Swing analysis method and swing analysis device

A swing analysis method comprising:

• providing an inertial sensor comprising an acceleration sensor and a gyro sensor, the inertial sensor being attached to a golf club, wherein
• the acceleration sensor is configured to measure acceleration individually in three axial directions orthogonal to one another, the three axial directions being constituted by a first axial direction along a first axis, a second axial direction along a second axis, and a third axial direction along a third axis,
• the gyro sensor is configured to measure angular velocity individually about the first, second and third axes orthogonal to one another, and
• the inertial sensor is configured to communicate wirelessly with an arithmetic processing circuit;
• wirelessly acquiring, by the arithmetic processing circuit, measurement data that is measured by the inertial sensor in a swing motion with the golf club, wherein
• the measurement data comprises the acceleration measured by the acceleration sensor and the angular velocity measured by the gyro sensor;
• and
• calculating, by the arithmetic processing circuit, torque data indicating a temporal change of a torque exerted on the golf club by analyzing the measurement data using a head speed coordination system having the first axis including a speed vector of a head of the golf club, the second axis being orthogonal to the first axis and extending along a shaft of the golf club, and the third axis being orthogonal to both the first axis and the second axis;
• determining, by the arithmetic processing circuit, a swing feature value of the swing motion, based on the torque data in a shorter period among a period from a top position to a natural release timing position in the swing motion and a period from the top position to a halfway down position in the swing motion, wherein
• the torque data is a force exerted for rotation about the third axis, and
• when the torque data in the shorter period is divided at a dividing time point of a plurality of time points including:
• a middle time point in an elapsed time in the swing motion;
• a middle time point in a moving distance of the golf club;
• and
• a time point when a maximum value is recorded in the torque data,
• the swing feature value is a ratio of an integrated value of the torque in a first period before the dividing time point and an integrated value of the torque in a second period after the dividing time point;
• and
• performing, by the arithmetic processing circuit, swing analysis, based on the determined swing feature value.