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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
1-2 g
low
1-2 g
low
1-2 g
higher
several grams
Natural Frequency
2000 – 5000 Hz
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
one magnitude higher than upper limit of range
several magnitudes 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
Packed run
Packed run
10m/s
10m/s
15m/s
25m/s
1
2
-
-
2
3
-
-
4-6
30-60
60-120
100-200
Walking 1 1 2-5
Running:
Heel-toe
Heel-toe
Toe
On asphalt
On grass
On asphalt
1-3
1-3
1-3
2-4
2-4
2-4
5-17
5-10
5-12
Gymnastics:
Landing from 1.5m
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
3-7
2
3
3
3
8-14
5
14
8
7
25-35
8
24
10
9
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)
-0.5
0
0.5
1
1.5
2
2.5
0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5 5.5 6 6.5 7 7.5 8 8.5 9 9.5Time (sec)
Acceleration (g)
Elbow Joint Angle (rad)
Bar Acceleration - Bench Presses (20 Kg Load, Intermediate Frequency)
-0.5
0
0.5
1
1.5
2
2.5
0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5 5.5 6 6.5 7 7.5 8 8.5 9 9.5Time (sec)
Acceleration (g)
Metronome (approx. 1 rep/2sec)Elbow Joint Angle (rad)
Bar Acceleration - Bench Presses (20 Kg Load, High Frequency)
-0.5
0
0.5
1
1.5
2
2.5
0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5 5.5 6 6.5 7 7.5 8 8.5 9 9.5Time (sec)
Acceleration (g)
Metronome (1 rep/1.2 sec)
Elbow Joint Angle (rad)
Bar Acceleration - Bench Presses (20 Kg Load, Slow Frequency)
-0.5
0
0.5
1
1.5
2
2.5
0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5 5.5 6 6.5 7 7.5 8 8.5 9 9.5Time (sec)
Acceleration (g)
Elbow Joint Angle (rad)
Bar Acceleration - Bench Presses (20 Kg Load, Intermediate Frequency)
-0.5
0
0.5
1
1.5
2
2.5
0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5 5.5 6 6.5 7 7.5 8 8.5 9 9.5Time (sec)
Acceleration (g)
Metronome (approx. 1 rep/2sec)Elbow Joint Angle (rad)
Bar Acceleration - Bench Presses (20 Kg Load, High Frequency)
-0.5
0
0.5
1
1.5
2
2.5
0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5 5.5 6 6.5 7 7.5 8 8.5 9 9.5Time (sec)
Acceleration (g)
Metronome (1 rep/1.2 sec)
Elbow Joint Angle (rad)
Bar Acceleration - Bench Presses (50 Kg Load, Slow Frequency)
-0.5
0
0.5
1
1.5
2
2.5
0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5 5.5 6 6.5 7 7.5 8 8.5 9 9.5Time (sec)
Acceleration (g)
Elbow Joint Angle (rad)
Bar Acceleration - Bench Presses (50 Kg Load, Intermediate Frequency)
-0.5
0
0.5
1
1.5
2
2.5
0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5 5.5 6 6.5 7 7.5 8 8.5 9 9.5Time (sec)
Acceleration (g)
Metronome (approx. 1 rep/1.9 sec)
Elbow Joint Angle (rad)
Bar Acceleration - Bench Presses (50 Kg Load, High Frequency)
-0.5
0
0.5
1
1.5
2
2.5
0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5 5.5 6 6.5 7 7.5 8 8.5 9 9.5Time (sec)
Acceleration (g)
Metronome (1 rep/1.2 sec)
Elbow Joint Angle (rad)
Bar Acceleration - Bench Presses (50 Kg Load, Slow Frequency)
-0.5
0
0.5
1
1.5
2
2.5
0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5 5.5 6 6.5 7 7.5 8 8.5 9 9.5Time (sec)
Acceleration (g)
Elbow Joint Angle (rad)
Bar Acceleration - Bench Presses (50 Kg Load, Intermediate Frequency)
-0.5
0
0.5
1
1.5
2
2.5
0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5 5.5 6 6.5 7 7.5 8 8.5 9 9.5Time (sec)
Acceleration (g)
Metronome (approx. 1 rep/1.9 sec)
Elbow Joint Angle (rad)
Bar Acceleration - Bench Presses (50 Kg Load, High Frequency)
-0.5
0
0.5
1
1.5
2
2.5
0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5 5.5 6 6.5 7 7.5 8 8.5 9 9.5Time (sec)
Acceleration (g)
Metronome (1 rep/1.2 sec)
Elbow Joint Angle (rad)
Bar Acceleration - Bench Presses (70 Kg Load, Slow Frequency)
-0.5
0
0.5
1
1.5
2
2.5
0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5 5.5 6 6.5 7 7.5 8 8.5 9 9.5Time (sec)
Acceleration (g)
Elbow Joint Angle (rad)
Bar Acceleration - Bench Presses (70 Kg Load, Intermediate Frequency)
-0.5
0
0.5
1
1.5
2
2.5
0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5 5.5 6 6.5 7 7.5 8 8.5 9 9.5Time (sec)
Acceleration (g)
Metronome (approx. 1 rep/2.2 sec)
Elbow Joint Angle (rad)
Bar Acceleration - Bench Presses (70 Kg Load, High Frequency)
-0.5
0
0.5
1
1.5
2
2.5
0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5 5.5 6 6.5 7 7.5 8 8.5 9 9.5Time (sec)
Acceleration (g)
Metronome (1 rep/1.2 sec)
Elbow Joint Angle (rad)
Bar Acceleration - Bench Presses (70 Kg Load, Slow Frequency)
-0.5
0
0.5
1
1.5
2
2.5
0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5 5.5 6 6.5 7 7.5 8 8.5 9 9.5Time (sec)
Acceleration (g)
Elbow Joint Angle (rad)
Bar Acceleration - Bench Presses (70 Kg Load, Intermediate Frequency)
-0.5
0
0.5
1
1.5
2
2.5
0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5 5.5 6 6.5 7 7.5 8 8.5 9 9.5Time (sec)
Acceleration (g)
Metronome (approx. 1 rep/2.2 sec)
Elbow Joint Angle (rad)
Bar Acceleration - Bench Presses (70 Kg Load, High Frequency)
-0.5
0
0.5
1
1.5
2
2.5
0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5 5.5 6 6.5 7 7.5 8 8.5 9 9.5Time (sec)
Acceleration (g)
Metronome (1 rep/1.2 sec)
Elbow Joint Angle (rad)