van der Meché F G, van Gijn J
Brain. 1986 Dec;109 ( Pt 6):1169-78. doi: 10.1093/brain/109.6.1169.
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.
肌张力，如果定义为肢体被动运动时所感受到的阻力，通常由牵张反射的出现来解释。因此，肌张力减退将由这些反射的减弱或消失引起。这一观点从未受到过重现临床情况的实验的挑战。在本研究中，对72条对照腿和35条“肌张力减退”腿进行了两项临床测试，即膝关节的被动伸展和屈曲运动以及小腿在重力作用下的自由下落。测量了股四头肌的肌电图活动。尽管特别注意使受试者放松，但大多数对照腿在被动运动时仍显示出自主肌电图活动。在自由下落过程中，少数正常受试者存在长潜伏期反射，但这些腿的下落速度在没有任何肌电图尖峰的腿所测得的范围内。如果下落时间延长到放松腿的上限以上，这是自主活动的结果，肌电图测量证实了这一点。因此，长潜伏期牵张反射在“正常肌张力”的临床评估中不起作用。“肌张力减退”腿并不比放松的对照腿下落得更快这一观察结果进一步支持了这一结论。如果患者的腿感觉松弛，这是无力阻止自主活动的结果。临床检查中的被动运动很有价值，但仅用于检测痉挛或僵硬。
