Put simply, it’s like cracking a whip… the handle stops sharply, and that sudden stop sends energy forward, making the tip snap fast. Your lead leg works the same way: it ‘brakes’ hard and early, and that sudden stop is associated with faster hip and torso rotation.
Early and rapidly developed horizontal force in the lead leg, particularly in the x-direction, is significantly associated with how fast both the pelvis and torso are rotating at their peak. The fact that the pelvis window sits squarely in the RFD phase is particularly interesting. It suggests that the rate at which the lead leg builds horizontal force early in the swing may be setting the table for everything that happens rotationally upstream.
Now for what we didn’t find, because null findings are just as important as positive ones.
No significant windows were found for vertical GRF or overall force magnitude in relation to any outcome variable. More notably, no significant windows were found for GRF in *any* direction predicting bat speed. Why is this?
One possible explanation is related to arm mechanics. By the time energy travels up the kinetic chain and reaches the arms, things like elbow tuck, flexion, and extension timing can either amplify or dampen how much of that energy actually makes it to the bat. The lead leg may be doing everything right, but if the arms aren’t efficiently transferring that energy, bat speed becomes its own story.
A second potential explanation is that anthropometric differences between hitters, things like arm and leg length, could alter how force travels up the kinetic chain and ultimately influences bat speed. Two hitters producing identical lead leg force profiles may still arrive at very different bat speeds simply due to how their individual proportions affect energy transfer along the way. Since we didn’t account for these differences in the model, it’s possible that any underlying relationship between lead leg force and bat speed was obscured in the aggregate.