Proprioceptive training and injury prevention

Proprioceptive training and injury prevention

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Can proper proprioceptive training reduce your probability of injury? 

A mounting body of evidence indicates that proprioceptive training can improve athletes’ strength, coordination, muscular balance, and muscle-reaction times, and two new studies link proprioceptive work with a reduced risk of injury during sporting activity. Future investigations are likely to find that improved proprioception can also boost athletic performance.
To fully understand proprioception and proprioceptive training if you’re an athlete, bear in mind that as you carry out your athletic movements your overall muscular activity, the range of motion at your joints, and your body posture are all the products of sensory-nerve activity which is received, ‘coded’, and acted on by your brain and spinal cord (aka, your ‘central nervous system’, or CNS). Your CNS actually gets the information needed to control your movements from three ‘subsystems’ within your body – your ‘somatosensory system’, your ‘vestibular system’, and your ‘visual system’.

What the somatosensory system does
Containing nerves located in the skin, bones, muscle-tendon junctions, and joints, the somatosensory system can detect touch, pressure, pain, and joint motion and position. In your joints, the somatosensory system actually possesses both quick-adapting (QA) and slow-adapting (SA) ‘mechanoreceptors’ (nerve endings which detect physical actions). If a joint is stimulated continuously by pressure or motion, the QAs decrease their signaling of the CNS, while the SAs keep the CNS fired up. Mechanoreceptor experts believe that the sensation of joint motion is mediated primarily by QAs, with SAs playing more of a role in telling the CNS about joint position and sensation. In the human knee joint, mechanoreceptors have been identified which respond specifically to joint acceleration and deceleration (‘Proprioception of the Ankle and Knee,’ Sports Medicine, Vol. 25(3), pp. 149-155, 1998).
Naturally, human muscles also contain mechanoreceptors which report to the CNS about muscle length and tension and which work with the joint QAs and SAs to give the brain and spinal cord complete information about what is happening around the body. Overall, the somatosensory system is often called the proprioceptive system, and the work it carries out is called proprioception. Proprioception is sometimes referred to as the collection of sensations regarding joint movement (kinaesthesia) and joint position.

‘The visual system provides the CNS with visual clues for use as reference points in orienting the body in space’
As mentioned, the somatosensory system is aided by two other systems – the vestibular and visual systems. The vestibular system, which picks up information from the vestibules and semicircular canals of the inner ear, helps to maintain overall body posture and balance. The visual system of course also plays a large role in the maintenance of balance; to gain an appreciation of its importance, simply stand with your full body weight supported on one foot – and then close your eyes. The resultant ‘leg shaking’ and postural sway are a testament to the visual-system’s ability to help with overall balance and coordination. Basically, the visual system provides the CNS with visual cues for use as reference points in orienting the body in space.
The key role played by the somatosensory system helps to explain why some athletes tend to injure certain joints over and over again. For example, when an athlete sprains an ankle, he/she usually damages not just the ankle ligaments but also the somatosensory system’s mechanoreceptors which are dispersed throughout the ankle joint. As a result, kinaesthetic acuity for the ankle joint (the ability to detect ankle-joint movements) diminishes (‘Kinesthetic Awareness in Subjects with Multiple Ankle Sprains,’ Physical Therapy, Vol. 68, pp. 1667-1671, 1988), and so does the ability to detect the actual position of the ankle joint in space (‘Position Sense Following Joint Injury,’ Journal of Sports Medicine and Physical Fitness, Vol. 21, pp. 23-27, 1982). As a result, the ankle remains relatively unstable long after the torn ligaments have healed. This chronic ankle instability can be reversed if the right rehabilitative techniques are utilised, however.
Although athletes and coaches sometimes think that the proprioceptive, vestibular, and visual (PVV) systems are somewhat ‘hard-wired’ and rather inflexible, scientific research has actually indicated the reverse. As you might expect, research in this area has concentrated on whether certain forms of training might enhance joint-position sense, kinaesthesia, balance and coordination, muscle reaction times, and overall muscular strength. Naturally, researchers have been intrigued by the possibility that improved PVV function might reduce the risk of injury – and even improve performance.

