Hypothetical strategies to combine motor plans and experiment.

A. Different hypotheses for coordination between a fast (F) and a slow (S) partner. The arrows represent the information flow in the respective hypotheses. The co-activity (CA) strategy corresponding to independent motor plans, leader-follower (LF) here based on the faster partner’s individual motor plan, weighted adaptation (WA) generalizing the LF hypothesis based on a weighting of their initial strategies in the task, and interactive adaptation (IA) where both partners dynamically adapt their original motor plan, possibly differently, due to the interaction with an uncertain partner. B. Experiment to investigate vigor in individuals and in mechanically connected dyads. The two partners have to reach one of targets {A1, …, A5} on their individual monitor using wrist flexion/extension of the right arm. Their real-time wrist angles qi and qj are mapped to individual red cursors ci and cj. In the dyadic session, their hands are coupled through a virtual elastic band of either low (KL) or high (KH) stiffness. C. Experimental protocol. The initial solo session consists of four blocks: one passive block (P) with exoskeletons’ motor off to estimate the individual vigor, two blocks with low (VL) or high (VH) resistive viscous load to vary the cost of movement (i.e. effort), and a passive washout block (W). The subsequent dyadic session involves six blocks in coupled mode, and a final block in passive mode to analyze after-effects of the practice with mechanical connection. The KL and KH blocks, performed in random order, first allow participants to familiarize with the interaction, and the KxVx blocks are to investigate all combinations of the connection stiffness and viscous load.

Kinematics and vigor in the solo conditions.

A. Trajectories of reaching movements of different amplitude averaged across the population in the first passive block (P1) of Fig. 1C. B. Resulting individual and averaged amplitude-duration relationships. C. The vigor scores in the three solo conditions {P1,VL,VH} are all correlated, indicating that vigor can be defined robustly across varied effort conditions.

Dyads kinematics, interaction torque and dyadic vigor.

A. Predicted trajectories and interaction torque of a dyad with independent motor plans for the faster and slower partners for A5 in the KL condition. B. Participant’s trajectories averaged across the population for KL and for the five targets. C. Average absolute interaction torque with low (KL) and high (KH) connection stiffness. D. The average interaction efforts in KL and KH are independent on the difference in individual vigor between the partners of a dyad (here during P1). E. Amplitude-duration relationships of each participant’s movements and of their average during the KL condition. F. Effect of the two different viscous loads on movement duration, for the KL connection, averaged across all participants. G. Movement durations are not different between the fast and slow groups in the connected conditions (here KL). H. Percentage of time saved by the fast and slow groups between P1 and KL, and between VL and KLVL, where positive values indicate faster movements. I. Correlations between the vigor scores obtained during the three KL connected conditions. Vigor scores were computed using the average movement duration between members of the dyad.

Predictions of dyad movement duration obtained with the model arising from the IA hypothesis.

A. Illustration of predicted velocity profiles and measured averaged durations for movements towards A5 in the KL condition. The solid and dashed-dotted blue lines represent respectively the average and samples of the distribution of the slow partner, with the average movement duration of the slow partners as a dashed dotted blue vertical line. The solid green line illustrates the average velocity profile of the fast partners moving alone, with the green dashed-dotted vertical line the corresponding duration in the experiment. Finally, the solid black represents the velocity profile predicted by IA, and the black dashed-dotted vertical line the average experimental duration of dyadic movements. B. Comparison between predicted movement time and data for all the connected conditions.

Kinematics and vigor before and after human-human interaction.

Data before (P1) and after (P2) the dyadic session are represented using dashed-dotted and solid lines, respectively. A. Population average of the position and velocity for each target. B. Averaged amplitude-duration relationships for the fast and slow partners in P1 and P2. C. Time saved between P1 and P2 for the fast and slow partners and for each target. D. Difference in vigor scores between P1 and P2, for the fast and slow partners. E. Illustration of the fast and slow partners adaptations. When connected, the average vigor of the slow partners increases to match the one of the fast partners, while the fast partners’ group changes its internal ranking in vigor to match the ranking of the slow partners. These adaptations are retained for both groups in after-effects (AE).