So, I was thinking when doing Bulgarian split squats, many people say we want to keep a vertical shin and not allow the shin to travel forward to reduce quad involvement. However, when trying to predict the forces in a simple way (I know, I need force plates, etc.), I made 3 images comparing a 90 degree vs ~45 degree shin angle showing a GRF (vertical green) and the friction force (horizontal green) to create the resultant (orange dot) and from there I drew the moment arms of the glutes and quads.
I was trying to see that assuming we know the center of pressure (red dot), if we do the Bulgarian split squat with the COM (white dot) further away from the COP (1st image) the GRF (vertical green arrow) might not be as high and the friction force (horizontal green arrow) might be longer and this can actually increase the quads contributing despite being in a vertical shin angle.
However, if we try to bring the COP a little closer to the COM by allowing the shin to move a little and lean forward the friction force will not be as long and the GRF will be taller and this can increase the glutes contribution and have the quads contribute a little less despite the knee moving forward.
The last image shows that if we allow the knee the travel forward and not lean forward (further distance from COP to COM), now the quads are contributing more than the glutes.
What do you guys think of this? For training glutes should we pay attention to our COP and COM? We ideally would want them to be close together right?
I know this is in static equilibrium and when drawing force vectors like this the further they are from the joint the more inaccurate it’s getting but I thought it was a fun little way to draw this idea out.
Open to discuss!