The Swedish wobble board
One of the very first studies to look at the potential for balance improvements was carried out in the mid-1980s with professional Swedish soccer players who had functional instability in one or both ankle joints (‘Effects of Ankle Disc Training on Muscular Strength and Postural Control,’ Clinical Biomechanics, Vol. 3, pp. 88-91, 1988). The ‘proprioceptive training’ carried out by the athletes was incredibly simple; they simply stood on one leg on a small ‘ankle disc’, which was nothing more than a ‘wobble board’ – a wooden platform resting on a hemispheric structure, with the flat edge of the hemisphere attached to the underside of the platform. Such a device is of course unstable in all planes of motion; the slightest deviation from stability in the ankle joint is greeted with an instantaneous ‘tip-over’ by the wobble board. Note that while standing on an ankle disc is popularly called ‘proprioceptive training’, it is actually PVV training, since the visual and vestibular systems help the somatosensory system maintain control and balance on an unstable surface (unless, of course, the eyes are closed – which forces the visual system to drop out).
As the athletes stood on the ankle discs, the knee of the support leg was maximally extended, the arms were crossed over the chest, and the non-support leg was flexed at the knee and raised. Principal scientists Hans Tropp and Carl Askling of the University Hospital in Linkoping instructed the athletes to maintain balance by making corrections in their ankle joints, not by activating the knee or hip or re-positioning the upper body.
During the first 10 weeks of the study, the soccer players trained by standing one-footed on the disc for 10 minutes per foot, five times each week. After 10 weeks, the athletes trained on each foot for five minutes, three times weekly. A control group of 30 soccer players performed the same routine soccer training as the ankle-disc group, but without the ankle-disc exercises.

The results?
After just six weeks, the disc trainers were more stable in a one-leg stance (stability was measured on a force platform, not on the ankle disc), and the stability gains were even greater after 10 weeks. Over the subsequent 10 weeks, when total training load was cut by 70%, there were no further improvements in stability (there were no losses, either). Meanwhile, the routinely training control subjects failed to improve stability at all.
Interestingly enough, isokinetic muscle strength was also significantly improved for ankle-disc users after 10 weeks. Utilisation of the ankle disc had upgraded ankle-pronation strength at both slow and relatively high speeds and ankle-dorsiflexion strength at high speeds. Ankle-disc trainers also reported that the feeling of ‘giving way’ in the ankle was significantly improved.
Thus, this very early study suggested that rudimentary training on an unstable device like a wobble board could enhance both postural control and ankle-muscle strength. Tropp and Askling reasonably concluded that it would be better to give athletes with chronic ankle injuries an ankle disc, not a brace. Note that this investigation did not determine whether disc-trained athletes were also more stable during movement (stability was measured as the athletes stood motionless on the force platform). It is logical to think, though, that the heightened stability might have carried over to the stance phase of running, when the foot is firmly planted on the ground and the body swings over and beyond the foot like an inverted pendulum. If this were the case, running economy would be improved. Remember, too that ankle-disc training had upgraded ankle-pronation strength; ankle pronation is one way in which the foot applies propulsive force to the ground during the stance phase. Thus, it’s not too far-fetched to think that wobble-board training might also have some impact on propulsion and stride length.

Muscle-reaction times
The Swedish study was eventually followed by research which looked at the effects of PVV training on muscle-reaction times, ie, how quickly muscles could respond correctively to changes in body position. This has become a key thrust of PVV research, for two reasons: (1) If muscles can react more quickly after a postural or joint perturbation, there should be better control of movement and a lower risk of uncontrolled motions which might provoke injury. (2) If muscles react more quickly, an athlete can function with greater power – more force production per unit time. This could ultimately improve performance.
In the first P-V-V-muscle-reaction-time study to be published in the scientific literature, researchers from the Sports Medicine Center and the Biomechanics Laboratory at the Mayo Clinic in Rochester, Minnesota, focused intently on the actions of the ankle joint and its associated muscles and ligaments, for good reason. Perhaps surprisingly to non-specialists, the so-called lateral ankle complex, which consists of the anterior talofibular, calcaneofibular, and posterior talofibular ligaments, is the anatomical site which is most frequently injured in athletes. In fact, injuries of the lateral ankle complex account for 38 to 45%(!) of all injuries in sports (‘Epidemiologic Perspective,’ Clin Sports Medicine, Vol. 1, pp. 13-18, 1982). As it turns out, 85% of ankle injuries are sprains, and approximately 17% of all participation time lost in association with sports injuries is the result of ankle sprains (‘Role of External Support in the Prevention of Ankle Injuries,’ Medicine and Science in Sports and Exercise, Vol. 5, pp. 200-203, 1973). Among ankle sprains, 85% are ‘inversion sprains’ of the ankle’s lateral ligaments; inversion sprains occur when the ankle rolls onto its lateral edge while the sole of the foot points inward. About half of ankle-sprain patients who are seen by health professionals tend to have recurrent ankle sprains. As you can see, ankle injuries are a major problem in sports.
The Mayo scientists hypothesised that training on an ankle disc (wobble board) would enhance ankle-joint proprioception, and that this improved proprioception would then allow more information about ankle-joint position and speed of movement to be sent to an athlete’s CNS. Importantly, upgraded proprioception might also mean that the info would be sent more quickly. The quicker signals to the CNS could then lead to faster, ankle-stabilising and sprain-preventing actions of ankle-joint muscles, at least in theory.

