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ACCELEROMETRY

Learn about the use of accelerometers in measuring acceleration in biomechanics, including different measurement techniques, types of accelerometers, and considerations when using accelerometers with humans. Explore a case study on elbow joint angle and acceleration patterns in the bench press with various loads and frequencies.

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ACCELEROMETRY

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  1. 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.)

  2. Acceleration - The time derivative of velocity or the second time derivative of position Accelerometer - A device used to measure acceleration

  3. 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.

  4. 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.

  5. 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

  6. 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.

  7. Piezoresistive and Piezoelectric Accelerometers

  8. Piezoelectric Accelerometer

  9. Inductive Accelerometer

  10. Accelerometer Characteristics

  11. 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?

  12. Mounting Accelerometer to Bone • Screwed to bone • Strap at location on segment with minimal amount of soft tissue between accelerometer and bone

  13. Model to Study Behavior of Accelerometer with Various Interfaces and Landing Surfaces

  14. Independent Variables • Mounting • Screws (location?) • Strapping • Light • Intermediate • Strong • Landing surface (adhered to cement) • Foam rubber (soft) • Tartan • Linoleum (hard)

  15. Results • atrue and askin only identical in a few cases • askin<, =, or > atrue • askin< or = atrue for light strapping • askin> and < atrue for strong strapping • For hard surface askin< atrue • For softest surface 50% of askin> atrue

  16. Peak Acceleration Values Acceleration < for larger body mass? Increase # of joints between force and measurement =  acceleration Acceleration < for softer surfaces

  17. 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

  18. 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?

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