Apparatus is disclosed for monitoring, measuring and/or estimating a force applied to a body or body part of a vertebral mammal. The apparatus includes an acceleration sensor for measuring acceleration of the body or body part relative to an inertial frame of reference and for providing data indicative of the acceleration. The acceleration sensor includes at least one inertial sensor, a memory device adapted for storing the data, and a processor adapted for processing the data to evaluate a reaction force that correlates to the data. The processor may be configured to execute an algorithm for evaluating the reaction force, based on one or more correlation components such as mass, speed and/or velocity associated with the body or body part. A method of monitoring and/or estimating a force applied to a body or body part of a vertebral mammal is also disclosed.