Vestibular actions on back and lower limb muscles during postural tasks in man

Abstract

The vestibular system was activated by galvanic electrical stimulation in 19 normal subjects. With the head turned to one side so that the stimulating anode was on the posterior mastoid process, stimulation caused standing subjects to sway backwards in the sagittal plane. Electromyography showed bilateral activation of erector spinae, gluteus maximus, biceps femoris, soleus and intrinsic foot (toe flexor) muscles. When head direction or electrode polarity was reversed so that the anode was anterior, all those muscles became less active and the subjects swayed forwards. With the head facing forward, stimulation caused sideways sway in the coronal plane, towards the anode, with excitation of the erector spinae on the anode side and reduced activity on the cathode side. The limb muscles were activated on the side opposite the anode and showed complex responses on the anode side. Responses were detectable in the erectores spinae muscles in sitting subjects. No responses in limb muscles were detected in the sitting posture. Subject responses in erector spinae recorded at L3/L4 had latencies from 59 to 110 ms, using a 2 mA stimulus. Latencies in lower limb muscles were longer. The results suggest a role for the vestibular system and descending brain stem motor pathways to the erectores spinae muscles in the control of postural orientation of the back when sitting and standing. The conduction velocity in the motor pathway was estimated to be 13 +/- 10 m s(-1) (mean +/- S.D., n = 12 subjects).

Figure 1. EMG responses in soleus and erector spinae muscles to galvanic vestibular stimulation: signal processing

The subject was standing with the head turned around 60 deg to the right and the stimulating anode was placed on the right (posterior) mastoid process. A, EMG responses recorded from the right soleus muscle. Each record is the average of 120 responses. The upper record is an average of the raw EMG responses; the middle record consists of the same responses full-wave rectified and then averaged. The lower record consists of the rectified responses integrated with a 10 ms time constant and then averaged. The vestibular system was stimulated (2 mA) from time 0 to 0.4 s. A period of 0.1 s was recorded before stimulus onset in order to provide a measure of background EMG level (which is large because the subject was leaning forward). A ML response is present at a latency of 85 ms. B, for comparison, EMG responses from the right erector spinae muscle recorded at L3/L4. Each record is the average of 160 responses. A ML response is present at a latency of 60 ms.

Figure 2. Soleus ML response latency: dependence on galvanic stimulus strength

The subject was standing with the head turned around 60 deg to the left and the stimulating anode was placed on the left (posterior) mastoid process. The ML response latencies were measured from averaged rectified and integrated EMG records from the right soleus muscle. Data from two experiments performed 1 year apart on the same subject are plotted. ML latency is shortest for stimuli of 2 mA or larger. Weaker stimuli evoke responses at longer latencies.

Figure 3. Responses in erector spinae and lower limb muscles to vestibular stimulation inducing sway in the sagittal plane

The subject was standing with the head turned about 60 deg to the side and leaning forward. A, excitatory EMG responses recorded from erector spinae at L3/L4 and lower limb muscles on the side to which the subject was facing and with the stimulating anode facing to the posterior. B, inhibitory EMG responses recorded from erector spinae and lower limb muscles with the stimulating anode facing to the anterior. Each record is the average of 160 or more responses. All responses have had the background EMG level (recorded from – 0.1 to 0 s) subtracted (the levels were 62, 20, 43, 84 and 33 μV, from the top to the bottom records).

Figure 4. Responses in erector spinae and lower limb muscles to vestibular stimulation inducing sway in the coronal plane

The subject (the same as in Fig. 3) was standing facing forward and leaning forward. A, EMG responses recorded from muscles on the same side as the stimulating anode. B, EMG responses recorded from muscles on the side of the stimulating cathode. Each record is the average of 120 or more responses. Background EMG level has been subtracted (the levels were 13, 13, 67, 39 and 23 μV, from the top to the bottom records). In the soleus and foot flexor muscles the ML responses are preceded by large SL responses.

Figure 5. Responses in erectores spinae muscles in a sitting subject

The subject was sitting and leaning forward. A, EMG responses recorded from the right erector spinae at L3/L4. B, EMG responses recorded from the left erector spinae at L3/L4. Upper records were obtained during sway in the sagittal plane (anode on the right mastoid process, head turned to the right). Lower records were obtained during sway in the coronal plane (anode on the right, head facing forward). Each record is the average of at least 120 responses. Background EMG level (10, 22 μV) has been subtracted.