Biomechanical analysis of stretch reflex responses : an approach to spasticity measurement
Spasticity is a clinical condition that may develop in people with central nervous system injuries. It is believed that spasticity results from changes in the excitability of the stretch reflex pathways manifesting clinically as a velocity dependent increase in resistance to passive movement (RTPM) and exaggerated tendon jerks. Stretch reflex excitability is influenced by neural (e.g. feed-forward and feedback mechanisms) and biomechanical components (e.g. muscle length). The objective of this work was to quantify the stretch reflex parameters of the biceps brachii under different initial conditions (amplitude of applied torque, initial muscle length, initial voluntary activity, head position) in non-impaired (NI) volunteers and stroke patients (SP) with diagnosed upper limb spasticity and objectively evaluate their differences. A biomechanical device was designed to provide a 90 ms initially applied torque controlled stretch to the biceps brachii. The stretch reflex response was recorded using surface electromyography and angular displacement with a potentiometer. Stretch reflex characterisation was done on EMG data collected 150 ms before and to complete 450 ms after the perturbation. The outcome measures were the amplitude of the rectified reflex response and, the latency, rise time and duration reflex response. Lower amplitudes, shorter latencies and longer durations were observed in the poststroke populancn when compared to the non-impaired volunteers. Amplitude results were unexpected. However latencies and durations suggest increased stretch reflex excitability. Significant differences dependent on the initial conditions were found within the non-impaired volunteers. No differences were found in the post-stroke population. These latter results suggest lack of modulation of the stretch reflex excitability after stroke. More research is necessary to understand the relationship between the changes in the stretch reflex excitability and the clinical concept of spasticity and the importance of their quantification to improve the quality of life of people with neurological lesions.