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Stretchable and biocompatible hydrogel-based strain sensors have been considered as promising candidates for flexible wearable electronics in the light of their properties being similar to those of biological tissues. To date, excellent mechanical performances and high sensitivity are still challenging to satisfy the practical applications. In this work, a flexible strain senor is made of patterned liquid metal (LM) sandwiched by poly(acrylamide-co-acrylic acid)/Zr4+ (P(AAm-co-AAc)/Zr4+) hydrogel with a bilayer structure. The carboxyl–Zr4+ coordination bonds endow the prepared hydrogel substrate with moderate water content, as well as spectacular and adaptable mechanical properties. In addition, LM is doped with silicon oxide particles as a conductive layer to improve the sensitivity of the flexible sensor. The flexible sensor combines sensing performances of high sensitivity (maximum gauge factor = 7.16), an ultralow detection limit (0.1%), and a wide strain sensing range (up to 400%). LM-hydrogel strain sensors demonstrate reliable detection of repeated large strains and small vibrations of human movements. As a proof of concept, the sensor can be utilized to detect finger rehabilitation training in different periods owing to the controllable mechanical response. Furthermore, by introducing machine learning, the device with the sensor detects the writing words quickly and accurately, indicating that human activities have been recognized with proper training, which is an important premise for human–machine interaction. [ABSTRACT FROM AUTHOR]