‘A unique aspect of this study was that a customised platform was utilised to simulate a lateral ankle sprain’
To test their hypotheses, the Mayo scientists recruited 20 healthy, ‘right-foot dominant’ adults (10 men and 10 women aged 18 to 37 years) who had normal range of motion at their ankles and normal strength in their ankle, knee, and hip muscles; these 20 recruits were then divided into a control group (five men and five women) and an experimental group (same set-up). As in the Swedish study, the actual ankle-disc training was pretty basic, but unlike the Scandinavian subjects the Mayo experimental-group participants exercised only their right ankles and legs, not their left ones. The Mayo people carried out various standing exercises on their ankle discs (full body weight supported by right leg only), working for a total of 15 minutes per day for eight weeks.
A unique aspect of this study was that a customised platform was utilised to simulate a lateral ankle sprain – before and after the eight weeks of ankle-disc training (‘Ankle Disk Training Influences Reaction Times of Selected Muscles in a Simulated Ankle Sprain,’ The American Journal of Sports Medicine, Vol. 25(4), pp. 538-543, 1997). One half of this platform had a hinged trap-door which could produce a sudden lateral ankle inversion of 20 degrees. When the trap-door sprung open unexpectedly to create a sudden ankle inversion, the Mayo scientists took EMG signals from four key shin and ankle muscles – the anterior tibialis, posterior tibialis, peroneus longus, and flexor digitorum longus – to see how they reacted to the sudden, potentially injury-producing change in ankle position. Control subjects were tested in the same way before and after the eight-week period; during the eight weeks, they carried out no ankle-disc or strength training.

How the muscles responded
At the beginning of the study, there were no differences in EMG activity (when the trap-door was sprung) for any of the four muscles between the control and experimental groups; interestingly enough, it took about 70 milliseconds for the muscles to ‘fire up’ after the trap-door accelerated downward. Things were quite different after the eight weeks of disc training, however. In the experimental group, there was a significant delay in the onset time of activity for the anterior tibialis muscle, and there was also a delay in the onset time for the posterior tibialis muscle and an elongation of its peak EMG time (essentially, the amount of time required to reach maximal activity). In other words, ankle-disc training changed the rapidity with which key ankle muscles responded to sudden changes in ankle position and also altered the rate at which force was produced in a key ankle muscle – the posterior tibialis.
Why did the anterior and posterior tibialis muscles delay their activities following the ankle-disc training? Bear in mind that the anterior and posterior tibialis muscles are the two major ‘inversion muscles’ of the ankle joint, ie, when they actively shorten, the produce ankle-joint inversion. Such activity would be bad during a sudden, unexpected ankle inversion (as in the Mayo trap-door test or as a result of a step on an uneven surface during running), since it would tend to magnify the inversion and put more strain on the oft-injured lateral ankle complex. In effect, proprioceptive training taught the CNS to ‘hang out’ a bit longer during sudden ankle inversions before firing up the a&p tibialis muscles, improving ankle-joint stability and helping to minimise the risk of injury to the ankle.
Note that the Mayo scientists didn’t get quite what they expected. While they hypothesised quicker muscular reactions in response to proprioceptive training, they actually got slower reactions by two key ankle muscles, the anterior tibialis and posterior tibialis. However, those more lethargic reactions actually helped to stabilise the ankle during sudden shifts in position. In addition, the slower muscle actions were of course the result of quicker adjustments to ankle-position change made by the nervous system as a result of proprioceptive training; the nervous system had learned to inhibit a&p tibialis action much more quickly than before ankle-disc training.
Note, too, that ankle-muscle activity had become more efficient following proprioceptive training. Muscle activities which would have supported and reinforced extreme movements were restricted, leading to less total muscular activity during ankle inversion. This is the kind of change that might improve running economy, since unnecessary movement of the ankle would be controlled and energetic cost might be lowered. Both ankle inversion and eversion are normal parts of the gait cycle and thus represent opportunities for economy improvement. Runners who make such enhancements would be able to operate at specific speeds at a lower percentage of their max rate of energy consumption or max rate of oxygen usage, an effect which would make the effort easier to sustain.

