The Tight Game: Implicit Force Intervention in Inter-personal
Physical Interactions on Playing Tug of War
Azumi Maekawa∗Shunichi Kasahara∗

Hiroto Saito

The University of Tokyoazumi@star.rcast.u-tokyo.ac.jp

RCAST, The University of Tokyo
Sony CSL
kasahara@star.rcast.u-tokyo.ac.jp

RCAST, The University of Tokyo
hiroto.saito@star.rcast.u-tokyo.ac.jp

Daisuke Uriu

Ganesh Gowrishankar

Masahiko Inami

RCAST, The University of Tokyouriu@star.rcast.u-tokyo.ac.jp

Centre National de la Recherche
Scientifiqueganesh.gowrishankar@lirmm.fr

RCAST, The University of Tokyo
inami@star.rcast.u-tokyo.ac.jpFigure 1: 1) A tug of war between two players. 2) An external mediator is controlling the game without the players noticing. 3)
The players were totally engaged in the game since they believed it was a tight game.

ABSTRACT
Physical assistance can alleviate individual differences of abilities
between players to create well-balanced inter-personal physicalgames. However, ‘explicit’ intervention can ruin the players’ senseof agency, and cause a loss of engagements in both the player andaudience. We propose an implicit physical intervention system"The Tight Game" for ‘Tug of War’ a one-dimensional physicalgame. Our system includes four force sensors connected to therope and two hidden high torque motors, which provide realtime
physical assistance. We designed the implicit physical assistance byleveraging human recognition of the external forces during physicalactions. In The Tight Game, a pair of players engage in a tug ofwar, and believe that they are participating in a well balanced, tightgame. In reality, however, an external system or person mediates
the game, performing physical interventions without the players
noticing.
ACM Reference Format:
Azumi Maekawa, Shunichi Kasahara, Hiroto Saito, Daisuke Uriu, Ganesh
Gowrishankar, and Masahiko Inami. 2020. The Tight Game: Implicit Force
Intervention in Inter-personal Physical Interactions on Playing Tug of War.
∗ Both authors contributed equally to this research.

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For all other uses, contact the owner/author(s).
SIGGRAPH ’20 Emerging Technologies, August 17, 2020, Virtual Event, USA© 2020 Copyright held by the owner/author(s).
ACM ISBN 978-1-4503-7967-0/20/08.
https://doi.org/10.1145/3388534.3407301

In Special Interest Group on Computer Graphics and Interactive Techniques
Conference Emerging Technologies (SIGGRAPH ’20 Emerging Technologies),
August 17, 2020. ACM, New York, NY, USA, 2 pages. https://doi.org/10.1145/3388534.3407301

1INTRODUCTION

Tight games, especially in sports, excite both players and audiences.
On the other hand, predictable games that result from a huge differences in physical abilities between players are typically boring.
This research investigates how technology can be used to (artificially) create dramatic, close games and empirically survey how
both players enjoy the fixed games.
Realtime physically assistance technologies can provide us augmented experience with physical motions [Kasahara et al. 2019;
Maekawa et al. 2020]. While physical assistance can narrow the skill
gap between players, however, the obvious ‘explicit’ interventions
removes the sense of being in control—the sense of agency—fromthe players. Without the sense of agency, players are discouraged
from engaging in the game, since they noticed that the game isfixed by something (one) out of their control. Applying previous
knowledge, we endeavoured to artificially create a close game in aphysical game (sport) involving multiple human players, carefully
managing players’ sense of agencies.
We propose an implicit physical intervention system called The
Tight Game for the ‘tug of war’, a one-dimensional physical game.
Our system includes four force sensors in the rope and two hidden
high torque motors that provide realtime physical assistance. We
designed the implicit assistance by leveraging human recognitionof external forces during physical actions. In a game, a pair of

SIGGRAPH ’20 Emerging Technologies, August 17, 2020, Virtual Event, USA

Maekawa and Kasahara, et al.

3

IMPLICIT FORCE ASSISTANCE DESIGN

To perform unnoticeable interventions, we designed the realtime
force assistance by leveraging forces fitting to humans’ recognition.
Each motor output force Fmi are described as;
Fmi = Ri f (Fs 1 + Fs 2 ) + Fc(i = 1, 2)

Figure 2: 1) The players pull at the rope using handles. 2) Twoforce sensors on the handle measure forces at both the front
and rear ends, which detects how each player is applying
force. 3) A BLDC motor pulls at the rope from behind eachplayer.

Here, f (Fs 1 +Fs 2 ) is a non-linear assistive force which is dependent
on the pulling (front) force sensor values (Fsi ) of both players. This
provides a less noticeable force assistance to the players, because
the processing keeps force assistance in the range of force by humans based on the Weber–Fechner law in the output force and theperceived force in muscle activity (e.g., [Takemura et al. 2013]). Fc
is a constant force applied to prevent slack in the rope.
With this reactive force assistance, the system or/and the mediator can control the balance of the force assistance between twoplayers by changing R 1 and R 2 (Here, R 1 + R 2 = 1.0 ). From ourpreliminary experiment, we found that naive (sudden) control of
the assistance rate resulted in players noticing the existence of external force, since the weight of the rope changed asynchronouslywith the players’ actions. Therefore, we only triggered changes in
the assistance rate when the players were actively pulling on therope. As depicted in Fig 3, the system detects the players’ pullingas a the difference within a pair of force sensor values. Changes in
the assistance rates (R 1 and R 2 ) are made gradually to match themediator’s input. When the player who is to be assisted gives a bigpull, the assistance rate will quickly update to match the mediator’sinput.

4Figure 3: The system detects players’ impulse pulling fromthe force sensors. Only when the player who will be assisted(Player1) gives a big pull, the assistance rate updates quickly.
players pull at the rope and believe that they participating in a tightgame. In reality, however, an external systems or person mediates
the game, performing physical interventions without the players
noticing.

2THE TIGHT GAME SYSTEM

A unit of our system consists of 1) two pairs of force sensors on
the handle (Fig. 2-2) connected with the rope, and 2) a high torquemotor (Fig. 2-3) at the end of the rope. We adopted a large-diameterbrushless DC (BLDC) motor with a low reduction ratio. This motor
was chosen for its backdrivability and high torque output. Thesensors and motors are all connected to a main computer which
controls the torque outputs according to the force sensor signals.
In our system, the two motor units are hidden from the players
such that they unaware of the presence of the physical assistance.
In addition to the hardware units, the system also has a remoteuser interface (Fig. 1-2) that can be provided to a mediator. The
user interface allows the mediator to to control and intervene in
the game without the players noticing.

CONCLUSIONS

We demonstrated implicit force intervention in the game of tug
of war as an example of inter-personal physical interaction. Our
design of the force assistance allows the system or another user tocontrol the status of the game without the players noticing. With
our implicit physical intervention, we also demonstrated a method
for balancing individual physical differences between players without compromising the players’ sense of agency and engagement insports activities.

ACKNOWLEDGMENTS
We would like to thank Zendai Kashino for proofreading and usefulcomments. This work is supported by JST ERATO Grant Number JPMJER1701, Japan.

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