Assessment of soleus motoneuronal excitability using the joint angle dependent H reflex in humans

As variations in the amplitude of H reflex potentials can be influenced by changes in muscle length, motoneuronal excitability in terms of H reflex during free movement has long been argued. With the maximal M response controlled, the present study compared several H reflex parameters in order to assess motoneuronal excitability of the resting soleus for different ankle angles (plantarflexion 20°, neutral, and dorsiflexion 20°). All H-related parameters were dependent on joint angle, suggesting that soleus motoneuronal excitability in the dorsiflexed position was significantly suppressed. By contrast, soleus motoneuronal excitability in the plantarflexed position was not effectively modulated since H-related parameters did not differ from their neutral-position analogs. Methodologically, assessment of joint angle-dependent modulation of motoneuronal excitability requires meticulous control of M responses and selection of appropriate parameters that are insensitive to possible physical modulation and spatial shift of the M recruitment curve confounded by geometrical factors.