Downhill walking induces rapid shallow breathing.

To examine the effect of downhill walking a form of negative work on ventilation, we studied the exercise responses of 13 healthy subjects during uphill and downhill walking on a treadmill. Each test lasted 16 min and the peak work rate was 5.6 kph with either a positive or negative 14% grade. Throughout each test we recorded VO2, VE, f, VT, HR, systolic BP and Borg's rating of perceived exertion. At the target work rates of 5.6 kph +/- 14% grade, VO2 and VE were three times greater in uphill compared with downhill walking. However, in downhill walking, f was greater compared with uphill walking wherein VT approximated baseline values, reflecting rapid shallow breathing, and VT appeared to increase after reaching some critically-low level. These trends persisted when VO2 was held constant (p less than 0.01). HR and systolic BP increased and decreased with the positive and negative grade respectively. At a constant VO2 however, HR was significantly higher during downhill compared with horizontal walking (p less than 0.01) whereas systolic BP was not significantly different (p greater than 0.05). We conclude that there is a significant difference in the ventilatory responses between the two types of work performed on a treadmill. Specifically, downhill walking is associated with rapid shallow breathing which may be countered by a protective feedback mechanism at critically-low levels of VT.