Ground reaction force metrics are not strongly correlated with tibial bone load when running across speeds and slopes: Implications for science, sport and wearable tech

Abstract

IntroductionTibial stress fractures are a common overuse injury resulting from the accumulation of bone microdamage due to repeated loading. Researchers and wearable device developers have sought to understand or predict stress fracture risks, and other injury risks, by monitoring the ground reaction force (GRF, the force between the foot and ground), or GRF correlates (e.g., tibial shock) captured via wearable sensors. Increases in GRF metrics are typically assumed to reflect increases in loading on internal biological structures (e.g., bones). The purpose of this study was to evaluate this assumption for running by testing if increases in GRF metrics were strongly correlated with increases in tibial compression force over a range of speeds and slopes.MethodsTen healthy individuals performed running trials while we collected GRFs and kinematics. We assessed if commonly-used vertical GRF metrics (impact peak, loading rate, active peak, impulse) were strongly correlated with tibial load metrics (peak force, impulse).ResultsOn average, increases in GRF metrics were not strongly correlated with increases in tibial load metrics. For instance, correlating GRF impact peak and loading rate with peak tibial load resulted in r = -0.29±0.37 and r = -0.20±0.35 (inter-subject mean and standard deviation), respectively. We observed high inter-subject variability in correlations, though most coefficients were negligible, weak or moderate. Seventy-six of the 80 subject-specific correlation coefficients computed indicated that higher GRF metrics were not strongly correlated with higher tibial forces.ConclusionsThese results demonstrate that commonly-used GRF metrics can mislead our understanding of loading on internal structures, such as the tibia. Increases in GRF metrics should not be assumed to be an indicator of increases in tibial bone load or overuse injury risk during running. This has important implications for sports, wearable devices, and research on running-related injuries, affecting >50 scientific publications per year from 2015–2017.

Real-time feedback by wearables in running: Current approaches, challenges and suggestions for improvements

“…One real-time feedback method is continuous feedback, which involves feedback provision without interruption. Disadvantages of continuous feedback are that it can be Running speed Increases in running speed lead to higher peak values in most biomechanical load-related variables (J. G. Hunter, Garcia, Shim, & Miller, 2019;Matijevich, Branscombe, Scott, & Zelik, 2019) and may therefore be used as a proxy of mechanical intensity. However, running technique, surface and incline may also affect the mechanical load on tissues and running speed alone does therefore likely not provide an accurate indication of tissue loading.…”

“…Increases in insole pressure are often assumed to reflect increases in internal tissue loading, which is important to quantify for injury prevention. Matijevich et al (2019) recently showed that ground reaction forces do generally however not correlate well with bone (tibia) loading and only have a small contribution to bone load magnitude. Some ground reaction force metrics were even negatively correlated to bone load and may therefore even provide misleading information in some situations.…”

Mechanical misconceptions: Have we lost the “mechanics” in “sports biomechanics”?

“…Thus, inferences and speculation about running overuse injury risks are often being made based on the wrong joint reaction force estimates, resulting in misleading or unfounded conclusions [34]. Similar issues appear to exist in figure skating as well.…”

“…The sports discussed here were given as examples, but similar confusion between net joint force vs. joint contact force exists in other disciplines as well. The danger of this misconception is exemplified by Mills et al [39] study on gymnasts landing and Matijevich et al [34] study on runners, both of which demonstrate how decreasing GRFs (or GRF metrics, such as impact peaks) can actually correspond to greater joint contact forces; thus, the wrong choice of joint reaction force construct could lead to opposite conclusions.…”

“…Likewise, there are a growing number of consumer wearables that seek to provide feedback presumably on joint contact force or other musculoskeletal forces inside the body, or to identify injury risks due to repetitive tissue loading. However, many of these devices actually provide summary metrics related to net joint force (e.g., vertical GRF impact peak or loading rate, tibial shock, or other accelerometer-based correlates of the GRF), which is not the relevant joint reaction force in this case [34].…”