Ultrasound based methods can assess muscle fatigue by penetrating deep muscles and are not susceptible to electrical noise, but traditional ultrasound probes have bulky size that hinders integration and wearability, making them impractical to continuously monitor muscle status. This paper proposed a wearable probe based on piezoelectric micromachined ultrasonic transducers (PMUTs), where a polymer acoustic lens was incorporated to focus ultrasonic beam and enhance the emitted acoustic pressure (increased by >20%). The fatigue dynamics of upper arm muscles during sustained exertion was investigated by continuously monitoring thickness changes with the PMUTs-based probe, revealing a temporal pattern characterized by swift and subsequent gradual thickening. Several muscle fatigue metrics were derived by piecewise linear fitting, including the initial and secondary deformation rate and the transition time, offering insight into muscle status and fatigue resistance. Additionally, an outlier elimination method for thickness profiles was utilized, mitigating the influence of uncontrollable motion on findings. Leveraging PMUTs technology and acoustic lens integration, this proposed wearable approach holds promise for real-time assessment of muscle fatigue. Display omitted •A compact, cost-effective PMUT-based wearable probe was fabricated to continuously monitor muscle fatigue.•Polymer acoustic Lens was incorporated to focus and enhance the emitted acoustic pressure (increased by >20%).•An outlier elimination technique for thickness profiles was introduced to mitigate uncontrollable motions' impact.•Fatigue metrics were derived to characterize muscle fatigue resistance or indicate fatigue onset.