LR = Loading Response
TS = Terminal Stance
"Vertical GRF peaks at LR & TS"
WHY? Because those two points are when the greatest amount of acceleration is being applied into the ground.
Therefore, vertical GRF is greatest during: Loading Response (1st peak) & Terminal Stance (2nd peak)
F = m x a
The force is greater due to increased acceleration (a)
WHY THE DIP AT MIDSTANCE?
At midstance no acceleration is being created, the foot is just fixed while weight passes over it
"These peaks are due to the body acceleration into the ground following inital contact & pushing down into the ground to propel the body upwards."