‘As it turned out, these individuals with prior ankle injuries responded quite differently to the ankle-disc training’
The Mayo-Clinic study described above was carried out with individuals with no history of ankle sprain. In a fascinating follow-up investigation at Mayo, researchers asked eight subjects with a history of inversion ankle sprain to undergo the same ankle-disc training utilised in the first study, with the previously sprained ankle used as the experimental ankle and the uninjured ankle employed as the control. The subjects had experienced an average of two prior sprains in the experimental ankle, and at least six months had passed since the last ankle injury (‘The Effect of Ankle Disk Training on Muscle Reaction Time in Subjects with a History of Ankle Sprain,’ The American Journal of Sports Medicine, Vol. 29(5), 2001).
As it turned out, these individuals with prior ankle injuries responded quite differently to the ankle-disc training. When the trap-door dropped, the anterior tibialis muscles in the experimental legs responded more – not less – quickly after the eight weeks of disc exercise, potentially exacerbating the inversion movement. The mechanism underlying this difference is not clear. One suggested possibility is that the previous ankle injuries damaged nerve endings in the somatosensory systems of the subjects, leading to poorer-quality feedback to the CNS during ankle-disc exercise and thus a lack of adaptation. This explanation is not completely satisfying, however, since it is clear that the CNS did adapt to the disc training and chose – as part of the adaptation – to activate the anterior tibialis more quickly than usual. Another possibility is that ankle-sprain prone individuals have substandard somatosensory systems to begin with, and that somatosensory deficiencies cause the high frequency of ankle spraining. If this is the case, such individuals might require more challenging PVV training to bring their neuromuscular reactions up to par.

Proprioceptive cross-training
The second striking finding in the Mayo follow-up was that the anterior tibialis muscles in the untrained legs also responded more quickly to ankle inversion! In other words, there was a proprioceptive cross-training effect; the CNS took the pattern of control established in the trained leg and applied it to the completely unworked one. This cross-over effect has large implications for rehabilitation from athletic injury (training a non-injured leg proprioceptively could help the injured leg – not yet able to train – return to normalcy more quickly once training is resumed) and medical catastrophes such as strokes (similar scenario, with the unaffected side of the body ‘helping’ the side which has lost motor control following the stroke).
The training protocols in the Mayo and Linkoping investigations were very simple, involving little more than standing with one leg on a wobble board. Recently, researchers at Westfaelische Wilhelms University in Muenster, Germany developed a far more sophisticated, multi-station PVV training programme, and the results were rather astounding: athletes using the programme simultaneously improved joint-position sense, coordination of body posture, and muscle-reaction times after just six weeks of training (‘A Multi-Station Proprioceptive Exercise Program in Patients with Ankle Instability,’ Medicine and Science in Sports and Exercise, Vol. 33(12), pp. 1991-1998, 2001).

