At present, commercially available wearable sensors are only capable of tracking an individual’s physical activities and vital signs (such as heart rate). However, these devices fail to provide insight into fatigue itself. Fatigue is a broad term with many meanings. Fatigue in this paper refers to physiological changes where the muscle becomes fatigued due to increased lactate concentration during high physical activities. Real-time measurements of thermal sweat out of the eccrine glands could enable such insight because it contains physiologically and metabolically rich information that can be retrieved non-invasively. Thermal sweat that secretes out of the eccrine glands contains multiple inorganic constituents such as salt and potassium, nitrogen compounds such as ammonia and urea, and sugars such as glucose and lactate. Most literature agrees that ammonia is a potential constituent for determining the state of muscular fatigue. Different methods were proposed for real-time ammonia monitoring. Out of all these methods, a concept was designed using MOS sensors which measures ammonia concentration in a gas phase. This design was tested by letting participants perform an incremental cycling exercise up to the point while monitoring the ammonia concentration, temperature, humidity, and respiratory exchange ratio to validate whether the sensor is able to determine fatigue. The results are inconclusive whether it is able to determine fatigue through real-time ammonia monitoring.