…but I can’t. 
I got these in a short assignment. They sound so straight forward, but I’m stumped (can’t blame me, I’ve just had 2 concussions in as many weeks):
During the squat exercise, which muscle (and which part) exerts influence over the motions of both the hip joint and the knee joint?
Describe the differences between the front squat and the back squat with respect to the joint angle at the hip. What effect would this change have on muscle activation?
Help anyone? 
1A. The quadricep and the rectus femoris? It functions in hip flexion and knee extension. My guess would be they mean the rectus femoris works eccentrically in the hip and concentrically in the knee. ???
2A. The front squat supports a more vertical position and reduces torque to the back. Depending on lever length, I believe (from here on out this is an opinion) the front squat would support increased recruitment of the hip extensors since full depth on a front squat would be a deeper drop of the hips, where as on a back squat a lot of the drop is from the torso??? I don’t know, man, just trying to help.
The first question is ambiguous, as both the hamstring and quadricep muscles “exert influence” on the motions of the knee and hip joint during a squat. We all what it’s like to squat with stiff hamstrings.
The answer to the second question really depends on where the bar is set on the back during the squat. The higher up on the back the bar is set, the more upright the torso during the movement. The lower the bar is set, the lower the torso will drop. The angle, however, is smaller during the traditional squat, and will activate muscle fibers in the quads and the posterior chain. A strict front squat gives a greater hip angle, with more emphasis put on the quadriceps and glutes, as the back acts more as a stabilizer.
That’s my take on it.
Torpitamike
Cheers, thanks for your help Torpitamike and Speedkills.
I wrote:
During the squat exercise, which muscle (and which part) exerts influence over the motions of both the hip joint and the knee joint?
The rectus femoris of the quadriceps group initiates hip flexion to begin the lowering (eccentric) part of the squat, while coming back up (concentric portion) it acts to extend the knee joint.
Also, all three parts of the hamstring group (biceps femoris, semitendonosis, semimembranosis) initiate flexion at the knee joint to begin the eccentric part of the exercise, while they also contribute (along with the gluteal group) during hip extension in the concentric part of the exercise.
Describe the differences between the front squat and the back squat with respect to the joint angle at the hip. What effect would this change have on muscle activation?
In a regular back squat (particularly if the bar placement is slightly lower), the joint angle at the hip tends to be more acute which allows the powerful posterior chain muscles (glutes, hamstrings, lower back) to contribute more to the movement.
In a front squat, due to the placement of the bar the lifter is able to have minimal forward lean during the movement (otherwise the bar will topple forwards and cause the lifter to lose his balance). Therefore, the joint angle at the hip must be greater than that of a back squat and consequently the posterior chain muscles cannot exert as much influence (eg. the lower back muscles can only operate as stabilisers as the back must remain completely vertical). Due to the lesser contribution of the posterior chain muscles, more emphasis is placed on the quadriceps group to complete the movement.
Real basic stuff, as it is only Stage 1 Exercise Prescription but it’s always the basic stuff that screws me up! 
The rectus femoris of the quadriceps group initiates hip flexion to begin the lowering (eccentric) part of the squat, while coming back up (concentric portion) it acts to extend the knee joint.
The rectus femoris, which is a biarticulate muscle only slighlty contributes during the concentric effort.The RF enters “active insufficiency” during hip flexion and thereby obligates greater contribution from the three vasti muslces(VL, Vr, & VI). The glutes maximus activity increases as the hip flexion increasees, since the biceps femoris enters active insufficeny due to knee flexion.
Describe the differences between the front squat and the back squat with respect to the joint angle at the hip. What effect would this change have on muscle activation?
The differnces in muscle activation in front and back squat is due to the different locations of the center of mass in these lifts.In the back squat, COM is posyerior to the body, and hence greater hip flexion is required to prevent tlilting.The greater hip flexion involves the posterior chain muscles to a greater extent so as to perform the hip extension motion.On the other hand, the COm being in front of the body for front enables the body to assume a much more vertical position therby negating the invovment of posterior muscles to a greater extent. I guess you echoed the same.