The velocity revolution in modern baseball has dramatically increased mechanical and metabolic demands on pitchers, elevating injury risk through cumulative upper extremity loading and impaired recovery. Traditional fatigue markers fail to capture real-time metabolic stress during pitching. Here, we report the first-in-human demonstration and validation of a minimally invasive wearable microneedle-based electrochemical biosensor for continuous interstitial fluid (ISF) lactate monitoring in Division I collegiate pitchers (n = 11), toward real-time performance tracking in real-world athletic settings. The wearable device integrates lactate oxidase-modified platinum microneedles with wireless telemetry and was applied to the non-dominant arm and calibrated against capillary point-of-care (POC) blood lactate. Pitchers underwent a standardized 2-h protocol including baseline rest, performance testing, warm-up, simulated acute pitching, and post-pitch grip strength assessment, with synchronous heart rate data acquisition. ISF lactate profiles revealed dynamic elevations during pitching (peak Δ ∼4.1 mmol/L from baseline 3.1 ± 1.1 mmol/L) that resolved within 20-30 min post-activity, correlating strongly with blood lactate levels and heart rate (HR) excursions. Unlike discrete blood sampling, continuous ISF monitoring captured transient lactate spikes associated with pitch intensity, speed, and inning transitions, revealing early signs of metabolic strain. Such ability to continuously monitor lactate in athletics holds considerable promise for physiological monitoring across professional sports towards performance enhancement and personalized athlete care.