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 Characteristics Character- istics Sensor Type Strain GaugePiezoresistivePiezoelectricInductive Masslow 1-2 g low 1-2 g low 1-2 g higher several grams Natural Frequency 2000 – 5000 Hz – Hz200 – 400 Hz Frequency Response 0 – 1000 Hz 0 – 5000 Hzlow; no shock measurement Shock Rating one magnitude higher than upper limit of range several magnitudes higher than upper limit of range Rangelimited to about one magnitude 0.01 to gabout 30 g Advantagesmeasure static and dynamic acceleration excellent rangeaccuracy in low frequency accelerations Disadvan- tages 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.a true and a skin only identical in a few cases 2.a skin a true 3.a skin < or = a true for light strapping 4.a skin > and < a true for strong strapping 5.For hard surface a skin < a true 6.For softest surface 50% of a skin > a true

Peak Acceleration Values ActivityCommentsMagnitudes of Acceleration (g) HeadHipTibia Skiing: Powder snow Packed run 10m/s 15m/s 25m/s Walking112-5 Running: Heel-toe 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?