Answer
An athlete spends a factor of 4.8 times more time above $\frac{y_{max}}{2}$. Than below $\frac{y_{max}}{2}$ for the duration of the jump.
Work Step by Step
Let $t_2$ be the time it takes to go from $\frac{y_{max}}{2}$ up to $y_{max}$. Note that $t_2$ is equal to the time it takes to fall freely from $y_{max}$ down to $\frac{y_{max}}{2}$.
$\frac{y_{max}}{2} = \frac{1}{2}~g~t_2^2$
$t_2 = \sqrt{\frac{y_{max}}{g}}$
Let $t$ be the total time it takes to go from the floor up to $y_{max}$. Note that $t$ is equal to the time it takes to fall freely from $y_{max}$ down to the floor.
$y_{max} = \frac{1}{2}~g~t^2$
$t = \sqrt{\frac{2~y_{max}}{g}}$
Let $t_1$ be the time it takes to go from the floor up to $\frac{y_{max}}{2}$.
$t_1 = t - t_2 = \sqrt{\frac{2~y_{max}}{g}} - \sqrt{\frac{y_{max}}{g}}$
We will find the ratio of $t_2$ to $t_1$.
$\frac{t_2}{t_1} = \frac{\sqrt{\frac{y_{max}}{g}}}{\sqrt{\frac{2~y_{max}}{g}} ~- ~\sqrt{\frac{y_{max}}{g}}}$
$\frac{t_2}{t_1} = \frac{1}{\sqrt{2} - 1} = 2.4$
We then multiple 2.4 by 2 to get 4.8, as this solution is only for the last period of the total jump. Therefore, an athlete spends a factor of 4.8 times more time above $\frac{y_{max}}{2}$ during the total jump compared to when he is below $\frac{y_{max}}{2}$ .
