Habituation And Fatigue In The Stretch Reflex Of The Ankle

Abstract

Many studies have investigated the role of inversion stretch on the reflex response of the ankle musculature, however none have considered the effect of habituation of the reflex over time. Furthermore, the influence of fatigue on the habituation of the stretch reflex at the ankle has yet to be evaluated. PURPOSE: To investigate the effect of fatigue and habituation on the stretch reflex of the ankle musculature in response to a sudden inversion perturbation. METHODS: A total of 40 subjects recruited from the university community participated and were divided into a treatment group [10 males (age: 20.7 ± 2.2 yr., height: 175.8 ± 5.9 cm, mass: 76.2 ± 10.1 kg) and 10 females (age: 19.7 ± 1.3 yr., height: 167.5 ± 8.4 cm, mass: 68.9 ± 10.4kg)] and a control group [10 males (age: 22.9 ± 2.3 yr., height: 181.3 ± 5.5 cm, mass: 85.0 ± 13.9 kg) and 10 females (age: 20.0 ± 0.9 yr., height: 164.1 ± 6.7 cm, mass: 63.0 ± 5.7 kg)]. All subjects performed 10 pretest trials for the reflex response to a sudden ankle inversion with electrodes placed on the Tibialis Anterior (TA), Peroneus Longus (PL), and Peroneus Brevis (PB) on their dominant leg. The treatment group was then fatigued on an isokinetic dynamometer to 50% of their max eccentric eversion force, while the control group sat quietly for 5 min. Subsequently, all subjects performed 10 posttest trials. The peak EMG value was calculated for each trial and all data were normalized to the 1 st pretest peak value. A group x trial mixed MANO VA was performed with the alpha level set at 0.05 a-priori. RESULTS: There was a significant group x trial interaction (p = 0.017) for the PL (p < 0.001) & PB (p < 0.001) muscles, indicating a fatigue effect on habituation of the reflex. A significant decrease over trials (p < 0.001) was noted for all three muscles. CONCLUSION: The decrease in reaction intensity over time indicates that habituation of the stretch reflex must be accounted for in future research. A high anxiety level present at the onset of testing followed by increased familiarization throughout a testing protocol, may help to explain this habituation. Furthermore, for the PL and PB muscles, a decrease was noted in the response intensity following the fatigue protocol, while the control group demonstrated dishabituation following the rest period. Therefore, fatigue effects must also be considered during reflex testing.