Functional muscle testing relies on the fact that a muscle can resist a higher eccentric load that it can concentrically contract. This is an important safety mechanism which enables a voluntarily maximally contracted muscle to still resist an external force. However, this depends on appropriate afferent input and any disturbance to neurological coordination can compromise this reflex. This shapes our testing procedure, where we ask the subject to maximally contract against a static load and the gradually increase an external force to assess the muscles ability to ‘lock’ against a forced eccentric contraction. It is essential the difference between the classic ‘Make test’ and the ‘Break test’.
Can you cheat with muscle testing? Yes you can – just as you can cheat with most orthopaedic test, motion palpation etc. But why would you? I class myself as an open minded sceptic and I keep questioning whether I am indeed subconsciously finding what I want to find rather than being objective, but having worked with and muscle tested some very large and incredibly strong athletes for a couple of decades, there is no way I can cheat with them. When they are strong they I have no chance of overpowering them with manual testing, but when they are inhibited I can very easily break their contraction.
And when you find and treat the source of the disturbed sensory input which compromised the efferent output to the muscle – and then return the muscle to full strength – you have an important insight into what may have compromised this patient’s adaptive capacity and caused their injury.