Role of Sensory Deficits and the Effect of Sensory Stimulation on Sensorimotor Control and Neuromuscular Performance in Athletes With Chronic Ankle Instability

Characteristics of the chronic ankle instability (CAI): Lateral ankle sprain is one of the most common musculoskeletal injuries in the sports clinics. The high prevalence of lateral ankle sprains, combined with a high recurrence rate and persistent post sprain symptoms make lateral ankle sprains and their sequelae a significant health impact and healthcare burden. Although lateral ankle sprains are generally considered benign injuries, it is estimated that 40% of individuals with lateral ankle sprains will subsequently develop chronic ankle instability within the first year after a lateral ankle sprain.

Chronic ankle instability is characterized by repetitive sprains, perceived instability, and giving way, which can be classified into two categories: mechanical and functional ankle instability. Mechanical instability of the ankle is a result of loss of mechanical structural constraints (i.e., ligaments); whereas functional instability refers to instability related to altered postural control and reduced functional capacity due to impaired sensorimotor control including altered muscle recruitment around the ankle and impairment of the proprioceptive system etc.. In addition to interfering with sporting performance, and CAI has been found to be associated with lower quality of life and increased likelihood (68%-75%) of developing ankle arthritis. Hertel reported that individuals with ankle instability had higher risk of recurrent ankle sprains, and the treatment for symptoms of CAI could be time-consuming and costly. Numerous impairments have been reported associated with CAI, ranging from loss of joint mobility and muscle strength, compromised proprioception and sensation, poor postural control and neuromuscular performance, and altered movement patterns. However, the mechanisms and management strategies are still not clearly understood in many aspects. And thus, a great variety of intervention has been proposed with inconsistent results reported.

Conceptual model for the development of CAI: The theory linking poor neuromuscular control and CAI has been discussed for more than half a century. It was believed that after the ankle sprain, some of the joint receptors and peripheral nerve fibers around the ankle joint were affected, resulting in impaired reflex responses and functional stability of the ankle. More researchers later reported that CAI not only presented with sensory deficits, but also deficits in motor control, and these changes in sensorimotor system would eventually lead to loss of function. In 2019, Hertel et al. (2019) revised the current model for the development of CAI. It is proposed that the initial ankle injury would influence the neural system, disturbing the balance and stability between the sensation, perception and motor function. These disturbances would lead to self-adjustment of the sensorimotor system to attempt to adapt to the altered condition, and consequently might result in long term changes in neural network, and expose individuals to persistent symptoms of instability. This newly revised model integrates both peripheral and central sensorimotor systems to explain the development of CAI; but it clearly shows that the central sensorimotor processing in individuals with CAI is less explored, particularly for the impact of CAI on the somatosensory cortex.

Individuals with CAI have poorer neuromuscular performance: The effect of CAI on the neuromuscular control and posture stability has been extensively studied. The research topics of interest included altered reflexes and neuromuscular inhibition, muscle activation patterns (timing, recruitment order, and amplitude) and joint mechanics of the lower extremity segments during sudden inversion perturbation or jump landing, and posture control characteristics during balance testing tasks. It is generally agreed that people with CAI had lower Hmax/Mmax ratios (H reflex/M reflex ratios) of the soleus and peroneus longus muscle than those un-injured controls, indicating that neural excitability of the spinal level in those with CAI was depressed; this population also presented with altered muscle activation of the peroneus, tibialis anterior and soleus muscles, and changed balance control strategies. Recent publications of systematic review and meta-analysis by Hoch and McKeon (2014) and by Thompson et al. (2018) showed that there was a strong evidence to support dynamic balance, peroneal reaction, and eversion strength deficits contributing to CAI. Because of the strong evidence showing motor impairment in CAI, the dominant intervention strategy for this population has been motor performance driven so far.

Deficits in somatosensation in people with CAI Proprioceptive deficits in CAI have been the main focus of research for decades. Thompsons et al. (2018) indicated that there was moderate evidence supporting both active and passive proprioception deficits in CAI as compared with controls. According to the latest systematic review and meta-analysis by Xue et al. (2021), CAI-affected ankles had poorer ankle kinesthesia in both inversion and plantarflexion when compared with the contralateral side; and individuals with CAI showed kinesthesia defects and active joint reposition deficits in the frontal plane movement (both inversion and eversion) when compared to the healthy controls. Not until recent years that more attention has been drawn to deficits in cutaneous sensation. Hoch et al. (2012) found that the plantar surface was less sensitive to vibratory tactile stimulation as compared to non-CAI people at the head of the first metatarsal, the base of the fifth metatarsal, and the heel, along the innervation of the branches of the sural and tibial nerves. Burcal and Wikstrom found similar results that people with CAI displayed higher Semmes Weinstein monofilament thresholds at the head of the first metatarsal, base of the fifth metatarsal, and sinus tarsi31. The possible involvement of impaired cutaneous sensation in CAI have led to the recent approach of sensory-targeted intervention in this population.