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 occurs when the femur rotates towards the front of the pelvis. Using the hip flexors, this motor pattern is not the strongest. The to tal 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
Adduction is the movement of the leg across the body towards the centerline. The muscles engaged are the adductors, pretty simple huh? The second largest muscle in our body is the adductor magnus. It attaches to the pelvis and the inside of the femur and is the main contributor to adduction. It can also act as a hip extensor, although it has less mechanical advantage than the glute max. Other noteworthy muscles include the adductor longus, adductor brevis and the gracilis, the only adductor to cross the knee. Although adduction is the movement of the femur towards the centerline of the body, the main purpose of the adductors in sport is for knee stabilization. How often does the knee of an athlete standing on one leg tuck inward? This is because the adductors are engaged to stabilize the knee and they are pulling the femur towards the midline. Knee stabilization is a dynamic balance between the adductors and the abductors.
Abduction is the movement of the leg away from the centerline of the body. A convenient way to remember abduction is that when the leg is abducted, it's being taken away from the body. The TFL and gluteus medius are the main hip abductors and contribute significantly to hip abduction for knee stabilization. When an athlete stands on one leg, their abductors and adductors should both engage to keep the knee in a neutral position. Failure to do so can cause knee pain and stress at the joint. Lack of flexibility in either adductors or abductors can cause dysfunction between them. If an athlete's adductors are too tight, their abductors will be at a mechanical disadvantage and generally fail to engage adequately. Flexibility in the abductors and adductors are essential for any athlete with significant hip or knee movement and is generally difficult.
The motor pattern of knee stabilization is easily loaded onto only one of these two muscle groups. For example, if an athlete does a lot of squatting and they start to tuck their knees inward during the movement (valgus knee), their adductors will handle most of the load of the movement. This load bearing causes them to strengthen and tighten and a feedback loop of increased strength and tightness occurs until the abductors are so weak and elongated that they cannot contribute. This type of pattern is incredibly common in all kinds of athletes. It's important to maintain the strength and flexibility of both of these muscle groups to try to avoid this feedback loop.
Hip rotation occurs when the femur rotates around it's major axis. For the other hip movement patterns the femur is moving relative to the pelvic, but in this movement pattern there is no relative movement, just a rotation. If the knee is bent when the hip goes into rotation, the foot will flare out/in. The hip can do internal and external rotation with the definition being: which direction is the top of the femur rotating towards? For external rotation, the foot moves towards the centerline but the top of the femur rotates outward. For internal rotation, the foot moves outward, away form the centerline while top of the femur rotates inward. Hip rotation is important because it controls the position of the lower leg. Since the knee is a one dimensional joint (spoiler!), changes in foot position come as a result of changes in hip rotation.
Knee Flexion and Extension
Knees are so simple; they flex and then they extend. The simplicity of a hinge joint is that there is one degree of freedom and the motion is quite restricted. The hamstrings control the flexion (bending) of the knee and the quadriceps control the extension. For cycling, we focus largely on the extension of the knee during the first half of the pedal stroke. A powerful down stroke is led by the quadriceps and the result is the stereotypical cyclist's physique, big quads. Because of the restricted nature of the knee joint, it's prone to pain. Other joints, such as the hips and ankles, have more 'space' to move. If your joint mobility is limited in the hip, you can always flair your knee out to the side to make space. But for the knee, there is no room. The result is regular pain around the patella and many cyclists experience this pain everyday. Many muscles connect around the knee and any one of them could cause the divergence from regular movement. The best bet to avoid knee pain is to ensure flexibility through the entire lower body.
Dorsiflexion and Plantar Flexion
Dorsiflexion is a fancy word for lifting the foot up towards the shin. The opposite, toes down, is plantar flexion. In the cycling pedal stroke, dorsiflexion and plantar flexion are a major movement pattern along with hip extension/flexion and knee extension/flexion. It's been noted in many studies that ankle range of motion decreases when intensity increases. This is likely due to the relatively small calves (compared to quads and glutes) fatiguing quickly and the body reacting by engaging them less. It has been shown that calves generally contain a large number of type II, powerful but fast fatiguing, muscles which could be the explanation for this motor pattern phenomenon. The one main exception to this rule is during sprinting. Sprinters will typically flick their ankles at the bottom of the pedal stroke to eek out a little more power before entering the recovery phase. Since this action is at most 15 seconds, the fatigue in the calves generally doesn't build up enough to reduce their contribution.
Another interesting note is that ankle flexion is generally the free variable in bike fitting. If a fitter changes a rider's saddle height, we may expect changes in joint angles in all of the hips, knees, and ankles, but generally, the ankle changes its range of motion while the hips and knees remain constant. Maintaining good ankle range of motion is a key to allowing the hips and knees to move through their desired range of motion. It's been shown that poor ankle flexibility can cause knee pain by failing to absorb some of the length from the saddle to the pedal.
Pronation and Supination
Pronation and supination are kind of weird. There are actually two sets of joints in the ankle. The upper joint controls ankle flexion and another joint just below the talus bone controls the side to side movement of the ankle. This side to side motion is called pronation and supination. Pronation is bringing the foot towards the outside of the shank and supination is bringing the foot towards the midline. While the hip has six separate movements, the ankle's movement can be summarized by only four. Specifically, pronation and supination combines some of the movement akin to hip adduction/abduction AND hip rotation, but the movement of the ankle is more restricted than the hip as a result.
Excessive pronation and supination can cause ankle joint issues because the weight of the force is not directly through the ankle, and it can also cause dysfunction in the ankle muscles. For cycling, the main concern with pronation and supination is the associated hip movements. Since the knee is a simple hinge, changes in side-to-side ankle position causes the knee to flair in or out. This results in changes in hip abduction/adduction and rotation due to the restriction of the cleat and pedal interaction. In a runner, the body can move to compensate for pronation/supination, but in a cyclist, the response must come from changes in hip movement patterns. This change in hip movement can cause improper fatigue cycles or muscular dysfunction. An interesting question for bike fitters is: which came first? Did the hip dysfunction cause the pronation or did the pronation cause the hip dysfunction? Either way, they are inextricably linked and should be carefully monitored to avoid overuse injuries.
Putting It All Together
Every lower body movement pattern is some combination of these simple movement patterns. I chose to exclude the toes, but I've hit all the others! Let's see how these simple movements are combined in the pedal stroke. Starting at 12 o'clock (top dead center), hip extension and knee extension are the major force producing motor patterns all the way through about 5 o'clock. The ankle may dorsiflex in this portion of the pedal stroke, but is generally used as a stabilization joint rather than a force producing joint. Passing through the bottom of the pedal stroke, knee flexion starts to occur right at 5 o'clock through 7 o'clock and later. Plantar flexion is likely to occur to unweight the pedal on the back side of the pedal stroke and hip flexion occurs moderately after 9 o'clock through the top of the pedal stroke. And we return back to the hip and knee extension for the next pedal stroke.
Improper movement patterns may include excessive pronation/supination and hip rotation and abduction/adduction. There will always be some of the movements in the pedal stroke, but patterns where there is non-linear knee tracking are of greatest concern. Studying the relationship between these improper movement patterns and their solutions is of great interest to researchers who look to make systematic changes to prevent injuries in riders.
Take some time to move through all these different movement patterns one at a time. Then evaluate your pedal stroke and see if your movement patterns are satisfactory. The biggest reason for improper movement patterns is a lack of range of motion (stretch!), but there may also be some motor pattern issues related to neural firing or muscular dysfunction. If simple stretching does not start to alleviate the motor pattern issues, it's probably best to see an expert.