What the German programme involved
In the Muenster model, 12 different exercise stations were utilised. We list the stations, the exercise carried out at each station, and the progression for each exercise below:
Station 1: Exercise mats. Athletes stood one-footed on the mats. As the athletes improved their coordination and stability, the thickness of the mats was increased.
Station 2: Posturomed. Athletes stood one-footed on a treadmill-like mobile platform, and as their skill advanced the mobility of the platform increased.
Station 3: Ankle disc. Athletes maintained balance in a single-limb stance on an ankle disc. Pads were initially placed under the disc to control movement, but as the athletes improved the pads were removed.
Station 4: Pedalo. If you haven’t seen a Pedalo, you have a treat in store for you. A Pedalo is a small exercise device fitted with four small wheels. In the centre of the wheels are two diminutive pedals, which can be peddled to move the Pedalo forward or backward. Individuals operate the Pedalo from a standing position, with one foot on each pedal. Balance must be maintained as the pedals go up and down and the Pedalo moves back and forth. As the athletes improved their Pedalo skill, movement speed and the frequency of changes in direction increased.
Station 5: Exercise band. Subjects maintained a single-limb stance and then abducted the non-weight-bearing leg, moving it away from the midline of the body, against the resistance of an exercise band (stretch cord). Difficulty was increased by forcing subjects to stand on a carpet instead of a hard floor and then by asking subjects to maintain the stance position on mats of increasing thicknesses.
Station 6: Air squab. Not a kind of pigeon, an air squab consists of a unstable, pillow-like surface for standing and a thick elastic band which attaches to and runs between the knees. The task at hand was to maintain balance using double- and single-limb stances on the unstable pillow; as skill improved, the athletes had to sustain balance with a one-leg stance while the non-weight bearing leg abducted itself, pulling on and disrupting the knee of the weight-bearing stance leg (who thinks of all these things?).
Station 7: Wooden inversion-eversion boards. Standing on small boards which could easily tip inward (eversion) or outward (for ankle inversion), Muenster subjects maintained balance first with a double-limb stance and then with a one-limb position. The progression included additional knee flexion and extension and the addition of arm movements.
Station 8: Mini trampoline. On a small trampoline, the subjects maintained balance in a one-limb stance. After the Muenster athletes became skilled at doing this, they added various arm movements to challenge the stability-producing properties of the support leg.
Station 9: Aerobic step. The subjects maintained balance with a one-footed stance on an aerobic step. Once this became easy, they alternated back and forth between plantar- and dorsi-flexion in the ankle of the weight-bearing limb, first on a level aerobic step and then on an inclined step.
Station 10: Uneven walkway. The Muenster athletes walked across unstable walkways, constructed of cork, tennis balls, or sand bags.
Station 11: Haramed. The German subjects had to maintain balance on a fairly narrow platform which was mobile in both vertical and horizontal directions. The progression consisted of moving from a double-limb to single-limb stance and from a wide supporting surface to a very narrow one.
Station 12: Biodex Balance System. The athletes maintained balance on a computer-controlled, moveable platform. The difficulty of the exercise was increased by increasing the tilting movement of the platform.
30 subjects (18 females and 12 males) who had chronic ankle instability but no ankle pain participated in the Muenster training; these individuals normally trained five times per week. An ‘exercise group’ of 20 individuals used the above stations in a six-week training programme, while a control group of 10 people participated only in test procedures before and after the six-week period. Exercise-group athletes started each workout with a five- to 10-minute warm-up and then exercised at each of the 12 stations for 45 seconds per station. After each 45-second exertion, there was a 30-second recovery as subjects moved to the next station. The whole programme was performed twice to exercise both limbs, right and left, in the same way.

A 60% improvement
After six weeks, the exercise-group athletes had a significantly improved sense of ankle position, upgraded postural stability in a medio-lateral direction, and delayed muscle-reaction times in key ankle muscles in response to sudden changes in ankle position. During the year following the study, exercise-group subjects decreased their frequency of ankle inversions by almost 60%. 
So far, we have seen that PVV training can increase leg-muscle strength, can improve postural control, can enhance joint-position awareness, and can upgrade the quality and quickness of neural reactivity to changes in joint position. Surprisingly enough, PVV training can also correct muscular imbalances between legs!
We know that thanks to a recent study in which researchers at the University of Tubingen in Tubingen, Germany, divided 30 experienced exercisers into two equal groups; 15 individuals took part in a strength-training programme, while 15 others engaged in PVV workouts. Both groups trained two to three times per week for four to six weeks until 12 workouts had been completed (‘Gain in Strength and Muscular Balance after Balance Training,’ International Journal of Sports Medicine, Vol. 22, pp. 285-290, 2001).
The strength-training group employed just two exercises, leg presses and leg curls, using two sets of five repetitions at 80% of maximum strength for each exercise. In contrast, the PVV-group athletes used a variety of exercise stations, including:
(1) A .9-meter-diameter rubber Pezzi-Ball, upon which the backs of both ankles were initially perched, with the subject’s back just above the floor and full body weight supported by the forearms (face looking upward). The legs were then alternatively bent and stretched to move the ball back and forth, and toward the end of the 12-workout period the exercise was completed with one leg rather than two. Five 20-second reps were used for each workout, with a short intervening recovery.
(2) A swinging plate (balance exercises on one leg)
(3) An unstable plate (one-limb balance while trying to catch ball)
(4) Mini-trampoline (one-limb balance while catching ball)
(5) Rolling board (ROLA). As the name implies, this is a board which can move freely from one end to the other over the top of a ball. As athletes moved back and forth, they attempted to catch balls thrown in the air.
(6) Pedalo (described earlier) and a stepper. Subjects moved forward and backward, first with eyes open and then with eyes closed.
Before and after the six-week training period, all subjects were asked to stand on one foot on a narrow one-inch beam for as long as possible. The mean standing time for the PVV group increased by 146% after the six weeks of PVV training but advanced by just 34% in the strength-trained group.

