Raphael Brandon explains why instability devices may not be the answer to every client’s proprioception needs
One of the fundamental principles in fitness and sports training is specificity. This means that adaptations to a training exercise are restricted to the specific characteristics of that exercise. The principle holds good across different aspects of fitness. In strength training, for example, performing isometric exercises will result in improvements in strength only at the specific joint angle at which the isometric contractions are performed. For dynamic movements, training with very heavy loads – and as a consequence slow movements – results in big improvements in force production but small improvements in speed. On the other hand, fast exercises undertaken with light weight (or no weight) will increase speed of movement, but not force production.
Skill training, too, is task specific. Learning to kick a rugby ball will not improve the accuracy of your golf swing. Only skills with similar movement patterns will have any transfer across to each other, such as an overarm ball throw and a tennis serve. Gymnasts have fantastic balance skills, especially those who compete in the beam event. However, if you asked a gymnast to sit in a competition- standard sprint kayak and paddle, they would probably be getting wet after the first pull on the water.
In the same vein, a sprint canoe athlete has fantastic balance skills in the boat, but ask them to perform a seated balance exercise on a Swiss ball and it will take them some time to learn to control the ball. This is because each balance exercise is a specific task that requires its own patterns of motor coordination. Any change in positioning or equipment turns the balance exercise into a new one.
Instability is no panacea
This principle of specificity is one that therapists need to be very aware of when they prescribe balance exercises in a rehabilitation setting. A recent US study examined the effects of a Bosu training programme on various physical skills (the Bosu is an inflated hemisphere used for balance training)(1). Postural sway on a force plate was assessed, along with vertical jump height, agility run time and length of time for a single-leg balance on the Bosu. Predictably, the postural sway and single-leg balance time improved, but there was no change in performance of the vertical jump or agility run.
Further, your client’s improved ability to balance on one leg on an instability device will have limited training effect on their dynamic athletic movements. When designing sports rehabilitation prescriptions – the goal of which, by definition, is to improve dynamic function and coordination of athletic movement – it may be worthwhile considering just how much time is devoted to using instability devices.
It is also important not to jump to unsubstantiated conclusions about a client’s level of proprioception and core stability from what you see them doing on the instability device. Think back to the canoeist. Does the canoeist really have poor core stability because they cannot control a Swiss ball, when we know they have fantastic balance control in the boat? There is no logic to support that idea. The only reasonable conclusion is that the athlete has yet to develop their specific Swiss-ball control skills.
Which proprioception drills?
We all know why we include proprioception training within rehab: the injury process can reduce proprioceptive function, so it makes sense to retrain the injured joint to ensure a return to full function and reduce the risk of a recurring injury. But how do we select the most appropriate proprioception exercises? Let us consider the biomechanics of stability and postural control.
Balance can be lost for different reasons. For instance:
* the movements of the limbs or the trunk/head can cause a large deviation in the centre of mass in relation to the base of support. For example, if you stand on one leg and suddenly lean your head and trunk over to the opposite side, it is quite hard to retain your balance
* the foot can slip, suddenly changing the degree and position of contact with the ground in relation to where it was expected to be.
In both cases, the balance is lost because the body has moved suddenly, and the relative position of the supporting foot to trunk/head has changed.
The problem with instability devices is that they create the balance challenge by making the ground move underneath you, so the reaction for which the body is being trained does not bear any resemblance to sporting or natural movements (except possibly small earthquakes).
I therefore tend to recommend exercises where the ability to balance on one leg is challenged by movements of other limbs or the head trunk. For example, stand on one leg with knee slightly bent. Then pulse your free leg quickly in and out, and forward and back. These rapid changes in movement cause you to increase your control on the hip and ankle of the support leg.
Controlling joint forces during dynamic movements, such as jumping, landing, changing direction and moving sideways, is a major role of proprioception. Efficient and correct joint ‘position sense’ helps the motor system coordinate the required level of muscle activity to stabilise the joint and produce force for movement.
This is particularly important for ankle and knee ligament injury rehabilitation. For example, your client may have sprained an ankle because they were unable to control the lateral forces during a jump landing, or because an unexpected change in ground surface resulted in a perturbation of ankle position that was too rapid to be controlled. In both cases, standing balance tasks on instability devices may produce training effects of lateral loading and controlling quick perturbations of the ankle. But other training methods may be more effective, especially those that involve more dynamic movement control of the whole lower limb and also those that practise the key skills of landing force and lateral movement control.
Another recent piece of US research(2) examined the effectiveness of a combined strength and dynamic balance programme on the ability of female volleyball athletes to balance and control landing forces. The programme mainly comprised exercises that challenged movement control. For example, jump forward, land and quickly stop still in squat position and hold, or single- leg hop to side and stop still quickly in single- leg squat position and hold.
The programme also included dynamic Bosu exercises. For example, stand on Bosu, then quickly drop into a deep squat and hold balance, or jump on to Bosu with two feet and quickly stop still in squat position and hold. This programme produced reductions in landing forces during jumping and reduced lateral sway during single-leg hopping tests. These training effects are highly specific to reducing injury risks and enhancing athletic ability.
Another key related concept for balance and proprioception training is ‘anticipation’. Commonly injury arises when an awkward movement or change in body position occurs suddenly, precluding the chance to anticipate (and thereby prepare for) the move.
Besier et al (2001)(3) showed that the joint forces involved in a cutting step (side step) were significantly greater when the athlete had to react to a visual signal to cut than when the athlete was free to make the side step at a time of their choosing. The forces of the planned movement were less than the forces of the unanticipated one. This means that the motor system does not have to coordinate such a high level of muscle activity for planned dynamic movements.
The practical application of this piece of that it is worth including proprioception exercises in which the client must react to unanticipated forces. The following are three good examples.
Bosu reaction wobbles
Athlete stands on Bosu on two feet, facing away from therapist. The athlete assumes the ‘ready’ position, knees slightly bent, good posture, arms out slightly and relaxed. The therapist uses their foot to kick the Bosu in random directions, making the surface wobble around. The athlete has to react and brace against the Bosu force to control balance.
Complete a few sets of reaction wobbles, each lasting 30 to 60 seconds. Kick the Bosu with a random rhythm as well as random direction.
Single-leg Bosu/ trampette reaction braces
Athlete stands on one leg on a Bosu or trampette, facing away from therapist. The athlete sets themselves with good balance in the ‘ready’ position. The therapist randomly pushes the athlete’s leg above and below the knee. Pushes can be inwards or outwards. Athlete must react to the contact force and brace the leg musculature to increase the stability of the knee and ankle.
Complete a few sets of reaction braces, each lasting 30 to 60 seconds. Push the leg with a random rhythm and direction, allowing the client to regain balance and maintain form between pushes.
Reaction box agility drill
This drill can be used at the end of a rehab progression for a knee and ankle injury. The athlete is asked to move around a small box marked out by four cones, one at each corner. The therapist stands at the front of the box and points to one of the cones. The athlete must react to the signal and change direction to the target cone the moment the therapist makes a new signal. As the client is facing forwards, by making them change direction you will be training multidirectional movement skills, but each reaction requires an unanticipated change of direction, challenging joint position control.
Five to 10 sets of 10 changes of direction will make a good workout. Take 45-60 seconds between each set, otherwise it can be really hard work.
1.Yaggie and Campbell, J S&C Res, 2006, 20(2), 422-428.
2.Myer et al, J S&C Res, 2006, 20(2), 345-353.
3.Besier et al Med Sci Sp and Ex, 2001, 33(7) 1176-81.