieve here. Simply put, we are trying to decrease the time spent in the amortization phase whilst attempting to generate as much force as possible. Considering this, it’s vital that we know what exactly the SSC adapts physiologically.
Flanagan and Comyns (2008) presented evidence that eccentric/concentric coupling produces a more forceful contraction than would appear from concentric contractions alone. Utilising the SSC clearly allows an increased contraction to be generated as a result of a prior eccentric contraction. Flanagan and Comyns continued to suggest that the use of a preceding eccentric phase moves the force-velocity curve (we’ll get to that…) to the right. It should also be considered that plyometric exercise can manipulate the threshold of Golgi-Tendon Organ activation which can maximise the elastic property of muscle (Kutz, 2003). That might all sound like scientific garble, but when we recall what we are ultimately aiming to do, something seems to be working!
The force-velocity curve is a perfect reference point to highlight the impact of plyometrics on power (Figure 2.):
Figure 2. The force-velocity curve, including a trace of power.
Adams et al. (1992) highlighted the impact of plyometric training on increasing power. Using the force-velocity curve can give a truer understanding as to why this is so. We need to understand the principle that the heavier an object, the slower it will move when a force is applied to it. This is highlighted by the force-velocity curve. Note how a high velocity results in the production of a low force whilst a high force results in a movement at a slower velocity. In order to optimise power – as seen on the curve – a compromise needs to be found between force and velocity. This is the main lesson we can take from the force-velocity curve.
Now, consider our aims again. As mentioned above, Flanagan and Comyns (2008) highlighted the possibility of moving the force-velocity curve to the right. Essentially this means that we can produce a greater force at a quicker velocity – thus presenting an increase in power. Hakkinen et al. (1998) highlighted the possibility of training to produce neural adaptations which can increase strength production and as such, force. Kutz (2003) expanded on this suggesting that changes in neural function can affect force production by increasing the rate at which motor units are stimulated whilst also increasing the number of motor units that are activated. Through the correct application of plyometric training we are able to enhance the rate of force development. Compounded to the attempt to shorten the amortization phase, it is evident how plyometric training can improve power.
Here’s how you’re gonna do it…
In order to train plyometrically, a thorough understanding of exercise technique is paramount. All too often you hear people talk about how dangerous plyometrics are. Ultimately, if you know what you’re doing, they aren’t!
The primary movement in lower limb plyometrics is that of a triple flexion (stretch phase) followed by a triple extension (shortening phase). To fully utilise the SSC through the articulating muscles, both posture and technique must be trained to allow stable movement with a quick amortization phase (McNeely, 2007). Meira and Brumitt (2005) highlighted that considerations are possible to reduce knee injuries when training plyometrically. High ground reaction forces (GRF’s) increase chance of sustaining an injury (Prapavessis and McNair, 1999) and so optimising technique can reduce GRF’s and as such injury risk.
Figure 3. Proper landing mechanics of the lower limb when performing triple flexion/triple extension exercises. Adapted from Meira and Brumitt (2005).
McNeely (2007) highlighted that overall good posture will ensure:
· Knees are aligned over the toes
· The trunk is inclined forward slightly
· The head is up, looking forwards
· The back is flat
McNeely (2007) continues to suggest that participants should be proficient at this technique before plyometric training begins.
Figure 4. Poor landing technique with valgus knees in the lower limbs. Adapted from Meira and Brumitt (2005).
So, you know how to land with good posture but that’s not exactly much use if we can’t think of a varied and suitable training programme! There are four primary principles that can be applied to plyometric training to allow movement both to the front and to the side on both one and two legs. I like to call these principles the ‘height-distance’ principles. Each principle manipulates the centre of mass in order to affect GRF’s and to target various muscles and parts of those muscles that are used. The principles follow:
· Low and short – the jump is low and the distance travelled from each ground contact is short. An example may be short, side to side, one leg hops. Probably the most appropriate principle to use with beginners due to lower GRF’s.
· Low and long – the jump height is the same as that in a low and short motion yet the distance travelled is long. In order to apply this, double-leg, rapid bounds may be appropriate.
· High and short – the jump height is high yet little or no distance is travelled. Though jump height is high, the focus should still be on a short amortization phase. Double-leg jumps off a box with a jump upon landing would be appropriate to use in experienced athletes.
· High and long – a large distance is covered with a high jump height. Consecutive hurdle jumps would be a good activity to utilise this principle should the individual be highly conditioned.
When putting plyometrics into practice, consider technique before attempting any drills. If technique is good and the athlete is suitably conditioned, the above principles can allow a varied and effective programme to be developed.
An appropriate programme is highlighted by McNeely (2007) which is a medium intensity session for an intermediate athlete (Figure 5.). This can help present the way to set out a training programme in relation to the principles above and the intensity of a session highlighted later in Table 2.
Table 1. Adapted from McNeely (2007) to highlight a varied programme for an intermediate athlete working at a medium intensity.
Looking at Figure 5., notice how each exercise incorporates a different ‘height-distance’ principle. Single response vertical jumps allow the use of either short principles, according to the coach’s choice. Hurdle hops allow low and long or high and long activities with box jumps presenting a possibility of both high principles. This highlights the ease to vary a plyometric session – a feat that should be possible for almost any trained coach!
Health and safety… bear with me!
As with all walks of life these days, health and safety is paramount! McNeely (2007) and Kutz (2003) highlighted various protocols to help ensure a safe plyometric session:
Landing – A well-trained landing (highlighted above) can reduce stress on bones, joints and connective tissues (McNeely, 2007).
Landing Surface – Whilst plyometrics can be performed almost anywhere, it’s vital that the surface has an absorbent component. Things like concrete or asphalt can lead to high GRF’s and resultantly cause injury (McNeely, 2007).
Strength – As already referred to, the athlete must be suitably conditioned to the drills being performed. It is important to consider workout intensity (McNeely, 2007).
Fatigue – Plyometrics should not be performed without 48 hours rest or in fatigued athletes (Kutz, 2003)
Progression – Obviously progression is important, but too much too soon is not a good idea! (Kutz, 2003). Attempt to progress slowly, emphasising technique and fast amortization phases.
Plyometrics can be intense…
Now let’s think about intensity. It’s important that athletes are trained to use all of the ‘height-distance’ principles, yet all of these require varying GRF’s and as such can be classed as different intensities. McNeely (2007) presented the following table to highlight an appropriate number of ground contacts when training at these differing intensities:
Table 2. Adapted from McNeely (2007) to represent appropriate training intensities for athletes with varied experience.
Considering the safety components above, the intensity of the session needs to be considered in relation to the athlete’s strength, current condition (fatigue) and the progression of their training. To truly maximise performance, the ‘height-distance’ principles should be implemented to ensure the athletes trains at different, appropriate intensities for each session.
Without taking the sessions for you, I think that’s all the help I can give… may the force be with you!
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