Clinical application of twitch interpolation technique as an objective measure of peripheral and central paresis

The BMRC-Scale is most common used to estimate the degree of paresis in clinical routine. However, quantification of muscle strength is crude and detection of discrete paresis and minor changes in the course of the disease are difficult. Therefore we introduced the twitch-interpolation technique to measure peripheral and central paresis in the clinical setting. The method involves the interpolation of electrical elicited twitches on a maintained voluntary contraction (VC). During truly maximal VC no extra force is generated. Residual twitches despite of maximal effort by the patient reveal the inability to recruit all motor units available. The isometric VC is continuously recorded by a dynamometer. After determination of the maximal VC (MVC) patients are instructed to keep their muscle farce at a level specified by a horizontal line on the display of the dynamometer. Subsequently, three electrical stimuli are applied at each level of VC. The changes in twitch parameters at different levels of muscle activation provide information about the extend of motor unit recruitment. In addition to the measurement of the maximal muscle force, the method allows the prediction of true maximal force (TMF) based on the known observation of a linear relationship between decrement in twitch force and increased voluntary force. In patients with central paresis even slight dysfunction is detected by residual twitches. In myopathic diseases the use of the available motoneuron pool can be estimated. Finally the method is recommended to report central and peripheral paresis in patients with amyotrophic lateral sclerosis.