Physical inactivity is associated with elevated risks for many chronic diseases and premature death. Both rodent and human studies indicate that sex steroids—testosterone and estrogen—play a role in the biological regulation of physical activity. The sex steroids likely affect brain physiology and/or the cellular condition of the skeletal muscle of the hind limb muscles used to complete a wheel running task. Differences in speed patterns between physiologically different sex steroid levels may indicate a deficiency in the skeletal muscle's capability to effectively contract during wheel running, while differences in the accumulated duration pattern during wheel running may indicate a deficiency in central motivation to complete a prolonged wheel running bout. The purpose of this study was to assess wheel running speed and duration patterns in male C57BL/6J mice under normal and physiologically‐deficient testosterone conditions. Wheel running data were collected for ten days (n=14 physiologically normal; n=12 testosterone deficient) and each wheel revolution was counted and time‐stamped to accumulate a daily turn‐by‐turn wheel revolution record. Testosterone deficiency was induced via bilateral orchidectomy ten‐days prior to wheel running assessment. Ten‐day average speed (m·min−1) and accumulated duration (% of total activity during the dark period) were assessed in four 3‐hour epochs during the dark period (epoch 1: 6pm–9pm, epoch 2: 9pm–12am, epoch 3: 12am–3am, epoch 4: 3am–6am). Differences in speed and duration data across epochs and between treatment groups were compared by separate 4 × 2 mixed‐design ANOVAs. Wheel running speed was significantly [F(3,72)=6.5, p=0.001] different across epochs and between treatments. Wheel running speed remained consistent across all epochs in intact mice but decreased from a zenith during the initial epoch (mean±sd: 30.1±5.0 m·min−1) to a nadir (26.9±3.8 m·min−1) in deficient animals during the latter epochs. Intact mice ran faster (31.4±2.8 m·min−1) than deficient mice (26.1±5.4 m·min−1) throughout the dark period as a whole. Wheel running duration was significantly [F(3,72)=28.8, p<0.001] different across epochs and between treatments. The intact mice recorded higher percentages of wheel running during the initial epoch and decreased as the dark period progressed. Testosterone deficient mice ran the most (highest percentages) during the first and last epochs. In conclusion, wheel running patterns were significantly different following alteration to circulating testosterone levels in male mice. Changes in speed and duration patterns during wheel running indicate that both brain physiology and the cellular composition of skeletal muscle may be affected by the loss of testosterone.Support or Funding InformationThis project was supported by the Pilgram Marpeck School of STEM intramural research fund at Truett McConnell University.This abstract is from the Experimental Biology 2019 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.