Biomechanics at the most basic level is pretty simple. There are key movement patterns that exist based on the body's geometry and every movement is some composition of these movements. This phenomenon allows us to describe the movement of the body based on a few key values. For OpenSim models these values determine the unique position of the body in space. Let's talk about the simple movement patterns of the lower body and some example composite movements at the end.

Hip Flexion and Extension

Hip flexion is bringing the femur towards the front of the body.
Hip flexion occurs when the femur rotates towards the front of the pelvis. Using the hip flexors, this motor pattern is not the strongest. The total muscle mass of the hip flexors is relatively low and the ability to generate force in this direction is difficult compared to others. The hip flexors attach to the pelvis or to the spine and can become dysfunctional if an individual sits too much. Upon sitting, we enter fairly tight hip flexion and our hip flexors start to atrophy in that shortened state. When we stand up again, they don't return to their original position. If this occurs consistently over time without stretching, our pelvis will start to rotate forward and we enter anterior pelvic tilt. Soccer players are likely to have anterior pelvic tilt because the action of kicking the soccer ball involves significant force through the hip flexors. As they fatigue, they shorten and anterior pelvic tilt is the result. The main concern with excessive anterior pelvic tilt is that both the hip flexors and the hip extensors (glutes) will be the incorrect length to function properly.
Hip extension is the opposite of hip flexion. It occurs when the femur rotates towards the back of the pelvis. The main movers in hip extension are the glutes, specifically the gluteus maximus, the largest muscle in the body. Compared to the relatively weak hip flexors, the glute max has a massive amount of total muscle mass and force production. Our bodies can generate a lot of force with our glutes and they are a main driver in activities such as vertical jump or squatting. Any position where the motion starts in hip flexion and ends in a neutral hip angle will have significant glute contribution. I wrote a separate article on the glutes and hip extension in cycling, which is worth a read. Cycling is an example of a sport where hip extension is important and as a result, many riders will have posterior pelvic tilt due to tight glutes. Posterior pelvic tilt is a backwards rotation of the pelvis into a 'tucked butt' position. This is caused by shortened glutes and hamstrings pulling the pelvis back and down. Just as with anterior pelvic tilt, posterior pelvic tilt can inhibit proper function of both the glutes and the hip flexors.
The hamstrings can also contribute to hip extension in some scenarios. Since hip extension is a rotational interaction between the pelvis and femur, we wouldn't expect a muscle connecting the shank and pelvis to contribute. Most of the hamstrings connect to the lower leg, below the knee and to the bottom of the pelvis. If an athlete's knee is locked, then changes in pelvic tilt can be caused by hamstring engagement because the knee joint is locked and the lower leg acts as a rigid extension of the upper leg. If the knee is free to move, then the hamstrings do not interact on the pelvis in the same way.

Hip Adduction and Abduction

Hip adduction brings the femur towards<br>the centerline of the body.