Recovery of reflexes at the elbow after surgery on severe brachial plexus injuries

The goal of this study is to quantify the recovery of spinal reflexes at the elbow after neurosurgical intervention in patients with brachial plexus injuries. So far, main focus was on the recovery of muscle force and little on sensory- and reflex system. As reflexes play an important role during normal movement, it is of interest to determine to what extend reflexes have been restored after surgery. Arm admittance (dynamic relationship between displacements in a response to forces) at endpoint level (hand) was estimated using force perturbations in two directions (horizontal) applied by a two-joint robotic manipulator. Three different task instructions were used to provoke different intrinsic and reflexive behavior, being a position task (PT), a relax task (RT) and a force task (FT) where the subject was required to minimize hand displacements, not react to the perturbations and minimize force deviations (being compliant) respectively. Ten patients with brachial plexus lesions participated in this experiment and were suffering from varying degrees of arm dysfunction. All had successful recovery for the biceps (MRC grade 3 and higher) after surgical nerve repair of the n. musculocutaneous. Estimated intrinsic and reflexive parameters were compared to those of a control group (n = 10, age and sex matched). The task instructions had great influence on the admittance, especially between the PT and RT. In all patients, reflexive activity was found corresponding to assumed muscle spindles (velocity- and position feedback) and Golgi tendon organ (force feedback) function. For the PT, the difference in parameters between patients and control subjects was largest. Overall, patients exhibit more intrinsic stiffness at the shoulder and elbow than the control subjects, an indication of co-contraction, and less reflexive feedback at the elbow. There are two possible explanations for this: 1) The intrinsic and reflexive properties did not recover to the combination as before the injury and are not cooperating correctly, and 2) The patients are relying more on intrinsic control than on reflexive control (different control strategy). Whereas the control group uses an energy efficient approach, i.e. less intrinsic and more reflex activity, the patients appear to use a more maximal activation approach resulting in co-contraction. It is possible that the exercises performed during rehabilitation which focus on muscle force do not provoke enough reflexive behavior. More research, e.g. experiments that are designed to disable co-contraction, is needed to verify if the use of co-contraction is learnt or a necessity. Conclusion: reflexes do recover after surgery to severe Brachial plexus injuries, the amount of reflex function is less or less effective than for the control group.BMEBioMechanical EngineeringMechanical, Maritime and Materials Engineerin