The use of force platforms in measuring functional stability of the knee and guiding the training program

The use of force platforms in measuring functional stability of the knee and guiding the training program.

The use of force platforms – like the Vald Forcedeck – in elite sports and rehabilitation is rapidly increasing. A measurement takes less than a minute, and the results are available immediately. What is less well known is that a force platform allows for the immediate and objective measurement of functional stability. What exactly is functional stability? A body (part) is sufficiently stable if it is stiff enough to prevent a force on a joint from causing unwanted deformation of the joint.

In our daily movements, the body behaves like a spring. A step or jump begins with a countermovement. To initiate a countermovement, we must relax our muscles. This makes the joint unstable and allows it to bend. Gravity causes the body to be pulled downwards. Then, the downward velocity must be slowed and converted into upward velocity. Our body consists of different parts that can move independently. With the right coordination, the body ensures that it can move as a unit.

Presentation of the force related to height. The red arrows follow the Counter Movement (eccentric) force, the green arrows follow the Jump up (concentric) force.

Proceedings of force production during a squat (left), walk (middle), and run (right). The circled area indicates the moment where recoil of elasticity occurs.

Instability can be recognized when insufficient force is built up during a portion of the Counter movement. The system is then unstable. Nevertheless, it must slow the downward movement. Force must therefore be built up very quickly at the last part of the moment, while the joint is in its most unstable (flexed) position. During a controlled, relatively slow movement, such as a squat, the instability does not yet pose a problem. However, as soon as the movement becomes much faster, for example, landing from a jump, changing direction during a sprint, or descending stairs, the body is too late to build up sufficient force and stability, and a small misalignment will result in a significant risk of overload and damage to the joint.

Bilateral (left) and unilateral (right leg, right) late force build-up during eccentric movement of a CMJ. Both tests were performed on elite football players who were not injured at the time of the test. The player with bilateral instability tore his cruciate ligament a few months later, while the player with unilateral instability had to retire from professional football due to repeated thigh muscle injuries.

A jump isn’t necessarily required for force plate testing. During a quickly executed squat, the force produced is very comparable to that of a jump. This test can therefore be used early in rehabilitation and later compared to a jump test.

Force production during a squat (left) is quite comparable to that of a CMJ (right). The two graphs are superimposed in the center graph.

In most cases, instability is caused not by strength but by poor timing. The question now is how this stability can be improved, and the answer lies in space research. This research examines the influence of reduced and increased gravity on coordination, also known as hypo- and hypergravity. Our movement is pre-programmed and automated, so we don’t have to think about it. This works fine until we encounter an unexpected situation, for example, missing a step while descending stairs. Something similar happens during increased gravity. The first time, you’ll be too late to contract your muscles, but we learn very quickly, and the next time we contract our muscles sooner. A new coordination is learned, which we then apply in normal gravity; the body is functionally more stable.

Loss of strength during a squat performed with FieldPower (blue) and without additional resistance (dark blue). During a squat with FieldPower, strength decreases and increases faster and earlier, changing the timing. This is remembered by the body for the next squat, where muscle contraction occurs earlier and stability is improved.

This is the core of FieldPower Hypergravity Training. Using force platform measurements, we can demonstrate that coordination, and thus stability, is significantly improved immediately after a session, even during very rapid braking, such as landing after a jump. The beauty of FieldPower Hypergravity Training is that the training stimulus can be delivered in a very controlled manner, so that even people with low physical demands, for example after surgery, can benefit from it safely.

Improved stability during landing after a jump following a session of FieldPower training.

On the author:
Drs. Martin Huizing is an exercise scientist and trainer. He has been working with and researching innovations in sports and rehabilitation for 25 years, with a special focus on ACL injuries.