A lower number of touchdowns
Stability was measured on a ‘stabilometer’, a platform device which tipped the ankle a maximum of 13 degrees while the subject stood on the platform with a one-footed stance. The number of ‘touchdowns’ – occasions when the non-weight-bearing foot was required to touch the platform in order to re-establish body stability – decreased significantly in the PVV group after six weeks but remained the same for the strength-trained subjects.
Although the strength-training exercises were designed specifically to improve knee-flexion and knee-extension strength, improvements in these two areas were not significantly different between the strength and PVV groups after the six-week period. Even though body weight provided the only resistance during PVV work, PVV training did a great job of improving leg strength (it was just as effective as conventional strength training), even when strength was measured on a seated exercise device which in effect took the PVV systems ‘out of the loop’ (little postural feedback from the somatosensory system is necessary during exercise when an athlete’s body is supported by an exercise machine).
Most remarkably, PVV training tended to balance out strength in the right and left legs of the subjects, improving strength in the dominant leg but hiking strength in the non-dominant leg even more. In other words, PVV training corrected muscular imbalances between legs. Traditional strength training with leg presses and leg curls was unable to accomplish this; in fact, in many cases the traditional training tended to broaden the imbalances between legs.
This latter finding may be particularly important for athletes who run, since most running athletes have one leg which is stronger than the other and which therefore produces longer steps. Bringing the weaker leg up to par with its more robust sibling might increase stride length (the distance travelled in two steps) and improve max running speed, a key predictor of performance in both endurance and sprint-type running events, as well as in a variety of different competitive sports.

What about injury prevention? 
So far, we have seen that PVV training can have a dramatic impact on coordination, stability, and strength. Does it also lower the risk of injury?
Well, the evidence is in, and there is good news. In a recent study, young female European team-handball players carried out ankle-disc training for 10 to 15 minutes per practice session over a 10-month season. 22 teams participated in the study, with 11 teams and 111 players randomised to the intervention (ankle-disc) group and 11 teams with 126 players placed as controls (‘Prevention of Injuries in Young Female Players in European Team Handball. A Prospective Intervention Study,’ Scandinavian Journal of Medicine and Science in Sports, Vol. 9(1), pp. 41-47, 1999).
Over the course of the 10-month season, use of the ankle disc decreased the numbers of both traumatic and overuse injuries significantly. Ankle-disc trainers had an 80% lower frequency of injuries during games and a 71% reduction in injuries during practices. Overall, players in the control group had a six-fold higher risk of acquiring an injury during the season! This is particularly important in a high-injury sport like European team handball, which boasts an injury rate of one per 20 hours of activity.
In a separate, controlled, prospective study carried out with soccer players, 300 athletes utilised a proprioceptive training programme over the course of three seasons, while 300 other players served as controls (‘Proprioceptive Training and Prevention of Anterior Cruciate Ligament Injuries in Soccer,’ Journal of Orthopaedic & Sports Physical Therapy, Vol. 31 (11), pp. 655-660, 2001). The proprioceptive training utilised in this study was quite progressive. The athletes started by training for 2.5 minutes per leg, four times a day and three times per week with simple one-legged stances on firm ground. Once proficiency was shown, they moved on to one-legged positions on a balance board with front-to-back and side-to-side instability, each leg alternately, using the same frequency of training as before. As coordination and balance continued to improve, the athletes graduated to one-limb stances on a board with multi-planar instability. 
The subjects also performed: 
(1) anterior and posterior step-offs with one foot held on the board, 
(2) forward lunges with deep knee bends with one foot in place on the board (the board foot was positioned alternately for front-to-back, side-to-side, and oblique instabilities), and – to make things really interesting – 
(3) step-downs onto the unstable board from a chair, with a maximal knee bend resulting in the leg positioned on the board. At the most advanced stage of training, athletes also stepped back and forth between two unstable boards and hopped onto and off a board while maintaining good stability.
The results of all this PVV work? Soccer players who stuck with the PVV training had about one-seventh the risk of a serious ACL (anterior-cruciate-ligament) injury, compared with control athletes who carried out no PVV work.

