Corticospinal Excitability of the Lower Limb Muscles During the Anticipatory Postural Adjustments: A TMS Study During Dart Throwing

Abstract

Objective: To investigate whether the changes in the corticospinal excitability contribute to the anticipatory postural adjustments (APAs) in the lower limb muscles when performing the ballistic upper limb movement of the dart throwing. Methods: We examined the primary motor cortex (M1) excitability of the lower limb muscles [tibialis anterior (TA) and soleus (SOL) muscles] during the APA phase by using transcranial magnetic stimulation (TMS) in the healthy volunteers. The surface electromyography (EMG) of anterior deltoid, triceps brachii, biceps brachii, TA, and SOL muscles was recorded and the motor evoked potential (MEP) to TMS was recorded in the TA muscle along with the SOL muscle. TMS at the hotspot of the TA muscle was applied at the timings immediately prior to the TA onset. The kinematic parameters including the three-dimensional motion analysis and center of pressure (COP) during the dart throwing were also assessed. Results: The changes in COP and EMG of the TA muscle occurred preceding the dart throwing, which involved a slight elbow flexion followed by an extension. The correlation analysis revealed that the onset of the TA muscle was related to the COP change and the elbow joint flexion. The MEP amplitude in the TA muscle, but not that in the SOL muscle, significantly increased immediately prior to the EMG burst (100, 50, and 0 ms prior to the TA onset). Conclusion: Our findings demonstrate that the corticospinal excitability of the TA muscle increases prior to the ballistic upper limb movement of the dart throwing, suggesting that the corticospinal pathway contributes to the APA in the lower limb in a muscle-specific manner.

Keywords: center of pressure (COP); central command; motor evoked potential (MEP); motor imagery ability; postural control; three-dimensional motion analysis; transcranial magnetic stimulation.

Figure 1

Illustrations of the experimental setup. (A) Data collection of the motor evoked potential (MEP) in the lower limb muscles during dart throwing. (B) The timing of TMS application in protocol 2. TMS was applied at the visual cue, 100, 50, 0 ms prior to the TA onset and the TB onset. TMS, transcranial magnetic stimulation; TA, tibialis anterior; TB, triceps brachii.

Figure 2

Rectified and averaged EMG activities of the AD, TB, TA, and SOL muscles (A, n = 15), displacement of the COP (B, n = 15), and the joint movements in the upper and lower limbs (C, n = 9) in the time course. All the data were aligned to the EMG onset of the TB muscle. Error bar indicates the SD. EMG, electromyography; AD, anterior deltoid; TB, triceps brachii; TA, tibialis anterior; SOL, soleus; COP, the center of pressure.

Figure 3

Relationships between the onset of the TA muscle and COP (n = 15) or elbow joint movement (n = 9). The onset timings were calculated by the onset of the TB muscle (0 ms, the TB onset). COP, the center of pressure; TA, tibialis anterior; TB, triceps brachii.

Figure 4

Rectified EMG activities of the BB, TB, TA, and SOL muscles aligned to the EMG onset of the TB muscle (A, n = 7) and correlation of the EMG onset of the TA muscle compared to the BB or TB muscle (B, n = 7). Note that the EMG onset in (B) was calculated by the duration from visual cue to the EMG onset of each muscle. BB, biceps brachii; TB, triceps brachii; TA, tibialis anterior; SOL, soleus.

Figure 5

Original tracings demonstrating the AD, TB, TA, and SOL muscle in the EMG activities during the dart throwing (A), representative recordings of the MEP in the TA muscle (averaged five trials, respectively) (B), and the average changes at the time points tested (C) (n = 13). Error bar indicates the SD. *p < 0.01 significant difference from the control or visual cue. AD, anterior deltoid; TB, triceps brachii; TA, tibialis anterior; SOL, soleus; MEP, motor evoked potential.

Figure 6

Relationships between the VI or KI score and the MEP in the TA (n = 13) and SOL (n = 12) muscles at the timings of 0, 50, and 100 ms prior to the EMG onset of the TA muscle. VI, visual imagery; KI, kinesthetic imagery; TA, tibialis anterior; SOL, soleus, MEP, motor evoked potential.