Controlled unilateral isometric force generated by epidural spinal cord stimulation in the rat hindlimb

Epidural electrical stimulation (EES) has often been used to restore stereotypic locomotor movements after spinal cord injury (SCI). However, restoring freeform movement requires specific force generation and independently controlled limbs for changing environments. Therefore, a second stimulus location would be advantageous, controlling force separately from locomotor movements. In normal and transected rats treated with mineral oil or saline, EES was performed at L1-L6 vertebral levels, caudal to spinal segments typical for locomotion, identifying secondary sites capable of activating hindlimb musculature, producing unilateral force at the paw. Threshold for generating force was identified and stimulation amplitude and duration varied to assess effects on evoked forces. Stimulation at L2 and L3 vertebral levels elicited negative vertical forces from extensor musculature while stimulation at L4 and L5 elicited positive vertical forces from flexion musculature. Thresholds were unchanged with transection or hydration method. Peak force magnitude was significantly correlated to stimulus amplitude, and response duration significantly correlated to stimulus duration in all animals. No differences were found in correlation coefficients or slopes of the regression for force or duration analyses with spinal condition or hydration method. This model demonstrates the ability to induce controlled forces with EES after SCI.