Lumbar mobilization is a standard intervention for the management of low back pain, yet ways to quantify lumbar mobilization are limited. An inertial measurement unit (IMU) is a small and inexpensive device that can be used to quantify lumbar mobilization. The objective of this study was to determine the validity and reliability of an IMU in measuring the amplitude of displacement of a clinician's hand movement during oscillatory lumbar mobilization.An IMU was secured on a clinician's hand during application of mobilization forces at the L4 segment of 16 healthy participants. The validity of the IMU was tested against common laboratory methods of measurements (force plate and motion capture system). The reliability of the IMU measurements was determined between 2 clinicians (inter-rater reliability) and between 2 sessions (intra-rater reliability) by calculating percent error of measurement (%e) and limits of agreement (LOA). The reliability was considered high when |%e| ≤ 10% and |LOA| ≤ 20%; moderate when |%e| 10% to 20% and |LOA| 21% to 40%; and non-acceptable when |%e| > 20% and |LOA| > 40%.The IMU measurements had high correlation with the force plate measurements (rs = 0.94) and high agreement with the motion capture system measurements (%e = 4%, LOA = -11% and 20%). Both the inter-rater reliability (%e = 6%, LOA = -25% and 37%) and the intrarater reliability (%e = -1%, LOA = -29% and 27%) of IMU measurements were moderate.The IMU seems to be a valid device to measure the amplitude of a clinician's hand movement. The moderate reliability found in this study may not reflect poor reliability of the IMU as much as inconsistency in reapplication of lumbar mobilization.