Effects of wheel running in delaying the onset of motor deficits, measured by tests of performance on the horizontal rod and rear-paw clasping. The cumulative percentage of mice consistently failing each test is plotted as a function of age. A) Wheel running from 4 weeks of age delays the onset of motor deficits revealed by the horizontal rod (P < 0.05). Wild-type mice, regardless of housing condition, always pass the horizontal rod test. B) Wheel running also delays onset of the rear-paw clasping motor deficit (P < 0.05). Triangles: non-enriched HD mice. Squares: wheel running HD mice. Circles: wild-type (WT) mice (enriched and non-enriched groups pooled: no failures).