ACCELEROMETRY (Much of this material was excerpted from Nigg, B. M. & Herzog, W. (1994)....

transcript

ACCELEROMETRY

(Much of this material was excerpted from Nigg, B. M. & Herzog, W. (1994).

Biomechanics of the Musculo-Skeletal System. Chichester, England: John Wiley & Sons, Ltd.

and additional information was added.)

Acceleration - The time derivative of velocity or the second time derivative of

position

Accelerometer - A device used to measure acceleration

History:The use of accelerometers to measure

acceleration is a relatively new technique in biomechanics. Their use in biomechanics is not widespread, possibly due to the problems

associated with these measurements.

Measuring Acceleration:• There are various methods in biomechanics that

are used to measure either linear and/or acceleration. Some involve the capture of images at known intervals of time (e.g., cinematography, videography, and stroboscopic photography) form which the second time derivative of position is determined. These optical methods have inherent problems associated with errors in position data which result in erratic acceleration parameters.

Measuring Acceleration:• Electronic devices such as potentiometers and

optical encoders have also been used to indirectly measure position values from which linear and angular acceleration may be derived. Sampling frequency and accuracy of the instrumentation may also cause derived acceleration values to be unacceptable

Measuring Acceleration:• Accelerometers are the most recent

developments in the electronic measurement of acceleration. There are different types of accelerometers. The type is based on the measurement technique employed within the accelerometer. The types of accelerometer are strain gauge, piezoresistive, peizoelectric, and inductive. In all cases, strain on the accelerating mass causes a voltage change proportional to the acceleration.

Piezoresistive and Piezoelectric Accelerometers

Piezoelectric Accelerometer

Inductive Accelerometer

Accelerometer CharacteristicsCharacter-istics

Sensor Type

Strain Gauge Piezoresistive Piezoelectric Inductive

Mass low

higher

several grams

Natural Frequency

2000 – 5000 Hz

20000 – 30000 Hz 200 – 400 Hz

Frequency Response

0 – 1000 Hz 0 – 1000 Hz 0 – 5000 Hz low; no shock measurement

Shock Rating

one magnitude higher than upper limit of range

several magnitudes higher than upper limit of range

Range limited to about one magnitude

limited to about one magnitude

0.01 to 10000 g about 30 g

Advantages measure static and dynamic acceleration

measure static and dynamic acceleration

excellent range accuracy in low frequency accelerations

Disadvan-tages

limited range limited range Limited use for static measurements

limited range

Questions to Be Asked When Using Accelerometers with Humans

• Which acceleration should be determined:– Rigid part of segment?– Soft tissue part of segment?– Average acceleration of soft and rigid tissue?

• How well does measured acceleration correspond to actual acceleration of interest?

Mounting Accelerometer to Bone

• Screwed to bone

• Strap at location on segment with minimal amount of soft tissue between accelerometer and bone

Model to Study Behavior of Accelerometer with Various

Interfaces and Landing Surfaces

Independent Variables

• Mounting– Screws (location?)– Strapping

• Light• Intermediate• Strong

• Landing surface (adhered to cement)– Foam rubber (soft)– Tartan – Linoleum (hard)

Results

1. atrue and askin only identical in a few cases

2. askin <, =, or > atrue

3. askin < or = atrue for light strapping

4. askin > and < atrue for strong strapping

5. For hard surface askin < atrue

6. For softest surface 50% of askin > atrue

Peak Acceleration ValuesActivity Comments Magnitudes of Acceleration (g)

Head Hip Tibia

Skiing:

Powder snow

Packed run

60-120

100-200

Walking 1 1 2-5

Running:

Heel-toe

On asphalt

On grass

On asphalt

Gymnastics:

Landing from 1.5m

Round-off somersault

Straddle dismount

Take-off minitramp

On 7 cm mat

On 40 cm mat

On 7 cm mat

On 40 cm mat

Acceleration < for larger body mass?

Increase # of joints between force and measurement = acceleration

Acceleration < for softer surfaces

Example Experiment

Elbow Joint Angle and Acceleration Patterns in the Bench Press with Various Loads (20,

50, and 70 kg) and Frequencies (slow, intermediate, and high) of Lift

• With same weight what would you expect to see in range of motion of elbow joint motion as frequency changes?

• With same weight what would you expect to see in acceleration pattern of the bar as frequency changes?

• At what point in the lift would you expect to see maximum acceleration under different frequencies and loads?

• What would you suggest is the best pattern of bench press? Why?

Bar Acceleration - Bench Presses (20 Kg Load, Slow Frequency)