Supplied by Amy Fu, The Hong Kong Polytechnic University
In posterior translation, the force caused by the plate movement is recorded by the platform shear transducer. The subject is thrown forwards (dorsiflexed at the ankle), such that more load is taken by the front (toe) load cells. The subject responds by contracting the plantarflexor muscles (triceps surae) in order to generate a plantarflexor moment to restore stability. In an anterior, or forward, platform translation, the reverse happens, with the anterior tibials (dorsiflexors) being used to restore stability.
A typical posterior translation is shown in fig. 1. The left and right ankle moment are derived by from the difference between the front and rear load cells under each foot.The first deflection of the shear curve indicates the start of the platform translation. Note that the moment curves simultaneously rise, as body weight is thrust forwards. At a certain point, however, the gradient of the moment curves suddenly increases. This point is said to indicate the motor response of the plantarflexors, and the time difference between this point and the initiation of platfrom translation is taken to be the Motor Latency of the plantarflexors and their associated neural drive.
In a forward translation, the same happens with the moment curves reversed (fig. 2). In this case the latency refers to the latency of the dorsiflexors and associated motor control circuitry.
The Equitest software differentiates the moment curves in order to locate the point at which the gradient changes abruptly, and reports this point as the motor latency. The manufacturer claims that this biomechanical latency is preceded by the electromyographic latency of the muscle by about 50 ms. Since it does not involve measuring EMG, the test is potentially a much easier and more reliable method for determining motor latency.
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Last modified on 14-August-97.