Power measurement method and apparatus

1. An apparatus for estimating power used by a user in performing a vertical component of a movement, the apparatus comprising:

an accelerometer configured for attachment to a user and configured for measuring components of the acceleration along three orthogonal axes experienced by the user;

a processor configured to:

receive the measured components of the acceleration A x , A y , and A z , along three orthogonal axes x a , y a , and z a from the accelerometer,

estimate an orientation of the accelerometer by calculating an angle θ between the acceleration and an axis that has the component with the highest magnitude of acceleration by:

calculating the magnitude of each of the components of the acceleration from the measurements indicating the components of the acceleration A x , A y , and A z along the three orthogonal axes x a , y a , and z a of the accelerometer,

identifying the acceleration component with the highest magnitude,

determining the angle θ between the acceleration and the axis with the highest component of acceleration from

θ

=

arc

⁢

⁢

tan

⁡

[

A

2

2

+

A

3

2

A

1

]

where A 1 is the component with the highest magnitude, and A 2 and A 3 are other components of acceleration along the three orthogonal axes;

estimate a vertical acceleration component from the acceleration components and the angle θ, and

estimate the power used from the vertical acceleration component; and

a transceiver configured to receive the estimated power from the processor, and configured to transmit the estimated power to a remote computer.

2. The apparatus as claimed in claim 1 , wherein the movement is a sit-to-stand (STS) transfer and the accelerometer is configured for measuring the acceleration along the three orthogonal axes during the STS transfer.

3. The apparatus as claimed in claim 2 , wherein the processor is further configured to calculate local instability over j samplings based on a variance of a norm of the components of the acceleration by calculating:

var i−b i+a (√{square root over (A x (j) 2 +A y (j) 2 +A z (j) 2 )})

where a is a number of sampling instants after a sampling instant i at which the orientation of the calculator is calculated,

b is a number of sampling instants before the sampling instant i at which the orientation of the calculator is calculated.

4. The apparatus as claimed in claim 3 , wherein the processor is configured to indicate instability when

var i−b i+a (√{square root over (A x (j) 2 +A y (j) 2 +A z (j) 2 )})

is greater than 15 m/s 2 .

5. The apparatus as claimed in claim 1 , further comprising:

a remote display device of the remote computer configured to:

receive the estimate of power used during the STS transfer from the transceiver, and

display the estimate of power used during the STS transfer.

6. The apparatus as claimed in claim 1 , wherein the processor is further configured to compensate the measured components of acceleration for a change in orientation of the accelerometer.

7. A physical therapy apparatus for estimating power used by a user in performing a sit-to-stand (STS) transfer, the apparatus comprising:

an accelerometer configured for attachment to a user and configured for measuring acceleration experienced by the user during an STS transfer; and

a processor configured to:

receive the measurements of the acceleration from the accelerometer;

estimate vertical accelerations from the received measurements by:

estimating an orientation of the accelerometer from the received acceleration measurements; and

identifying the vertical accelerations in the received acceleration measurements using the estimated orientation; and

wherein the processor is further configured to:

based only on the estimated vertical accelerations, a mass of the user, and gravity, estimate the power used during the STS transfer; and

compensate for errors in the estimated orientation by computing a variance of a norm of components of the received acceleration measurements over a period of time; and

wherein the apparatus further comprises a transceiver configured to receive the estimated power from the processor, and configured to transmit the estimated power to a remote display device to cause the remote display device to display an indication of an instability when the computed variance is greater than a threshold.