The function of the lower limbs during walking is to support the whole-body against gravitational forces while generating movement patterns which progress the body forward. Walking is an activity that is primarily confined to the sagittal plane, which is also described as the plane of progression. During one gait cycle, there are two major phases: stance and swing. In a healthy individual walking at a normal walking speed, stance phase makes up approximately 60% of one gait cycle and swing makes up the remaining 40%. The lower limbs are only in contact with the ground during the stance phase, which is typically subdivided into 5 events: heel contact, foot flat, mid-stance, heel off, and toe off. The majority of stance phase (~40%) takes place in single-limb support where one limb is in contact with the ground and the contralateral limb is in swing phase. During this time interval, the lower limb must support constant changes in alignment of body weight while propelling forward. The hip, knee, and ankle joints move through cyclical kinematic patterns that are controlled by muscles which cross these joints. As postural changes occur, the body adapts by motor tuning an efficient muscular pattern that will accomplish the necessary kinematics required to walk.
Kinetic and kinematic measures together, are powerful tools that help infer joint patterns and understand how patterns may alter in the presence of physical or environmental changes. In kinetic measures of ground reaction force, the shape of the vertical ground-reaction force is consistent and well known. Researchers have spent decades trying to establish a direct connection between kinetic patterns and muscle activity. Since the musculoskeletal system is complex, identifying all individual muscle contributions is challenging, therefore net joint moments are most commonly examined. In 1980, a principle called the support moment was introduced. It described a total lower-limb pattern occurring at the hip, knee, and ankle during stance. According to this principle, the basic function of the lower limbs during stance phase is to resistant collapse; and to prevent this collapse, vertical support of the body requires net extensor activity at the hip, knee, and ankle joints. Other reports suggest that the necessary amount of force generated by a muscle to produce a given moment about the axes of rotation at a joint, is dependent on limb position.
Sagittal plane kinematics
Hip
At heel contact the hip is maximally flexed at approximately 30 degrees. The hip extensor muscles are active and prepared to extend the hip to prevent any uncontrolled trunk flexion over the femur. Once the foot is flat on the ground, the hip gradually extends in preparation for weight acceptance as the whole-body moves forward over the stance foot. Between 30-50% of the gait cycle, the hip flexor muscles are eccentrically acting as the hip continues to extend, until reaching maximal extension at approximately 10-15 degrees past neutral. This max extension takes place right before toe off. The hip flexors then concentrically act to initiate hip flexion for swing phase. Overall, approximately 30 degrees of flexion and 10 degrees of extension (from neutral) are needed at the hip joint for a normal walking pattern.
Knee
The knee has a more complex movement pattern compared to the hip. At heel contact the knee extensor and flexor muscles co-contract to provide stability for the knee joint since it is almost maximally extended at that point in time. Shortly after, as the foot becomes flat on the ground, the knee gradually flexes approximately 10-15 degrees reaching the maximum at about 15% of the gait cycle. This small amount of knee flexion is controlled eccentrically by the knee extensor muscles which serve the purpose of cushioning the rate of loading on the lower limb and preventing excessive knee flexion. Following through to mid-and terminal stance, the knee gradually extends with concentric activity of the knee extensor muscles and approaches near full extension as heel off occurs (30-40% of gait cycle). At this point the knee flexor muscles concentrically flex the knee again for swing phase. The maximum knee flexion that occurs during swing is about 60 degrees.