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Techniques for controlling the transmission power of mobile devices have been widely studied in MANETs and cellular networks. However, as mobile applications for WSNs emerge, the unique characteristics of WSNs, such as severe resource constraints, suggest that transmission power control should be revisited from a WSN perspective. In this work, we take an experimental approach to examine the effectiveness of transmission power control for WSNs that involve mobility at human walking speeds. We propose two lightweight transmission power control schemes to improve energy efficiency and spatial reuse. The first is an active probing based scheme that adjusts transmission power based on (the lack of) packet losses and applies to all low-power radios, while the second scheme requires radios that offer link quality indicators (LQI) to estimate the proximity between the transmitter and receiver. We evaluate both schemes using mobile nodes in an indoor and an outdoor environment. Results show that the energy efficiency of the proposed transmission power control schemes can be very close to that of the optimal offline strategy and our schemes significantly reduce the interference for spatial reuse. To our knowledge, this is the first work that evaluates the effect of transmission power control in mobile WSNs.