This study aims to understand the mechanisms of a novel intervention involving breathing short durations of low levels of oxygen for persons with multiple sclerosis (MS). This intervention with low levels of oxygen is called Acute Intermittent Hypoxia (AIH), the levels of oxygen experienced are similar to breathing the air on a tall mountain, for less than 1 minute at a time. Previous studies have shown that AIH is a safe and effective way to increase strength in persons with MS. Here the investigators aim to look at brain activation and ankle strength before and after AIH to gain a better understanding of how the AIH may improve strength in those persons with MS.