Latency measurements in a study using transcranial magnetic stimulation (TMS) with surface electromyographic (EMG) recordings from intercostal muscles have been used to evaluate patients with cervical myelopathy (Misawa et al. 2001). We have now extended this technique in a group of control subjects to examine the pattern of facilitation with changing voluntary effort at all six intercostal muscle levels.

With local ethical approval and informed consent, nine healthy volunteers (aged 29-59 years; 5 males, 4 females) were recruited and seated comfortably. TMS was delivered using a MagStim 200 stimulator connected to 9 cm circular coil centred over the vertex with the induced current flowing clockwise. Surface electrodes were placed in each intercostal space on the right side of the body at approximately 2 cm from the sternal edge (1st-4th spaces), at the mid-clavicular line (6th space), and half way between (5th space). Recordings were made at maximum expiratory effort (MEE), at 50, 25, 10 and 5 % of MEE and at maximum inspiratory effort (MIE). Subjects breathed through a tube with a slow leak connected to a pressure meter, which was used to give them feedback of their effort. TMS intensity was set to 1.2 X threshold for evoking motor-evoked potentials (MEPs) at 10 % MEE. MEPs were rectified and averaged for each trial and the areas of the resulting MEPs were measured at each recording site.

MEPs were recorded from each muscle in all subjects at each voluntary effort. The mean (± S.E.M.) latencies of MEPs recorded during MEE were 8.8 ± 0.9 ms (1st space), 9.4 ± 1.2 ms (2nd space), 8.9 ± 0.5 ms (3rd space), 11.5 ± 1.6 ms (4th space), 10.3 ± 1.6 ms (5th space) and 10.8 ± 1.5 ms (6th space). Areas of MEPs recorded during MIE were 126 ± 52 % of those recorded during MEE (1st space), 103 ± 47 % (2nd space), 64 ± 18 % (3rd space), 89 ± 14 % (4th space), 65 ± 14 % (5th space) and 75 ± 22 % (6th space); due to the high level of variability, the difference was significant (paired t test, P < 0.05) only at the 3rd space. The area of MEPs became larger with increasing voluntary effort at all recording sites (ANOVA with Tukey correction; P < 0.05). For example, at the 1st space the mean area of MEPs was 23 ± 4 % of that at MEE (at 5 % MEE), 38 ± 8 % (at 10 % MEE), 45 ± 7 % (at 25 % MEE) and 70 ± 7 % (at 50 % MEE). There were no differences (ANOVA on ranks; P > 0.05) in MEP areas (relative to MEE) between recording sites at any given voluntary effort.

The pattern of facilitation of MEPs with increasing effort is altered in hand muscles of patients with incomplete spinal cord injury (Davey et al. 1999). We anticipate that these control data from intercostal muscles will help us to differentiate both the neurological level and degree of completeness of high thoracic spinal cord injury.

This work was supported by the International Spinal Research Trust.