Muscle synergy similarities and differences in the intact, neonatal and adult complete spinal cord injured rats, after injury and following several rehabilitation strategies
Authors:   , Yang Qi , Giszter Simon F. 
Published: July 2021

Loss of lower limb motor functions is among the most commonly seen effects of spinal cord injuries (SCI). Even with the current modern medical care, SCI patients require intense care at high expense. Animal models used for research on SCI may help develop better and lower cost therapies. It is well known that neonatal rats with complete SCI are capable of generating autonomous lumbar stepping, some even achieve independent weight support when tested as adults. On the contrary, rats injured at adult ages have limited recovery after a complete spinal cord injury. What can account for the difference? We hypothesized that neonatal and adult SCI animals' muscle synergy patterns were distinguished from each other and from the intacts'. Although sharing a lot of similarity, compared to the adult SCI animals, neonatal SCI animals had less synergies merging. Some xiv individual synergies of the neonatal SCIs' might resemble the intact animals more than adult SCIs'. To test the hypothesis, we examined the muscle synergy pattern using locomotor electromyography (EMG) in adult animals injured as neonates (T9/10 complete SCI, n=9), intact adult rats (n=12) and the same adult rats 10~14 days after T9/10 complete SCI (n=9). We found that adult SCI animals' synergies tended to merge post injury compared to the intact animals. The intacts also deviated from the neonatal and adult SCI animals regarding the correlation of all the synergies and individual synergy to a common synergy template. While sharing some similarity, in some individual synergies' correlation values to the common template, the neonatal animals were more similar to the adult SCIs shortly after injury than to the intacts. The neonatal synergies appeared to be preserved into adulthood and revealed after adult SCI. If given systematical training, would the synergies of adult SCI animals change with function? And if they do, would the change be the same regardless of the rehabilitation paradigms and recovery outcome? In order to answer these questions, we studied the synergy changes following robot-driven epidural stimulation combined with treadmill training (ES) and robotic assisted treadmill only (TM) systematic long-term training and we compared these to the animals resting in the cage after complete adult SCI. We hypothesized that synergies would tend to merge right after injury. As time went by after injury, the synergies' spatial structure would be simplified. However, with successful rehabilitation, the further merging and simplification of the synergies were avoided. To investigate the only (TM) systematic long-term training and we compared these to the animals resting in the cage after complete adult SCI. We hypothesized that synergies would tend to merge right after injury. As time went by after injury, the synergies' spatial structure would be simplified. However, with successful rehabilitation, the further merging and simplification of the synergies were avoided. To investigate the synergies' relationship to therapeutic methods and efficacy, we trained three groups of animals with ES training, TM training and in cage resting respectively. We examined their synergies through 8 weeks' rehabilitation period. ES and TM training showed beneficial effects on recovery after SCI. Synergy changes were linked with functional locomotion recovery. Unlike cage resting, the effective training harnessed and separated the synergies merging post injury and each synergy ended with more refined spatial patterns. However, although both ES and TM helped SCI animals recovered successfully, their underlying neural mechanisms were not the same in terms of synergy changes. Our study suggested that the TM training likely did not reverse the merging process to the extent that the ES training did.