Lower thoracic spinal cord stimulation (SCS) results in expiratory muscle activation and may be a useful method to restore an effective cough mechanism in patients with spinal cord injury. Stimulation applied on the dorsal epidural surface at the T₉-T₁₀ spinal cord level results in the greatest changes in airway pressure with a single electrode lead. In dogs, two groups of studies were performed to evaluate the mechanism of the expiratory muscle activation during T₉-T₁₀ level stimulation. In one group (n=4), expiratory muscle activation caudal to the stimulating electrode was monitored by evoked muscle compound action potentials (CAPs) from the internal intercostal muscles in the 10^th, 11^th and 12^th interspaces, and portions of the external oblique innervated by the L₁ and L₂ motor roots. SCS, applied with single-shocks, resulted in short latency CAPs at T₁₀ but not at more caudal levels. SCS resulted in long latency CAPs at each of the more caudal recording sites. SCS was also applied before and after sequential sectioning of the dorsal columns, lateral (LF) and ventral (VF) funiculi just below the T₁₁ spinal cord level. Bilateral dorsal columns sectioning did not affect the short latency CAPs but abolished the long latency CAPs below the level of sectioning and also resulted in a fall in airway pressure generation from 58±2 to 31±4 cmH₂O (p<0.01). Sectioning of the LF and VF resulted in only small further decrements in pressure generation. In the second group of animals (n=11), sequential spinal root sectioning was performed to assess their individual mechanical contribution to pressure generation. Section of the ventral roots from T₈ thru T₁₀ resulted in negligible changes while section of more caudal roots resulted in a progressive reduction in pressure generation. We conclude that: a) SCS at the T₉-T₁₀ level results in direct activation of spinal cord roots within 2-3 segments of the stimulating electrode, and activation of more distal roots via spinal cord pathways, and b) pathway activation of motor roots make a substantial contribution to pressure generation.