Kinematic variables

Movement time was calculated for the entire task (total movement time) and separately for each movement phase. The start and end of movement phases were defined by the velocity of the hand marker (2% of the maximum velocity) [12, 27]. The number of movement units (NMU) was computed from the tangential velocity profile of the hand marker for the entire task (excluding the drinking phase) and separately for the first two phases (reaching and forward transport) and for the last two phases (backward transport and returning). A movement unit was defined as a difference between a local minimum and the next maximum velocity value that exceeded the amplitude limit of 20 mm/s, where the time between two subsequent peaks had to be at least 150 ms [12, 27]. NMU captures the repeated sub-accelerations and sub-decelerations during movement performance and can be defined as movement smoothness. The minimum number for movement units for the drinking task is 4 (one unit for each movement phase). The peak hand velocity and the percentage of time to peak velocity in the reaching phase was calculated from the hand marker data. The peak elbow angular velocity during elbow extension in reaching phase was also computed.

The joint angles of the wrist and elbow were determined by the angles between the vectors joining the hand, wrist, elbow and shoulder markers. The shoulder abduction angle was defined as the angle between the vectors joining the shoulder and elbow markers and the vertical vector from the shoulder marker toward the hip. Joint angles were calculated for maximal elbow extension in reaching, maximal wrist angle (dorsal flexion) in reaching and forward transport and maximal angle in elbow flexion and arm abduction during drinking phase. The inter-joint coordination was calculated as a cross-correlation of the shoulder flexion and elbow extension joint angles during the reaching phase. The trunk displacement was defined as the maximal forward displacement of the thorax marker in the sagittal plane from the initial position during the entire drinking task [12].