*2.2. Taxonomy of Running Cadence*

Up to 79% of long-distance runners are expected to sustain a running-related injury within a six month period, most commonly located at the knee [1,17]. Patellofemoral pain syndrome (PFPS) was observed as the most frequent issue encountered in running injuries out of the 26 most common running injuries [18]. PFPS is a condition that causes severe to mild knee pain, which often starts due to a dramatic change in the training regimen [19]. In addition to this reason, numerous biomechanical risk factors can contribute to causing PFPS. These factors include kinematic abnormalities, patellar maltracking, overuse, and excessive compressive stresses on the patellofemoral joint cartilage [20–24]. Running as an activity generates much larger cartilage stress at the patellofemoral joint as compared to other everyday activities [25,26]. A clear way forward to mitigate PFPS would be to find a method to reduce the magnitude of the patellofemoral joint force during running. By running at 5% to 10% above one's preferred cadence has been shown to be beneficial in reducing pain, increasing training ability in runners with PFPS and minimizing the risk of injury to the patellofemoral joint. For instance, Lenhart et al. report a decrease of 14% in peak patellofemoral joint force as a result of a 10% increase in cadence relative to the preferred cadence level [27]. Heiderscheit et al. report a significant decrease in the absorbed mechanical energy at the knee as a result of 5% to 10% increase in cadence relative to the runner's preferred cadence level [2]. These reductions in peak patellofemoral joint force and absorbed mechanical energy may reduce the risk of running-related knee injuries [2,27].

The importance of optimizing one's cadence as a long-distance runner is twofold. First, subtly increasing and optimizing cadence during a run assists runners with preventing common running-related injuries [2]. From an injury-prevention standpoint, this enables both professional and hobbyist long-distance runners to continue running. Second, optimizing and maintaining a consistent cadence throughout a run enables long-distance runners to conserve energy and as such enhance performance [28,29]. Unlike novice long-distance runners, advanced long-distance runners avoid deviations from their optimal cadence when they are in a fatigued state as such deviations lead to an increased energy cost [30–32].

Given the numerous benefits of optimizing cadence for long-distance runners, many mHealth technologies were created to meet this need. In order to provide effective feedback on running cadence, many have studied the relationship between music and setting the running cadence through auditory-motor synchronization [33,34]. These studies provide evidence of the ability of music to affect the running cadence and ultimately improve performance [33] and reduce injuries [2,34].
