Table 4 Categorization of interlimb tasks

Discrete/Continuous

Temporal

Discrete tasks involve actions with a definite beginning and end [117]. Conversely, continuous tasks involve actions that lack such recognizable events [118, 119].

Discrete: point-to-point reaching movements, pushing a button.

Continuous: tracking a moving target, steering a driving wheel.

Periodic/Non-periodic

Spatio-temporal

Periodic tasks are those in which a particular movement is repeated at (quasi-) regular intervals. In non-periodic tasks, the intervals are not regular or the action does not repeat.

Periodic: drawing multiple circles at the same frequency without stopping.

Non-periodic: drawing one circle or drawing multiple circles of increasing radius at the same frequency without stopping.

Isometric/Non-isometric

Spatial

Isometric tasks do not involve limb movement, but require the production of forces.

Non-isometric tasks require movement.

Isometric: pushing a wall that does not move.

Non-isometric: moving an object.

Active/Passive

Movement execution

In an active task, the user is required to perform or try to perform a specific action.

In a passive task, the user’s limb is moved by an external agent (e.g., a robot) and the user is instructed to relax and not interfere with the movement.

Active: move your arm by contracting your muscles.

Passive: a robot to moves your arm without you trying to interfere.

Simultaneous/Sequential

Temporal

If limbs execute actions at the same time, the task is simultaneous. If the action of one limb ends and is followed by the action of the other limb, it is a sequential task.

We note that the terms ‘synchronous’ and ‘asynchronous’ have been used as substitutes for simultaneous/sequential. However, we discourage their use, as they can be confused with in-/out-of-phase (see below).

Simultaneous: cutting a steak.

Sequential: opening a drawer with one hand and retrieving objects from inside the drawer with the other hand after the drawer is opened.

In-phase/Out-of-phase

Temporal

This adjective is relative to the start of the movement

For periodic tasks or single cycles of movement, a task is in-phase if the relative phase between the movements of the limbs is zero.  In out-of-phase tasks, the relative phase is non-zero.

Note that these categories also apply to movements in which the frequency of one limb is a harmonic of the other, as there is a minimum frequency that is common to both. This common frequency is used as reference to determine the in-/out-of-phase characteristic of the task.

In-phase: arm movements during breast-stroke in swimming; drummer that does one beat with one hand while doing two with the other.

Out-of-phase: arm movements during front crawl in swimming; drawing ellipses starting from different points on the perimeter.

Mirror symmetric/Visual symmetric/Point symmetric

Spatio-temporal

Within the neuroscience community, the term symmetric has been used to tasks in which homologous muscles are used and asymmetric to tasks in which non-homologous muscles are used. However, such convention is unintuitive for someone unfamiliar with the historical background of the study of bimanual coordination. Therefore, we suggest to define symmetric tasks as proposed by Malabet et al. [35]:

 • Mirror symmetric-the movement of one limb reflects the movements of the other as if a mirror were placed in the mid-sagittal plane.

 • Visual symmetric-the endpoints of the two limbs move in the same direction and with the same magnitude (this is also referred as asymmetric in the literature; however, we discourage this use as it can be confused with incongruent movements).

 • Point symmetric-the movement of the limbs are opposed relative to a point in space.

To avoid confusion with previous literature, the term incongruent should be used for tasks in which limbs perform asymmetric movements (i.e. that cannot be categorized as mirror, visual or point symmetric).

Mirror symmetric: jumping jacks.

Visual symmetric: moving a tray with two hands forward and backward, and left to right (without rotations).

Point symmetric: turning a steering wheel with hands on opposite sides of a diameter (180° apart).

Congruent/Incongruent

Spatio-temporal

Congruent tasks are symmetric and in-phase.

Incongruent tasks are those in which the task assigned to each limb differs with respect to a least one parameter [5].

Congruent: drawing two ‘identical’ circles at the same time.

Incongruent: drawing a circle with one hand and a line with the other; drawing circles with different radii with each hand.

(Physically) Coupled/Uncoupled

Spatial

Coupled tasks are those in which the limbs are mechanically or virtually (external to the body) connected. In order to be considered coupled, the connection should allow one limb to have an effect on the dynamics of the opposite limb.  During uncoupled tasks, limbs move independently (e.g. [120]).

Coupled: turning a steering wheel, sweeping, squeezing a rubber ball with both hands.

Uncoupled: drawing a circle in the air with each hand.