Neural control of joint stability during a ballistic force production task.

Changes in transmission in Ia afferent and corticospinal pathways have been reported to depend on various factors including task complexity and the phase of movement. Here, we test whether a unilateral voluntary force production task leads to specific changes in both Ia afferent (by the use of the H-reflex technique) and corticospinal (by the use of transcranial magnetic stimulation, TMS) pathways in response to different mechanical conditions. The participants were exposed to either one or three mechanical degrees of freedom (DoF) of an external object while performing a unilateral knee extension movement in a sitting posture. The amplitudes of the m. soleus (SOL) EMG to either type of stimulation were normalized to amplitudes obtained during voluntary tonic contractions at matched background EMG in order to assess movement-related alterations. The results at two phases during movement (initial phase and during task execution) were analyzed. The unstable 1 DoF condition led to elevated co-contraction of SOL and the tibialis anterior muscle. Phase- and movement-related modulations of muscle responses to both types of stimulation were present in both mechanical conditions. However, the stable 1 DoF condition caused a significant facilitation of normalized H-reflexes in the initial phase of movement as compared to 3 DoF (1 DoF: 206%, 3 DoF: 96%, P < 0.001). Conversely, larger normalized amplitudes in response to TMS were found in the initial phase of the 3 DoF condition as compared to 1 DoF (1 DoF: 107%, 3 DoF 160%, P < 0.001). Additionally, during task execution the normalized amplitudes of TMS and H-Reflex testing revealed a relative decrease when compared to the initial phase of movement. It is suggested that presynaptic mechanisms caused the changes in the normalized H-reflexes (and thus in Ia afferent transmission) and that altered normalized responses to TMS reflect changes in corticospinal transmission. Consequently, we reason that the transmission in both pathways is sensitive to the nature of the mechanical interaction and thus to biomechanical task demands. Finally, the voluntary nature of the task might have caused a decisive influence of anticipatory control mechanisms triggered by advance information about the modality of environmental dynamics.