The market for wearable devices with sensors that can connect to an app and collect fitness data has exploded in the past 18 months and Fitbit has emerged as the market leader. Market leader: Fitbit was responsible for 68 percent of the full-body trackers sold in the year ending Jan. 4 Unhappy customers have taken to the Fitbit forum to voice their unhappiness with how the company is handling the problem In a statement, the manufacturer has suggested users may have an allergy to nickel which is contained in the device's stainless steel back Another poster on the Fitbit forum outlines the treatment that they've gone through to try and cure the mystery rash that has appeared beneath their device