Explain why a runner’s acceleration cannot exceed 10µ ms^-2, where µ is the coefficient of friction between her shoes and the track.

Ok here I have a big problem, I have no idea which formula to use to model this type of motion or resolve these forces. I think they want me to deduce it or something, I would be tempted to use a=F/m=10mµ/m where F=10mµ, i.e. the model for a frictional force, with g=10 for normal reaction 10m and µ coefficient of friction. But this doesn’t take into account any of the forces exerted by the runner except for her weight, which looks like a clumsy model to me…

Any hints?