Table 2 Definitions of the kinematic variables of the drinking task

Temporal and end-point kinematics

Total movement time (MT) (s)

Time is calculated for the entire drinking task and separately for each phase. The start and end of the movement was defined as the point in time when the velocity exceeded or was below 2% of the maximum velocity in the reaching or returning phase, respectively. Detailed definitions for each phase are available in a previous publication [17]

MT reaching (s)

MT forward transport (s)

MT drinking (s)

MT back transport (s)

MT returning (s)

Number of movement units total (NMU)

Movement units were computed from the tangential velocity profile separately for first two movement phases (reaching and forward transport), last two phases (back transport and returning) and as a summed total of these four phases (NMU total). One movement unit was defined as a difference between a local minimum and next maximum that exceeded the amplitude limit of 20 mm/s, minimum time between two subsequent peaks was set to 150 ms. NMU indicates movement smoothness

NMU phase 1&2

NMU phase 4&5

Peak velocity (mm/s)

Peak tangential velocity of the hand marker in the reaching phase

Time to peak velocity (%)

Percentage of time to peak velocity in the reaching phase; indicates relative time spent in acceleration and deceleration

Peak elbow angular velocity (°/s)

Peak angular velocity in the elbow joint (extension) in the reaching phase

Angular and displacement kinematics (arm and trunk)

Shoulder abduction reaching (°)

Maximum shoulder angle in frontal and sagittal plane, between the vectors joining the shoulder and elbow markers, and the vertical vector from the shoulder marker toward the hip

Shoulder abduction drinking (°)

Shoulder flexion reaching (°)

Shoulder flexion drinking (°)

Elbow extension reaching (°)

Minimum or maximum angle of the elbow joint between the vectors joining the elbow and wrist markers, and the elbow and shoulder markers

Elbow flexion drinking (°)

Wrist angle (°)

Maximum angle of the wrist joint in reaching and forward transport phase between the vectors joining the hand and wrist marker, and the wrist and elbow marker

Inter-joint coordination, r

Temporal cross-correlation between the shoulder flexion and elbow extension during the reaching phase. Stronger correlation indicates that joint motions are coupled

Trunk displacement (mm)

Maximum displacement of the thorax marker from the initial position during the entire drinking task