‘There is a strong likelihood that PVV training reduces the frequency of knee and ankle injuries’
A third study has also linked PVV training with a reduction in the rate of ankle injury during soccer practice and competition (‘Prevention of Ankle Sprains,’ American Journal of Sports Medicine, Vol. 13, pp. 259-262, 1985). The results of these three studies indicate that there is a strong likelihood that PVV training reduces the frequency of knee and ankle injury in a variety of sports.
Note, too, that there is a key implication of this PVV-injury research. One of the problems with the PVV studies to date has been that stability, coordination, and strength are always measured under non-dynamic conditions – when athletes are not moving, and specifically when they are not moving during the actual motor patterns associated with their sports. A concern has been that stability achieved in a one-leg, static stance on a stabilometer does not mean that stability will also be achieved during the footstrike portion of the gait cycle, especially when an athlete is moving along at a high rate of speed.
The injury data help to allay some of these fears. After all, if PVV training is lowering the risk of injury during training and competition, it must be producing changes in neuromuscular function which show up during dynamic activity, ie, during training and competition. Otherwise, drops in injury rate would not occur. Just as PVV training can produce ‘cross-over’ adaptations from one leg to another, it must be able create adaptations which are present during static and also dynamic body positions. Nonetheless, an athlete should emphasise dynamic PVV training as much as possible (squats, lunges, and hops on uneven surfaces, for example, rather than statue-like standing positions) to ensure that dynamic adaptations will occur in optimal fashion.

These are the rules to follow
The PVV research is so strong that it would be foolhardy for serious athletes to exclude PVV training from their overall programmes. In carrying out PVV routines, athletes should utilise the following principles:
(1) Start your PVV efforts by simply learning to balance yourself one-footed on firm ground. As your coordination improves, utilise ‘perturbations’ such as catching a medicine ball, swinging your arms, swinging your non-weight-bearing leg, and pulling on your weight-bearing leg with a stretch cord to challenge your stability.
(2) Once you are terrific on firm ground, begin to utilise training devices like exercise mats, mini-trampolines, rocker boards, and finally wobble boards (ankle discs). These implements will challenge your PVV and muscular systems even further to improve their stability-improving properties.
(3) When using the unstable training devices, start with two-leg stances and – when good balance is achieved – progress to one-leg positions.
(4) Don’t be afraid to experiment. Instead of simply standing relatively statically on unstable training devices, employ squats, lunges, step-downs, hops, and other exertions which force your PVV system to operate during dynamic actions.
(5) When you think your balance is really great, attempt to carry out your routines with eyes closed. Eye closing will force your somatosensory and vestibular systems to work overtime and will thus produce larger-than-usual adaptations in those systems.
In your ‘big picture,’ your ‘training macrocycle,’ your year-long plan, or whatever you want to call your overall training scheme, don’t forget to put PVV training first – before all other forms of training. A strong base of PVV work will reduce your risk of injury as your training year progresses, and PVV preparations should also enhance your efficiency and strength and therefore increase the quality of the training which follows the PVV base. It is logical to start your overall training year with an emphasis on PVV work, followed by general strengthening, sport-specific strengthening, hill training (if your sport involves running), and finally explosive exertions. This kind of periodisation plan will significantly lower your risk of injury – and increase your chances of performing at your highest-possible level in your chosen sport. 

Owen Anderson