a Experimental protocol and sensor placement. The subject performs (1) sitting, (2) walking/running at 2.8, 5, 7.2, and 9 km h−1, and (3) walking at 6 km h−1 in a thermal chamber (28 °C, 70% relative humidity). Battery-free sensors provide temperature and strain measurements when interconnected to a smartphone. A gyroscope attached at the ankle is used as reference for gait monitoring. b Comparison of a wired and wireless monitoring system. c, d Images of near-field-enabled shirt (c) and pants (d) for sensor interconnection. e Battery-free temperature sensor node calibration compared to wired thermocouple. f Axillary temperature Ta recorded by the smartphone during the exercise protocol. g Representative strain sensor and gyroscope measurements during walking at 2.8 km h−1. Δt, stride time given by the peak-to-peak interval. h Δt measured by the strain sensor node and gyroscope during phase (2). i Comparison of Δt and stride time variability (STV) acquired by the strain sensor node and gyroscope. Error bars show mean ± s.d. (n = 120 measurements). j, k Δt and STV measured by the strain sensor node (j) and gyroscope (k) during phase (3).