Chronic ankle instability is associated with changes in the nervous system that amount to increased difficulty in activating the stabilizing muscles of the ankle. Neuromuscular Electrical Stimulation involves using electricity to activate those muscles in bursts, and is commonly used to improve muscle function in those with ACL injury. This study will provide 5 treatments over 2 weeks in patients with Chronic Ankle Instability and determine if Electrical Stimulation can change neural excitability, balance, neuromuscular control, and perceived function in these individuals.

Individuals with joint injuries, including ankle sprain and anterior cruciate ligament (ACL) injury have been observed to exhibit changes in central nervous system function that potentially predispose them for further injury (Needle et al. 2017). In ankle sprains, repeated sensations of rolling and giving-way known as chronic ankle instability (CAI) emerges in nearly 50 percent of those with a history of ankle sprain (Holland et al. 2019), with symptoms tied to changes in central nervous system function. As the understanding of these pathologies have expanded, researchers have begun to attempt to identify neuromodulatory interventions capable of addressing injury-induced maladaptive neuroplasticity, thus improving function (Bruce et al. 2020, In Press).

Among those with ACL injury, one of the most common interventions implemented to overcome muscle activation deficits includes neuromuscular electrical stimulation (NMES) (Lepley et al. 2015). This intervention is often used in the initial stages of post-surgical recovery to improve quadriceps function; however, it's use in other populations of joint injury (i.e. ankle sprain) is far more limited. Some previous research has looked at the effects of NMES on acute ankle sprains, as this is the timeframe in which muscle activation deficits would be most evident (Wainwright et al. 2019), but there is very limited evidence in those with CAI. It was potentially thought that activation deficits are less evident and strengthening may overcome these deficits in those with chronic injury; however, new insights have identified additional mechanisms by which NMES may be effective (Lepley et al. 2015). Aside from generating activation of a generally inactive muscle, NMES when performed at high intensities has been described to improve neuromuscular function through disinhibitory mechanisms. That is that increased somatosensation from the electrical stimulation raises the central nervous sytem's awareness of that muscle's activation, yielding decreased inhibition and ultimately increased neural excitability.

Our previous research using cortically-directed interventions demonstrated that improving neural excitability yielded better function in patients with chronic ankle instability (Bruce et al. 2020). This study will follow a similar framework; however, determining if these changes can be induced via a peripheral intervention. These findings have the ability to reframe the current treatment for CAI.

We are pursuing the following 2 specific aims:

1. To determine if NMES changes neural excitability (MEP size, H:M ratio, silent period) compared to a placebo treatment in participants with chronic ankle instability.

   H1: NMES will increase MEP size, H:M ratio, and decrease cortical silent period in individuals with CAI compared to the placebo treatment.

2. To determine if changes in neural excitability related to NMES or placebo treatment result in improved function (balance, muscle activation, outcomes) in participants with chronic ankle instability.

H2: Increased neural excitability will yield improved balance (postural stability indices), muscle activation, and patient-reported function.

Inclusion Criteria:

Subjects will be healthy subjects between the ages of 18-35. The primary inclusion criteria for this study is the presence of chronic ankle instability (CAI). According to guidelines from the International Ankle Consortium, this means subjects will report having a history of one or more ankle sprains (the first >1 year ago), and repeated sensations of giving-way as measured by a score >10 on the Identification of Functional Ankle Instability instrument (IDFAI).

Exclusion Criteria:

• History of fracture or surgery to the legs
• Injury to the lower legs within 3 months prior to reporting for testing that resulted in modified physical activity.
• Currently involved in an ankle rehabilitation program.
• Failing to meet standards for the safe practice of transcranial magnetic stimulation and transcranial direct current stimulation (See questionnaire). Briefly, this includes personal or family history of seizure or epilepsy; current medication use that raises risk of seizure; implanted metal, medication devices, etc.; history of brain or heart surgery; and sensitivity of the scalp or skin.