Actions of the scalene muscles for rotation of the cervical spine in macaque and human

Study Design: Multiple single-subject design in 2 parts: 1 in anesthetized monkeys and a follow-up in human cadavers. Objectives: To determine whether anterior, middle, and posterior scalene muscles rotate the cervical spine to the same (ipsilateral to the muscle) or opposite (contralateral to the muscle) Background: Some physical therapy and anatomy textbooks indicate that all 3 scalenes rotate cervical spine to the same side, some indicate that all rotate to the opposite side, and the rest ascribe different functions to the different scalenes. Methods and Measures: While under anesthesia, macaques (n = 3) already scheduled for euthanasia were implanted with stimulating electrodes in each scalene muscle on one side, and then a neuromuscular junction blocker was administered to prevent confounding movement from brachial plexus stimulation. Three observers independently rated the direction of rotation produced by electrical stimulation. Postmortem dissection of the macaques was used to which direction of passive rotation stretched each scalene. Postmortem analyses in 2 human cadavers were also conducted to determine which direction of rotation stretched the human scalenes. Results: Electrical stimulation in the macaque produced rotation to the same side for each of the 3 scalenes. Passive rotation to the opposite side put each scalene muscle of the macaque on stretch. In the human, rotation to the opposite side also stretched each scalene. Conclusions: All 3 scalene muscles produce rotation of the cervical spine to the same side. Maximum stretching of the scalenes should include rotation to the opposite side.