A method for motion analysis, the method being implemented by an electronic device in communication with a sensing device over a communication network, the sensing device being positioned at a knee joint of a rider of a bicycle, the method being characterized by steps of: receiving, from the sensing device, a series of pieces of sensed data that are generated by the sensing device according to movement of the knee joint during a time period of bicycle riding and that are related to at least one pedal cycle of the bicycle riding, each of the pieces of sensed data corresponding to a time point within the time period and including a set of three elements representing an orientation of the sensing device with respect to a fixed coordinate system, wherein a first element of the set of three elements is representative of an angle measurement of adduction/abduction of the knee joint, a second element of the set of three elements is representative of an angle measurement of flexion/extension of the knee joint, and a third element of the set of three elements is representative of an angle measurement of internal/external rotation of the knee joint; determining a trajectory formed by a plurality of data points each of which corresponds to a respective one of the pieces of sensed data, wherein the trajectory is determined according to either the first and second elements, the first and third elements, or the second and third elements, the trajectory including at least one loop, wherein the at least one loop represents a full course of the at least one pedal cycle; determining a closed curve resembling the at least one loop of the trajectory by using principal components analysis, PCA; calculating an area enclosed by the closed curve; and generating an estimation result regarding correctness of a riding posture of the rider based on the at least one loop of the trajectory and based on comparison of the area to a predetermined threshold, wherein the step of generating an estimation result includes sub-steps of: generating an estimation result indicating an erroneous riding posture when the area enclosed by the closed curve is greater than the predetermined threshold; and generating an estimation result indicating a normal riding posture when the area enclosed by the closed curve is not greater than the predetermined threshold. receiving, from the sensing device, a series of pieces of sensed data that are generated by the sensing device according to movement of the knee joint during a time period of bicycle riding and that are related to at least one pedal cycle of the bicycle riding, each of the pieces of sensed data corresponding to a time point within the time period and including a set of three elements representing an orientation of the sensing device with respect to a fixed coordinate system, wherein a first element of the set of three elements is representative of an angle measurement of adduction/abduction of the knee joint, a second element of the set of three elements is representative of an angle measurement of flexion/extension of the knee joint, and a third element of the set of three elements is representative of an angle measurement of internal/external rotation of the knee joint; determining a trajectory formed by a plurality of data points each of which corresponds to a respective one of the pieces of sensed data, wherein the trajectory is determined according to either the first and second elements, the first and third elements, or the second and third elements, the trajectory including at least one loop, wherein the at least one loop represents a full course of the at least one pedal cycle; determining a closed curve resembling the at least one loop of the trajectory by using principal components analysis, PCA; calculating an area enclosed by the closed curve; and generating an estimation result regarding correctness of a riding posture of the rider based on the at least one loop of the trajectory and based on comparison of the area to a predetermined threshold, wherein the step of generating an estimation result includes sub-steps of: generating an estimation result indicating an erroneous riding posture when the area enclosed by the closed curve is greater than the predetermined threshold; and generating an estimation result indicating a normal riding posture when the area enclosed by the closed curve is not greater than the predetermined threshold. generating an estimation result indicating an erroneous riding posture when the area enclosed by the closed curve is greater than the predetermined threshold; and generating an estimation result indicating a normal riding posture when the area enclosed by the closed curve is not greater than the predetermined threshold.