Human wrist motors: Biomechanical design and application to tendon transfers

G. J. Loren, S. D. Shoemaker, T. J. Burkholder, M. D. Jacobson, J. Fridén, Richard L. Lieber*

*Corresponding author for this work

Research output: Contribution to journalArticlepeer-review

128 Scopus citations


Moment arm, muscle architecture, and tendon compliance in cadaveric human forearms were determined and used to model the wrist torque-joint angle relation (i.e. wrist torque profile). Instantaneous moment arms were calculated by differentiating tendon excursion with respect to joint rotation. Maximum isometric tension of each wrist muscle-tendon unit was predicted based on muscle physiological cross-sectional area. Muscle forces were subsequently adjusted for sarcomere length changes resulting from joint rotation and tendon strain. Torque profiles were then calculated for each prime wrist motor (i.e. muscle-tendon unit operating through the corresponding moment arm). Influences of moment arm, muscle force, and tendon compliance on the torque profile of each motor were quantified. Wrist extensor motor torque varied considerably throughout the range of motion. The contours of the extensor torque profiles were determined primarily by the moment arm-joint angle relations. In contrast, wrist flexor motors produced near-maximal torque over the entire range of motion. Flexor torque profiles were less influenced by moment arm and more dependent on muscle force variations with wrist rotation and with tendon strain. These data indicate that interactions between the joint, muscle, and tendon yield a unique torque profile for each wrist motor. This information has significant implications for biomechanical modeling and surgical tendon transfer.

Original languageEnglish (US)
Pages (from-to)331-342
Number of pages12
JournalJournal of Biomechanics
Issue number3
StatePublished - Mar 1996


  • Moment arm
  • Muscle architecture
  • Sarcomere length
  • Tendon
  • Tendon transfer
  • Wrist joint strength

ASJC Scopus subject areas

  • Biophysics
  • Orthopedics and Sports Medicine
  • Biomedical Engineering
  • Rehabilitation


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