Assisting users and restoring human locomotion for patients with lower limb spasticity is a challenging task. Studies focusing on patients with abnormal walking behavior are scarce because there is an important variability from one patient to another. Those patients could benefit the most from rehabilitation and assistive mechatronic devices, there is no generic controlling scheme or any dynamical gain indicator. This contribution introduces a bio-kinematic index which is called Neuro-motor index (NMI), based on electromyographic (EMG) and joint angles measurements. NMI is derived from the nonlinear regression with a combination of two co-contraction indices (CCI), which allows addressing the variability of walking situations. This new index is evaluated on patients with cerebral palsy and a stroke. Then, the estimation error was calculated in comparison with the other co-contraction indices. This estimation shows that this index has the highest signification of joint angles prediction. Thus it can be suitable in adaptive rehabilitation control for spasticity cases.