Easier going.

To the Editor: I am responding to the letter, "No Easy Street," by Henry Grills in the April issue. I'd like to offer a simple explanation based on coefficient of restitution (CR), the ratio of departing speed to the approach speed between impacting objects, as in the case of the runner and

The lower the CR is, the lower is the departing speed, hence lower kinetic energy; therefore, the reactive force received by the runner is less when the pavement has lower CR. Concrete pavement has higher CR than asphalt pavements. The reactive force is not related to displacement of the pavement, rather its CR.

Another aspect of the issue is inelastic impact. Modern running shoes are so shock-resistant that the difference in pavement CR is not obvious. The runner's technique plays a small role in this as well.

However, the overwhelming choice of runners is soft pavements or tracks. I have yet to see a school or college running track made out of concrete.

GOUTAM BAGCHI

Potomac, Md.

To the Editor: Regarding Henry Grills' letter on running on asphalt vs. concrete, I doubt that "the only Factor involved is energy absorption."

I agree that the difference in surface deflection is negligible compared to that of the shoe, but there are other factors involved. Asphalt is usually smoother than concrete, so the shoe doesn't "catch" as badly. This produces a smaller version of the very noticeable effect of running on hard-packed dirt or fine gravel: It is less jarring to the body.

In general, it is imprudent to dismiss experience of runners based on theory.

PATRICK J. ROACHE

Socorro, N.M.