Hypotonia: an erroneous clinical concept?

Abstract

Muscle tone, if defined as the resistance felt during passive movement of an extremity, is generally explained by the appearance of stretch reflexes. Consequently, hypotonia would be caused by a decrease or disappearance of these reflexes. This notion has never been challenged by experiments that reproduce the clinical situation. In this study two clinical tests, passive extension and flexion movements at the knee joint and a free fall of the lower leg with gravity, were applied to 72 control legs and 35 'hypotonic' legs. EMG activity was measured in the quadriceps muscle. Despite special care to obtain relaxation in the subjects, the majority of control legs showed voluntary EMG activity on passive movement. During free fall, long-latency reflexes were present in a minority of normal subjects, but the velocity of falling in these legs was within the range obtained in the legs without any EMG spikes. If the fall time was prolonged beyond the upper limit of relaxed legs, this was the result of voluntary activity, confirmed by EMG measurements. Therefore, long-latency stretch reflexes play no role in the clinical assessment of 'normal tone'. This conclusion was further supported by the observation that 'hypotonic' legs did not fall faster than relaxed control legs. If patient's legs feel flaccid this is the result of weakness preventing voluntary activity. Passive movements during the clinical examination are of great value, but only to detect spasticity or rigidity.