Introduction Isometric manipulation is a current practice in osteopathy and treatment benefits have been reported in the literature. Such benefits could be assessed using experimental non-invasive cervical mobility measurements. The main objective was to quantitatively measure the effects of isometric manipulation on principal and compensatory cervical motions. Methods 101 healthy volunteers were included in this study. 51 healthy volunteers selected randomly underwent the experimental protocol before and after isometric treatment and were compared to 50 healthy volunteers who underwent a placebo treatment. Osteopathic diagnosis was performed on each healthy volunteer before and after the treatment. The experimental protocol included measurements by a motion capture system focusing on principal range of motion and compensatory motions. Results In both the isometric and the placebo sample, respectively including 51 (age: 29.2 ± 8.1, BMI: 22.2 ± 3.5) and 50 healthy volunteers (age: 27.4 ± 6.8, BMI: 22.9 ± 2.8), a pre-treatment diagnosis revealed a light cervical dysfunction in all subjects, mainly in levels C3 and C4. Altered ranges of motion thresholds (C3/C4 alterations) were identified: 113.2° for flexion, 130.0° for rotation and 90.2° for lateral flexion. After manipulations, the volunteers who underwent the isometric treatment presented a slight increase in amplitude for lateral flexion (p \textless 0.04), which was not found in the volunteers who underwent the placebo treatment. Compensatory motions showed differences pre and post isometric treatment without reaching significant values. Conclusion Principal ranges of motion were found significantly higher after osteopathic treatment when compared to the placebo treatment. Osteopathic palpatory diagnosis showed significant correlation with range of motions before treatment.