1. Yoichiro Sato1), Hiroshi Nagasaki2), Norimasa Yamada3)
Organized joint coordination forms a task-dependent trajectory of the body center of mass
Yoichiro Sato1), Hiroshi Nagasaki2), Norimasa Yamada3)
　 1) Hokkaido University of Science, Department of Physical Therapy
　 2) Tohoku Bunka Gakuen University
　 3) Chukyo University 1

2. “The degree of freedom problem (Bernstein, 1967)”
Introduction
Whole-body motions involve multi-joint movements.
Various combinations of joint movements are used to accomplish a displacement of body center of mass (COM).
The central nervous system select one combination of joint movements from an innumerable set.
The central nervous system needs to control various potential COM and joint movements.
“The degree of freedom problem (Bernstein, 1967)”

3. Introduction Coordination among joints should organize as
Solutions for the problem have been proposed.
In a sit-to-stand motion, the central nervous system would control primarily the body COM (Scholz & Schöner 1999).
In a task maintaining the equilibrium against perturbations, the central nervous system controls only the body COM (Scholz et al. 2007)
These proposed that the central nervous system controls
only the COM but not joint movements
Coordination among joints should organize as
a lawful shape
We hypothesized

4. Introduction
We selected downward squatting as a task to elucidate the mutual relationship between the COM trajectory and joint movements.
Individuals can perform this motion with various COM trajectory without falling as his COM represents his base of support (Massion 1992).
Combinations among joints are redundant to determine COM coordinates in this motion.
Elucidate the relationship between the COM path and shape of joint coordination with altering one segment angle.
the purpose was to

5. Methods – Participants & Procedures
Eight healthy males (aged 23 yrs; height 1.73 m; mass 67.6 kg)
The subjects performed squat downward by about 15 cm
Speed: subject's preferred
Conditions: the trunk angle relative to the vertical line varied at the final position; 0, 20, 30, 50, and 70 degrees
Five trials were performed under each condition after practice a few times 0°
20°
30°
50°
70°

6. Methods – Analysis
Three-Dimensional motion analysis system (Vicon 370, 60 Hz)
Infrared reflective markers were placed on
Right Shoulder, Hip, Knee, Ankle, and Toe
The data were smoothed by using a low pass filter at 5 Hz
The segment angles were calculated
The COM coordinates (X, Y) of the whole body in the
sagittal plane were estimated
X = m1x1 + m2x2 + m3x3
Y = m1y1 + m2y2 + m3y3
Relative anteroposterior displacements were calculated
m3 (x3,y3)
(X,Y)
m2 (x2,y2)
m1 (x1,y1)

7. Methods – Statistical analysis
Two separate repeated measure analysis of variance were performed for the relative anteroposterior displacement of the COM
The cutoff for statistical significance was set at P < 0.05

8. Results – Center of Mass Trajectory
0 °
20 °
30 °
50 °
70 °
Upward
Anterior
0.1 m
The relative anteroposterior displacement showed
no statistical difference

9. Results – Shape of Joint coordination
Thigh angle
constant
Shank angle
systematical alteration

10. Results – Joint coordination (Timing)
70°
50°
30°
20° 0°
Relationship between Knee and Hip flexions
Linear
under all conditons

11. Discussion – COM trajectory
COM trajectory showed no difference under all trunk conditions
Paths of the COM show the task-dependent shape
(Saunders et al, 1953; Bernstein, 1996)
COM trajectories showed approximately vertical paths in
almost subjects.
The verticality is the form that
minimized changes of joint torques
(Winter et al, 2001; Yamasaki et al, 2011)

12. Discussion – Shape of joint coordination
The shank inclination angle decreased as the trunk angle increased
Shank and trunk angles coordinate together
(synergy (Latash et al, 2002) )
The thigh angle remained constant
Thigh angle determines a depth of COM
during squatting

13. Discussion – Joint coordination (Timing)
Relationship between knee and hip flexions showed linear
Knee and hip joints flex simultaneously
during squatting
This finding would useful for healthcare exercises
Ex) Teaching/Coaching squatting exercise etc.

14. Conclusion
The findings would help to improve individual’s health

15. Thank you for your attention!