$($a$)$

What is the maximum frictional force $($in N$)$ in the knee joint of a person who supports $69\mathrm{.}0$ kg of her mass on that knee?

$($b$)$

During strenuous exercise it is possible to exert forces to the joints that are easily ten times greater than the weight being supported. What is the maximum force of friction $($in N$)$ under such conditions? $($The frictional forces in joints are relatively small in all circumstances except when the joints deteriorate, such as from injury or arthritis. Increased frictional forces can cause further damage and pain.$)$