Spiral humerus fractures associated with extreme muscular torsion loading have been well documented in literature. Throwing motions and arm wrestling are the two causes most often researched, while spiral fractures associated with gymnastics have received less attention. The purpose of this study is to explore the factors that may contribute to torsional failure of the humerus while performing a gymnastics move known as a muscle-up. Primary motivation for this study was the result of the author sustaining a spiral fracture to the distal aspect of her left humerus while attempting a muscle-up. To the author’s knowledge, no previous studies analyzing the forces imposed on the upper extremities during a muscle-up have been conducted.

Utilizing the author’s estimated anthropometric measurements and the kinematic and kinetic constraints of the muscle-up activity, the torque acting about the long axis of the humerus was determined in two ways. First, an analytical approach was used to calculate the forces and moments within a simplified linkage representation of the upper extremity for several representative muscle-up postures. The second method was a computer simulation that modeled the entire body with muscles in several different kinematic positions and outputted internal body elbow joint net moments.

The analytical approach resulted in torques between 12.0 N·m and 29.3 N·m. The kinetics derived with the computer simulation revealed joint reaction torques between 13 N·m and 38 N·m and net axial torques between 29.1 N·m and 69.1 N·m acting on the left humerus. The internal moments predicted using the computer simulation were above the author’s minimum predicted torque, 53 N·m, associated with humerus fracture initiation.

Although there may be many factors that contribute to spiral humerus fracture, in this study, it was determined that the kinematic positions of the muscle-up movement are sufficiently extreme so as to produce torques capable of resulting in spiral humerus fracture.

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