This study assessed the biomechanical effect of a counterforce brace on the common extensor origin (CEO) tendon at the elbow via the measurement of shear wave velocity (SWV) using ultrasound. The counterforce brace was hypothesised to reduce SWV, which is a proxy measure of tendon stiffness, whilst the wrist and finger extensors were contracting at different levels of maximum voluntary contraction (MVC). In this cross-sectional study, nineteen healthy participants (age±SD:​ 30±9) were included in the study. The counterforce brace was applied with either 0 or 80 mmHg pressure to the forearm. The SWV was measured under four different wrist extensors MVC levels:​ 0%, 20%, 30%, and 40%. The counterforce brace had no significant effect on CEO tendon SWV at rest (V-statistic = 86, p = 0.74), 20% (V-statistic = 105, p = 0.71), 30% (V-statistic = 87, p = 0.77), or 40% (V-statistic = 94, p = 0.98) of MVC. The Friedman test for repeated measures showed an increase in SWV with greater levels of wrist extension MVC (x² = 7.9, p = 0.048). In conclusion, the counterforce brace does not appear to have a biomechanical effect on the CEO of the elbow during resting conditions or whilst the wrist extensors are contracting. The SWV of the CEO, a proxy for tendon stiffness, increases with greater levels of MVC.