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| EXSC 663-Research Methods in Biomechanics |
Chapter 5 - Objectives
- Define the process of inverse dynamics for planar motion analysis
- Present the standard method for numerically computing the internal kinetics of planar human movements
- Describe the concept of general plane motion
- Outline the method of sections for individually analyzing components of a system or segments of a human body
- Outline how inverse dynamics aids research of joint mechanics
- Examine applications of inverse dynamics in biomechanics research
PLANAR MOTION ANALYSIS
Numerical Formulation
- Single Segment Analysis
- Consider an arm being held horizontally. What are the shoulder reaction forces and joint moment? Assume that the arm is stationary and rigid. The masses of the upper arm, forearm, and hand are 4, 3, and 1 kg, respectively, and their mass centers are, respectively, 10, 30, and 42 cm from the shoulder.
- Multisegment Analysis
- Determine the joint reaction forces and moments at the ankle, knee, and hip given the following data. These occured during the swing phase of walking, so the GRFs are zero. The ankle is at (0.303, 0.189) m, the knee at (0.539, 0.420) m, and the hip is at (0.600, 0.765) m.
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Mass (kg)
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I (kgm2)
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ax (m/s2)
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ay (m/s2)
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α (rad/s2)
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CM (m)
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| Foot |
1.2
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0.011
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-4.39
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6.77
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5.12
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(0.373, 0.117)
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| Leg |
2.4
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0.064
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-4.01
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2.75
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-3.08
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(0.437, 0.320)
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| Thigh |
6.0
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0.130
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6.58
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-1.21
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8.62
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(0.573,0.616)
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- Determine the joint reaction forces and moments at the ankle, knee, and hip given the following data. These occured during the stance phase of walking, so the ground reaction forces are nonzero. The ankle is at (0.637, 0.063) m, the knee at (0.541, 0.379) m, and the hip at (0.421, 0.708) m. The horizontal GRF is -110 N, the vertical GRF is 720 N, and its center of pressure is (0.677, 0,0) m.
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Mass (kg)
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I (kgm2)
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ax (m/s2)
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ay (m/s2)
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α (rad/s2)
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CM (m)
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| Foot |
1.2
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0.011
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-4.39
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6.77
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5.12
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(0.373, 0.117)
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| Leg |
2.4
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0.064
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-4.01
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2.75
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-3.08
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(0.437, 0.320)
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| Thigh |
6.0
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0.130
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6.58
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-1.21
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8.62
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(0.573,0.616)
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Limitations
- There are errors in the methods for collecting 2D data
- There are frictional forces within joints that remain unknown
- Some of the forces are attenuated since the segments are not completely rigid (especially a problem with the foot and trunk)
- The calculations are very sensitive to errors in GRFs, center of pressure measurements, segment inertial properties, joint center estimates, and segment accelerations.
- Individual muscle, tendon, and ligament forces remain unknown
- We have no knowlegde of cocontractions
Applications
- Finding characteristic patterns to predict whether those with artificial joints or pathological conditions will have sufficient support when walking or running
- Determining compressive loads on the spine during lifting and lower back pain
- Determine the compressive and shear loads in other joints
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