Cooling down after a long workout might not be doing all that much for you.
A seminal study on the subject looked at 52 healthy adults and their workout habits and found that, while warming up delays muscle soreness, cooling down does not.
The trial went as follows:
Four equally-sized groups received either warm-up and cool-down, warm-up only, cool-down only, or neither warm-up nor cool-down. All participants performed exercise to induce delayed-onset muscle soreness, which involved walking backwards downhill on an inclined treadmill for 30 minutes. The warm-up and cool-down exercise involved walking forwards uphill on an inclined treadmill for 10 minutes.
The results were unanimous: those who warmed up had reduced soreness 48 hours after exercising. However, the people that cooled down reported the same level of soreness that those who did not cool down had.
A different study, published in 2013 looked at 36 adults, who undertook high intensity exercise by doing lunges as they held barbells. To the untrained person, this workout guaranteed some level of soreness the next day.
Those who had warmed up on a stationary bicycle for 20 minutes had a higher threshold for pain; those who hadn’t warmed up but cooled down with the stationary bike reported a much lower threshold for pain.
So, does that mean you don’t need to cool down?
Not quite. While prolonged cool-down sessions don’t seem to help with soreness, going from high intensity and gradually to lower intensity exercise may help prevent buildup of blood in your veins which might lead to dizziness and fainting. Just make sure you're doing your cool-down for the right reason.