In the energy-harvesting field, one of the key issues is how to realize efficient energy extraction from ultra-low frequency excitation sources. To offer a solution to this issue, this paper presents a monostable electromagnetic energy harvester (EMEH) that is composed of a magnet-spring resonator encapsulated in a tube, a set of coil wrapped around the tube, and two endmost magnets affixed at the tube's two ends. The three magnets are arranged in such a way that the attractive interaction is applied on both sides of the movable center magnet. Theoretical simulations and experimental tests under harmonic excitations indicate that the proposed EMEH features monostability, exhibits typical softening response, and enables the shift of the operating frequency band toward the left (lower frequency). Under the hand-shaking induced excitation, the proposed EMEH can light up 48 light-emitting diodes (LEDs) or enhance the voltage across a 47 mu F capacitor from 0 V to 4 V within 2 s. The experiments conducted on a treadmill show that the fabricated prototype can generate approximately 0.5 mW of power with it attached on the leg vertically under walking and 0.7 mW of power when it is attached on the leg parallelly under running. (C) 2018 Elsevier Ltd. All rights reserved.