There is a role for explosive power training in therapy programmes. Sean Fyfe explains
Plyometric drills have been used in training programmes for decades to enhance athletic performance. This training modality is seen as the link between strength and sport-specific speed. Increasingly plyometrics is also being used in rehabilitation, and my own practical experience leads me to believe that it is an essential therapeutic tool when steering athletic clients back to a safe and successful return to sport after injury.
The literature to date is mostly anecdotal. Chmielewski et al in their 2006 literature review(1), called for research to examine whether plyometric exercise is useful in the prevention of reinjury and to validate the assumption that plyometrics promote a return to sport for injured athletes. It would also be very useful to have research on whether the body’s response to plyometric training is altered by injury. Finally, what research there is looks only at lower limb training, although the modality is starting to be used more for upper body training, too.
The lack of evidence in support of plyo- metrics within rehabilitation is no reason not to use them. But we need to start from a thorough understanding of what this training modality is about: what type of fitness gains are possible, and what cautions are necessary. We can then apply this knowledge to the reha- bilitation setting.
The science of plyometrics
Plyometrics train the neuromuscular system to operate more powerfully, so that the muscles are able to perform more work in less time (7), exploiting the elastic properties of the muscle- tendon unit to increase explosive power. One research definition puts it this way: Plyometric exercise is an activity that involves and capitalises on the mechanisms of the stretch-short- ening cycle to increase the efficiency of force production at a joint or increase performance(1).
Plyometric movements occur throughout sport, in running, jumping and throwing. But what differentiates plyometric exercise from normal strength training is its focus on developing the efficiency of the stretch-shortening cycle.
A plyometric movement can be broken down into three phases:
The loading phase
If we look at a simple plyometric movement such as a ‘tuck jump’ (jump and pull both knees into the chest while in the air), the ‘loading’ phase is the instant of landing prior to repeating the jump. This landing results in a stretch to the muscle-tendon complexes that do the work of slowing the body down against the pull of gravity (ie, those that are working eccentrically as feet hit the ground from the jump), which in this instance means:
* gluteus maximus
* gastrocnemius and soleus.
Under training, this stretch-upon-impact can result in the muscle developing greater elastic force in response to the stretch. The training effect arises from physiological adaptations at the micro-fibre level within the muscles and tendons: First, there is ‘muscle potentiation’: an increase in the proportion of crossbridges attached to the actin protein filaments(2)and a decrease in the cross-bridge detachment rate(3) ; second, the stretch prompts the local motor unit to produce excitatory neural feedback to the muscle(4); and thirdly, the stretch causes a storage of potential energy in the elastic components of the muscle-tendon complex, which allows for greater recoil of the muscle during the shortening cycle.
It is actually the tendon complex that undergoes the most length change during the loading phase. This stimulates the Golgi tendon organ (sensory nerve receptor in the tendon), which results in greater force being generated in the subsequent contraction.
As well as capitalising on the properties of the stretch-shortening cycle, plyometric exercises also tap into two other reflex mechanisms: length feedbackand force feedback. In both cases, local neural feedback/feed- forward loops provide information to the muscle, its synergists and antagonists, either to ratchet up or to damp down their response to the stretch. The main purpose of length feedback is to help increase joint stiffness, while force feedback regulates the coupling between joints(1, 5, 6), resulting in enhanced neuro-motor control and joint stability.
The coupling phase
This is the brief moment of transition between the loading and unloading phases, when joint angles and ground reaction forces are about to change direction. During this time, the muscle-tendon complex length is constant and the muscle is in a state of isometric activation.
The unloading phase
This is the period from when the muscletendon unit begins to shorten (concentric contraction) through to when the foot leaves the ground (lower limb) or when a resistance object such as medicine ball is released (upper limb).
According to Chmielewski et al(1), the value of plyometrics lies not in isolating a single mechanism that can improve explosive power, but in its ability to train the combination of mechanisms outlined above in the interests of performance gains.
Researchers have established a direct correlation between plyometric training and the following improvements in athletic performance:
* increase in vertical jump height(10, 11, 9, 12, 7, 13)
* decreased sprint times(14)
* increase in golf club speed and driving distance(15)
* running economy(16).
Plyometrics in isolation can also improve isolated muscular strength, but better results are achieved when used in combination with traditional strength training.(8,9)
How to use plyo techniques
There is no agreement on what exactly constitutes plyometric training. But broadly speaking there are two types of plyometrics: low intensity and high intensity.
Low intensity drills are conducted at a submaximal effort level, with an emphasis on good technique or body position. These exercises are used to train dynamic balance and proprioception.
High intensity drills are about achieving rapid force development and maximal output. Athletes should already be able to perform low intensity drills and should gradually progress towards the more complex and high intensity work.
Thus, a lower limb plyometric exercise programme should start on the ground only, and on two feet. As the athlete gains in competence, they move on to single leg activities, the use of boxes (to jump up to or down from) and weights.
