Article
Resting Forearm Position and its Relevance to the Dart Thrower’s Motion
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Authors
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Mark Ross
- Brisbane Hand and Upper Limb Research Institute, Brisbane Private Hospital, Brisbane, Australia
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Libby J. Anderson
- Brisbane Hand and Upper Limb Research Institute, Brisbane Private Hospital, Brisbane, Australia
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Amaan Sood
- Brisbane Hand and Upper Limb Research Institute, Brisbane Private Hospital, Brisbane, Australia
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Greg Couzens
- Brisbane Hand and Upper Limb Research Institute, Brisbane Private Hospital, Brisbane, Australia
| Published: | February 6, 2020 |
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Outline
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Objectives/Interrogation: We propose that the net effect of forearm rotational motion is to keep the oblique DTM (Dart Thrower's Motion) axis aligned with the visual axis during wrist motion in order to improve accuracy through visual control. We also propose that the default resting position of the forearm places the hand approximately 35° pronated relative to the vertical axis, which brings the oblique axis parallel to the visual axis.
Methods: For the first aim, we used the optical motion capture system (Optotrak Certus®) with infrared emmiting surface markers to perform an integrated 3D upper limb kinematic analysis on 4 subjects. For the second aim, we examined 20 normal volunteers using motion analysis technology to ascertain the comfortable resting position of the dominant and non-dominant limbs.
Results and Conclusions: The targeting phase (coupled, biplanar motion) occurs when the oblique plane axis aligns with the visual axis at approximately 35° of forearm pronation. The targeting phase is the only portion of oblique plane activity which demonstrates true DTM (coupling of flexion/extension and radial deviation/ulnar deviation). When asked to rotate into a comfortable resting position, subjects adopted a mean resting forearm posture of 37° of pronation. The average comfortable resting forearm position of the non-dominant limb was found to be angled approximately 6 degrees greater in pronation when compared to the dominant limb. This difference was statistically significant.
The majority of function in oblique plane activities takes place in the extension/radial deviation quadrant, in pronation. There is little utilisation of the flexion/ulnar deviation quadrant or supination during dart-throwing motion activities. The targeting phase is the only phase demonstrating true DTM (collinear coupling of flexion/extension and radial deviation/ulnar deviation). The comfortable resting position of the forearm is in approximately 37° pronation, which corresponds to the position required to align the oblique plane axis with the visual axis.
