Misalignment of the Desired and Measured Center of Pressure Describes Falls Caused by Slip during Turning

Abstract

In this study, desired center of pressure (dCOP) was introduced to evaluate dynamic postural stability. The dCOP is defined as a virtual point on the ground, where the moment around the body center of mass (COM) becomes zero when dCOP and the measured COP (mCOP) coincide. We hypothesized that, when the misalignment of the dCOP and mCOP (dCOP-mCOP) increases up to a certain value due to a large perturbation during walking, it becomes difficult to make a compensatory step and to recover balance of COM and to continue gait. Here we tested this hypothesis in slipping during turning. The study involved twelve healthy young adult males with an average age of 21.5±1.9 yrs. The subjects were asked to (1) walk straight and turn 60 degrees to the right with the right foot (spin turn) on a dry floor surface, and (2) walk straight and 60 degrees spin turn to the right on a slippery lubricated surface. The dCOP-mCOP during turning in the slip trial with fall were significantly larger, particularly in x-direction (i.e., the medial-lateral direction during straight walk), than that in no-slip trial and slip trial without fall. The receiver operating characteristic (ROC) analysis indicated that the dCOP-mCOP in x-direction is good indicator of falling (area under the curve (AUC) = 0.93) and the threshold in the dCOP-mCOP in x-direction to distinguish for fall or no-fall was 0.55 m. These results support our hypothesis in slipping during turning.

Fig 1. Inverted pendulum model and the desired center of pressure (dCOP) in sagittal plane.

The dCOP is defined as a virtual point on the ground, i.e., location where the moment around the body center of mass (COM) becomes zero when dCOP and the measured COP (mCOP) are coincident.

Fig 2. Experimental set-up and movement instruction: (a) straight walk and 60 degrees spin turn on the dummy sheet and (b) straight walk and 60 degrees spin turn on the slip sheet.

Fig 3. Representative trajectory of measured center of pressure (mCOP) and desired COP (dCOP) for straight walk and spin turn without slip on the dummy sheet in (a) x-direction, (b) y-direction, and (c) x-y plane.

TO and HS mean toe-off and heel-strike, respectively.

Fig 4. Representative trajectory of measured center of pressure (mCOP) and desired COP (dCOP) for straight walk and spin turn on the slip sheet with slip without fall in (a) x-direction, (b) y-direction, and (c) x-y plane.

Slip on the right foot occurred during turning; however, subject successfully recovered postural balance due to stepping with the left foot. TO and HS mean toe-off and heel-strike, respectively. CS means compensatory step.

Fig 5. Representative trajectory of measured center of pressure (mCOP) and desired COP (dCOP) for straight walking and spin turn on the slip sheet with fall in (a) x-direction, (b) y-direction, and (c) x-y plane; slip on the right foot occurred during turning, and then subject could not step with left foot and finally fell.

TO and HS mean toe-off and heel-strike, respectively.

Fig 6. Relative location of dCOP with respect to mCOP for spin turn trials (a) on the dummy sheet (without slipping), (b) on the slip sheet with slip but not fall, and (c) on the slip sheet with fall at which the dCOP–mCOP took the largest value during turning in each trial.

Original point corresponds to the location of mCOP, and each plot indicates dCOP location.

Fig 7. Mean value of the maximum dCOP-mCOP in x- and y-directions during spin turn.

Fig 8. ROC curve for prediction of fall.