A method for determining stretch values of a body part, wherein the body part includes first and second points that move relative to each other when the body part moves, the method comprises, - connecting a stretch sensor to the body near both the first and second points such that movement of the first point relative to the second point changes elongation of the stretch sensor,where the stretch sensor is a capacitive or resistive sensor, which changes its capacitance or resistance as a function of elongation, - while moving the body part, while moving the body part, obtaining stretch data from the stretch sensor by a processor associated with the stretch sensor, - processing, by the processor, the stretch data obtained from the stretch sensor with a sensor device to determine a stretch value of the body part, wherein the processor is configured to perform the method by: - analysing the stretch data as cyclic stretch data, - analysing the stretch data to identify portions of the cyclic stretch data which correspond to motions of the body part, where the portions of the stretch data are identified so that the portions contain first and second stretch data points (101, 102) associated with respective first and second motion-phases of the body part, - analysing the cyclic stretch data to identify first and second stretch values (101, 102) by determining minimum and maximum sensor values (121,122), - further analysing the cyclic stretch data to identify the portions ensuring that the portions contain an entire period of a cyclic harmonic signal containing first and second stretch data points (101, 102) associated with respective first and second motion-phases of the body part and to identify values related to other motion phases (143, 443), - analysing the identified portions of cyclic stretch data to determine first and second sensor values (121, 122) of the respective first and second stretch data points (101,102), - determining stretch values by determining the differences between the first and second sensor values (121, 122) associated respectively with the first and second motion-phases, and - transmitting the determined stretch values to a receiver associated with the processor. - connecting a stretch sensor to the body near both the first and second points such that movement of the first point relative to the second point changes elongation of the stretch sensor,where the stretch sensor is a capacitive or resistive sensor, which changes its capacitance or resistance as a function of elongation, - while moving the body part, while moving the body part, obtaining stretch data from the stretch sensor by a processor associated with the stretch sensor, - processing, by the processor, the stretch data obtained from the stretch sensor with a sensor device to determine a stretch value of the body part, wherein the processor is configured to perform the method by: - analysing the stretch data as cyclic stretch data, - analysing the stretch data to identify portions of the cyclic stretch data which correspond to motions of the body part, where the portions of the stretch data are identified so that the portions contain first and second stretch data points (101, 102) associated with respective first and second motion-phases of the body part, - analysing the cyclic stretch data to identify first and second stretch values (101, 102) by determining minimum and maximum sensor values (121,122), - further analysing the cyclic stretch data to identify the portions ensuring that the portions contain an entire period of a cyclic harmonic signal containing first and second stretch data points (101, 102) associated with respective first and second motion-phases of the body part and to identify values related to other motion phases (143, 443), - analysing the identified portions of cyclic stretch data to determine first and second sensor values (121, 122) of the respective first and second stretch data points (101,102), - determining stretch values by determining the differences between the first and second sensor values (121, 122) associated respectively with the first and second motion-phases, and - transmitting the determined stretch values to a receiver associated with the processor. - analysing the stretch data as cyclic stretch data, - analysing the stretch data to identify portions of the cyclic stretch data which correspond to motions of the body part, where the portions of the stretch data are identified so that the portions contain first and second stretch data points (101, 102) associated with respective first and second motion-phases of the body part, - analysing the cyclic stretch data to identify first and second stretch values (101, 102) by determining minimum and maximum sensor values (121,122), - further analysing the cyclic stretch data to identify the portions ensuring that the portions contain an entire period of a cyclic harmonic signal containing first and second stretch data points (101, 102) associated with respective first and second motion-phases of the body part and to identify values related to other motion phases (143, 443), - analysing the identified portions of cyclic stretch data to determine first and second sensor values (121, 122) of the respective first and second stretch data points (101,102), - determining stretch values by determining the differences between the first and second sensor values (121, 122) associated respectively with the first and second motion-phases, and - transmitting the determined stretch values to a receiver associated with the processor.