Sports Performance Research lnstitute New Zealand, Auckland University of Technology, New Zealand
Auckland Bioengineering Institute & Dept of Engineering Science, University of Auckland, New Zealand
Musculoskeletal tissues, such as bone, muscle, tendon and cartilage, respond and adapt to their local mechanical environment to maintain a stable equilibrium, or homeostasis (Carter et al., 1998). As well as maintaining healthy tissue, mechanical loads can lead to injury and disease. In a broad sense, injury occurs when the loads experienced by the tissue exceed the tissues mechanical strength. These loads might be traumatic, such as a direct impact or single loading event causing failure, or repetitive, where cumulative loads result in damage to the tissue.
Measuring and monitoring the mechanical loads experienced by musculoskeletal tissue is critical to reducing risk of injury as well as prescribing appropriate training strategies to recover from injury.
IMU Step is a tool to measure and monitor the impact loads and long-term load exposure of an athlete to provide subject-specific insight to assist decision making around training volumes and intensities, and return-to-play.
IMU Step measures tibial shock as a surrogate of the impact load that is experienced by the tibia during running and court-based activities, where impact loads dominate the activity or sport. The Bone Stimulus metric that is calculated within IMU Step is NOT a surrogate measure of the ground reaction force, which some people have suggested. The Bone Stimulus represents a cumulative mechanical stimulus that would lead to potential bone remodelling bone stress injury. In the literature this stimulus has been referred to as a Daily Load Stimulus. A reasonable question to ask here is whether we should be monitoring impact loads across an entire day? This might be ideal, but it is also impractical. We will argue here that the high magnitude events that occur during training will dominate the loading stimulus experienced by the musculoskeletal tissues. As such, we measure each training bout and include these events into a cumulative Bone Stimulus metric.
The purpose of this document is to provide an overview of the literature that underpins our current understanding of injury mechanisms and the background concepts and rationale for IMU Step and Bone Stimulus. A review of bone mechanobiology is also provided, for further reading and context.