Abbreviated title: Spinal modularity is preserved in chronic spinal cats.

Chronic spinal cats with neurotrophin-secreting fibroblast (NTF) transplants recover locomotor function (Boyce et al. 2007). To ascertain possible mechanisms, intraspinal microstimulation was used to examine the lumbar spinal cord motor output of four groups of chronic spinal cats: 1) untrained cats with unmodified-fibroblast graft (Opcontrol) or 2) NTF graft, and 3) locomotor-trained cats with unmodified-fibroblast graft (Trained) or 4) NTF graft (Combination). Forces generated via intraspinal microstimulation at different hindlimb positions were recorded and interpolated, generating representations of force patterns at the paw. EMGs of hindlimb muscles, medial gastrocnemius, tibialis anterior, vastus lateralis and biceps femoris posterior, were also collected to examine relationships between activated muscles and force pattern types. The same four force pattern types obtained in spinal intact cats (Lemay and Grill 2004) were found in chronic spinal cats. Proportions of force patterns in spinal cats differed significantly from those in intact cats, but no significant differences in proportions were observed between individual spinal groups (Op-control, NTF, Trained and Combination). However, the proportions of force patterns differed significantly between trained (Trained and Combination) and untrained groups (Op-control and NTF). Thus, the frequency of expression of some response types was modified by injury and to a lesser extent by training. Force pattern laminar distribution differed in spinal cats compared to intact, with more responses obtained dorsally (0-1000 μm) and fewer ventrally (3200-5200 μm). EMG analysis demonstrated that muscle activity highly predicted some force pattern