A method for determining fatigue of a user by an apparatus, the method comprising: acquiring, from at least one stride sensor (13-A, 13-B) designed to be attached to a shoe or a foot and to measure motion of the foot during a physical exercise, gait measurement data representing measured gait of the user during a physical exercise; computing at least one of step interval variability and stride interval variability from the gait measurement data; determining standard conditions for testing the fatigue, wherein the standard conditions are a reproducible combination of factors affecting the at least one of step interval variability value and stride interval variability; and determining the user's fatigue from said at least one of step interval variability and stride interval variability such that a higher step interval variability and a higher stride interval variability is associated with a poorer physiological state and using in the determination of the fatigue exclusively gait measurement data measured under the standard conditions, wherein determining of whether or not the standard conditions are satisfied is performed by using at least one of the following steps a) and b): a) the standard conditions are defined by heart activity measurement data and at least one threshold or range for the heart activity measurement data, and measured heart activity measurement data is compared with the at least one threshold or range; if the heart activity measurement data falls outside the range defining the standard conditions, the standard conditions are determined not to have been met; and if the heart activity measurement data falls within the range defining the standard conditions, the standard conditions are determined to have been met; b) the standard conditions are defined by running speed measured from the gait measurement data during the exercise with the prior knowledge of a step length or a stride length; if the measured running speed falls within a predetermined range defining the standard conditions, the standard conditions are deemed to be fulfilled; and if the measured running speed falls outside the predetermined range defining the standard conditions, the standard conditions are deemed not to be fulfilled. acquiring, from at least one stride sensor (13-A, 13-B) designed to be attached to a shoe or a foot and to measure motion of the foot during a physical exercise, gait measurement data representing measured gait of the user during a physical exercise; computing at least one of step interval variability and stride interval variability from the gait measurement data; determining standard conditions for testing the fatigue, wherein the standard conditions are a reproducible combination of factors affecting the at least one of step interval variability value and stride interval variability; and determining the user's fatigue from said at least one of step interval variability and stride interval variability such that a higher step interval variability and a higher stride interval variability is associated with a poorer physiological state and using in the determination of the fatigue exclusively gait measurement data measured under the standard conditions, wherein determining of whether or not the standard conditions are satisfied is performed by using at least one of the following steps a) and b): a) the standard conditions are defined by heart activity measurement data and at least one threshold or range for the heart activity measurement data, and measured heart activity measurement data is compared with the at least one threshold or range; if the heart activity measurement data falls outside the range defining the standard conditions, the standard conditions are determined not to have been met; and if the heart activity measurement data falls within the range defining the standard conditions, the standard conditions are determined to have been met; b) the standard conditions are defined by running speed measured from the gait measurement data during the exercise with the prior knowledge of a step length or a stride length; if the measured running speed falls within a predetermined range defining the standard conditions, the standard conditions are deemed to be fulfilled; and if the measured running speed falls outside the predetermined range defining the standard conditions, the standard conditions are deemed not to be fulfilled. a) the standard conditions are defined by heart activity measurement data and at least one threshold or range for the heart activity measurement data, and measured heart activity measurement data is compared with the at least one threshold or range; if the heart activity measurement data falls outside the range defining the standard conditions, the standard conditions are determined not to have been met; and if the heart activity measurement data falls within the range defining the standard conditions, the standard conditions are determined to have been met; b) the standard conditions are defined by running speed measured from the gait measurement data during the exercise with the prior knowledge of a step length or a stride length; if the measured running speed falls within a predetermined range defining the standard conditions, the standard conditions are deemed to be fulfilled; and if the measured running speed falls outside the predetermined range defining the standard conditions, the standard conditions are deemed not to be fulfilled.