The aim of progressions is to increase load and speed of movement. If the progressions are too advanced, the athlete will struggle to make gains. Take, for instance, the ‘drop jump’, in which you jump from a height to the ground and immediately make a maximal jump. If the box height is increased too early, the athlete will be forced to increase their ground contact time, undermining the point of the exercise (gains in both reaction speed and force).
Progressions can also be made to challenge the athlete’s stability at speed, through, for instance, single leg exercises, exercises from a lunge position or adding a rotational movement or change of direction upon landing.
Get the fundamentals right
No athlete should attempt plyometric work, high or low intensity, until they can competently perform key functional movements such as double and single-leg squats. If someone can’t squat properly, they certainly won’t be able to do squat jumps. And if they can’t squat on one leg with good form, then all single-leg plyo training should be out of bounds.
This is particularly important advice for young, developing athletes. If they fail to achieve physical competencies in functional movements before attempting more complex exercises, they will set themselves up for early and possibly career-threatening injury.
The role of plyometrics in rehab
We know that plyometrics improve neuromuscular function, including joint position sense (proprioception) and postural control. In a series of studies conducted from 2004 to 2006, Myer et al(17, 18, 19)showed that plyo work improved postural stability in single- leg stance; trunk stability when landing from a jump; and biomechanical measures relevant to lower limb injury risk. In 2002, Swanik et alfound that upper limb plyo training improved joint position sense(20).
However, these studies were performed on uninjured athletes, so it is unclear whether injury and its effects on the neuromuscular system would alter the outcomes. As mentioned above, the sports therapy profes- sion really needs some research into this question in order to have confidence in adding this training tool to our rehab repertoire.
In the meantime, we must draw on what knowledge we do have and apply it as best we can. The key training principle of specificity dictates that training for a sport should replicate the movement patterns and energy systems required for that sport. Since the aim of rehabilitation is to regain the pre-injury level of function of the injured site – or in many cases to improve on the pre-existing level of function – and also to return the athlete to their pre-injury state of fitness, specificity would seem to argue in favour of using plyometrics as a rehab tool. If an athlete has to run, jump, change direction and so on in their sport, then their rehabilitation should prepare them specifically for those actions. The crucial judgement for therapists lies in what level of activity to introduce at what point in the rehab process.
Low intensity plyometrics is a very useful tool to retrain dynamic balance and proprioception after injury, and once functional stability is regained, such exercises can be gradually introduced. In late stage rehabili- tation, when the injury site is strong and close to fully repaired, higher level plyometric drills that mimic the sporting environment may be included. It is essential that recovering athletes perform explosive type movements in a closed environment before returning to any field or court sport.
Proceed with caution
Therapists should be aware that there is a risk of aggravating injury with plyometric exercises during rehabilitation. However, I believe if the athlete returns to play an explosive sport without having performed plyometric exercises at a high level, they will be at greater risk of re-injury.
Always err on the side of caution when introducing progressions, making sure only ever to progress one element at a time: repe- titions, or drop height, or stability challenge etc. Delay progressions by an additional week if there is any doubt about the athlete’s readiness to cope with them.
How much and when
Low intensity plyometric rehab can be done daily as long as the exercises don’t provoke soreness to the injured area. This kind of plyo work does not cause high levels of stress to the muscle-tendon complexes, unlike high intensity plyo drills, which should be reserved for late-stage rehab. Although this is anec- dotal, I recommend that high intensity exer- cises should only ever be done every second day at the very most. If an athlete is able to perform high level exercises, they are by definition likely to be a week or two away from competing, and then they can resume their normal training regime.
It is very important to specify the number of ground contacts (repetitions) the athlete is to perform. Progression should follow the general strength training pattern, in which early stage exercises will be done using low resistances and higher numbers of repeti- tions. As loading increases, the number of reps will decrease.
Thus, low intensity plyometric exercises, where maximal muscular output is not demanded, can have a larger number of repetitions/ground contacts, but this number reduces as intensity rises. Increasing the number of ground contacts is a progression in itself. You might set for your athlete a low intensity drill such as single leg hops, 3-4 sets of 20 hops. By the time they are doing single leg squat jumps, you might be asking them to perform 3-4 sets of 6-8 repetitions.
The rugby player
Take the rugby player as an illustration. His injury is an isolated Grade 2 strain of the medial collateral ligament (inner side knee ligament), managed conservatively with a brace. As pain and ligament recovery allow, the player should first return to squatting, lunging, single leg squats and proprioception exercises on the ground. Once he has mastered the full range of movement and strength in these exercises, he can progress to proprioception work on an unstable surface such as a wobble board or mini-trampoline. He may also now add in some low intensity plyometric drills on two legs. These can be progressed to the injured leg alone, building up to high- intensity single leg work. At the same time, he will be progressing his running, cutting moves, direction changes, skill drills and fitness level.
In the past, plyometrics, particularly high intensity exercises, have not been seen as an important part of rehabilitation, probably because of the associated risk and decreased emphasis on exercise based therapy. However, I believe this modality is sound in principle and essential in practice to the success of late stage rehabilitation. As long as the exercise is mimicking the sporting activity and you are confident the athlete’s injury can withstand it physiologically, embrace it.
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