why by walking activity we can only provides an estimate VO2 max?

Answer 1

VO2 maintains a proportionally linear relationship with exercise intensity. Generally the harder we exercise the greater the VO2 registered. However, at around the 70% VO2max mark, increasing exercise intensity will not increase VO2 much further because the body finds other ways to increase lung capacity/ tidal volume, and also because almost all reserves have been recruited.

If we only measure VO2 by extrapolating from a few different walking intensities, we do not see the non linear portion of the line and hence will tend to overestimate the true VO2 max. The YMCA cycle ergometry test uses a bike and 3 different exercise intensities based on heart rate and is a pretty accurate method of estimating VO2 max.