Mechanical and electromyographic stretch responses in spastic and healthy subjects

The reflex mediated mechanical response was studied in the ankle flexors and ankle extensors of healthy and spastic subjects at maintained contractions from low to high concentration levels. This was done by a technique where muscle stretches could be applied during contractions with stretch reflex responses present or during contractions where the stretch reflex was absent. Stretch responses without stretch reflexes were obtained during contractions elicited by electrical stimulation. The validity of this method is discussed in details and it is concluded that the stretch responses during electrical stimulation can give a correct estimate of the non-reflex muscle response. The method is difficult to carry out in many human subjects and a number of precautions have to be taken. In healthy subjects a large reflex mediated mechanical response was found in the ankle flexors and ankle extensors, with the largest response at low and intermediate contraction levels. Surprisingly the reflex mediated mechanical response was found to be of equal size in the ankle extensors of spastic patients and control subjects at all contraction levels. In the ankle flexors no reflex mediated mechanical response was present in the patients contrary to the findings in the control subjects. A method was developed to predict the reflex mediated mechanical response from the reflex mediated EMG response. The method was successfully applied in the ankle flexors. In the ankle extensors the measured reflex mediated mechanical response was a factor of 2.5 lower than the EMG predicted mechanical reflex response. It was concluded that the method cannot be applied in situations where a large synchronized EMG response occurs--as it does in the ankle extensors. An increased EMG response was found in the ankle extensors in spastic patients, but this was not followed by an increased mechanical reflex response. This emphasizes that conclusions drawn from EMG results should be done with caution. Stretch reflexes are increased in spastic patients during clinical examination. This is in contrast to the findings under our experimental conditions, where the reflex mediated response during maintained contraction was decreased in the ankle flexors and unchanged in the ankle extensors of spastic patients. Others have found that the H-reflex is modulated in healthy subjects in relation to different motor tasks. It was proposed that healthy subjects set the reflex in a facilitated state in relation to ongoing contraction under our experimental conditions and perhaps in a more inhibited state in the clinical test situation.(ABSTRACT TRUNCATED AT 400 WORDS)