Soleus long-latency stretch reflexes during walking in healthy and spastic humans

The present study was carried out to investigate the long-latency soleus stretch reflexes M2 (peak latency of approximately 85 ms) and M3 (peak latency of approximately 115 ms) during walking in healthy and spastic multiple sclerosis (MS) patients. An 8 degrees stretch was applied to the ankle extensors of the left leg in 8 healthy subjects during normal walking speed and 9 spastic MS patients and 10 age-matched healthy subjects during slow walking. When present in walking healthy subjects, M2 and M3 were modulated in a similar way and with the same amplitudes as previously described for the short latency soleus stretch reflex (M1). The spastic patients' soleus M1 was significantly less modulated during walking. The patients' M2 long-latency response was modulated in the same way as the age-matched healthy subjects. All patients' M3 responses were absent or much suppressed during walking. The origin and functional importance of the short- and long-latency stretch reflexes in healthy and spastic persons are discussed in relation to the above findings and the behaviour of the stretch reflexes during matched isometric contractions. M3 is argued to be part of a transcortical reflex in healthy subjects.