Unloading reactions in functional ankle instability

Abstract

Past studies have suggested that unloading reactions may be a strategy to prevent ankle sprains. This study tested unloading reactions in individuals with functional ankle instability (FAI) in order to better understand this phenomenon. We provoked unloading reactions in 20 individuals with FAI and 18 healthy controls by delivering nociceptive electrical stimulation to the lateral aspect of the ankle during standing. Ground reaction forces, lower extremity kinematics, and EMG activity of five muscles of the lower limb were recorded. Individuals with FAI demonstrated increased and faster body weight unloading after stimulation. This hyper-reactivity may partially account for the sensation of the ankle “giving way” in those with a history of severe ankle sprain and subsequent functional instability.

Introduction

Lateral ankle sprains are common [1], and many individuals will develop functional ankle instability [2] following injury. FAI was described by Freeman [3] as a condition where individuals experience recurrent sprains or a feeling of the ankle “giving way.” Although this is argued to be caused by an impaired “stabilization reflex of the foot,” [3] the neuro-physiological mechanisms of FAI remain unclear.

Past studies have suggested that reflexive activation and pre-activation of the peroneal muscles may protect against sprain injuries [4], [5]. A reduction in vertical loading may also reduce the severity of injury, since ankle sprains are frequently associated with downward loads applied to the supinated foot [5]. To reduce the vertical load during a potential injury event, individuals may use flexion reflexes (FR), also known as “unloading reactions.” These reactions are provoked by cutaneous stimulation of high and low threshold afferents identified as flexor reflex afferents (FRA) [6].

Santos and Liu [7] recently examined unloading reactions in a group of healthy individuals by applying electrical stimulation at the lateral aspect of the ankles in standing. When stretched (foot supinated), individuals showed greater unloading reactions than when in a neutral position. Unloading reactions were characterized by simultaneous movement of the body downwards and a shift of body weight towards the non-stimulated foot. This suggested that unloading reactions might help to reduce the risk of ankle sprain injuries.

Usually, individuals with FAI show chronic synovial inflammation at the ankle joint (sinus tarsi) [8]. This can sensitize the neurons involved in pain transmission [9]. Pain or fear of ankle injury may possibly lead to a residual hypersensitivity. In turn this could account for the symptom “giving way”, a typical sign of FAI. Therefore, the purpose of the present study was to test the sensitivity of unloading reactions in individuals with FAI. Unloading reactions were induced by delivering nociceptive electrical stimulation on the lateral aspect of the affected ankle while standing with the stimulated ankle in a supinated position. Ground reaction forces, lower extremity kinematics, and EMG activity of five muscles of the lower limb were recorded and subsequently compared with a matched group of individuals with no ankle impairment. It was hypothesized that subjects with FAI would show increased magnitudes of unloading reactions (hyper-reactivity) compared with unimpaired individuals.