The measurement of tibial acceleration in runners—A review of the factors that can affect tibial acceleration during running and evidence-based guidelines for its use
Introduction
Running is a popular activity, but the high participation rate is accompanied by a high incidence of injuries [1]. The majority of running-related injuries occur in the lower limbs, are chronic in nature, and are related to cumulative loading [2]. The repetitive impacts associated with running is thought to play an important role in the pathophysiology of many common running injuries, especially bony fatigue fractures (commonly termed stress fractures) [[3], [4], [5]]. In runners, between 35% and 49% of all fatigue fractures occur in the tibia [[6], [7], [8], [9]].
While many factors influence bony remodeling and ultimately the manifestation of a fatigue fracture [10], biomechanics dictate the level of mechanical loading on bone during running [11,12]. When the foot strikes the ground, its velocity decelerates to zero and large ground reaction forces (GRF) are generated [13]. This momentum change produces compressive loading of the lower limbs, and results in an impact shock transmitted through the musculoskeletal system, with local segment peak accelerations occurring at successively later times [14,15]. To minimise damage to proximal structures the shock is attenuated, which is accomplished through an interaction of passive and active mechanisms [[16], [17], [18], [19], [20]]. A failure of the lower extremity muscles to adequately absorb the energy of impact may lead to an over-reliance on passive mechanisms for attenuation [20].
Direct in-vivo measurement of bone strain would be ideal for monitoring injury risk in runners, however this is invasive and impractical [21,22]. Measuring the tibial acceleration (TA) via segment mounted accelerometers is a commonly used proxy measurement for the impact forces experienced at the tibia by virtue of Newton’s second law (F = ma) [23,24]. While the relationship between TA and bone strain is unclear, and likely to be complicated by local muscle forces, peak TA measured via devices attached directly to the tibia bone have revealed reasonable correlations with key GRF parameters (vertical impact peaks r = 0.7–0.85; loading rates r = 0.87–0.99) [25]. While the correlations are weaker when using skin-mounted accelerometers, average loading rate (r = 0.274–0.439) and instantaneous loading rate (r = 0.469) of the vertical GRF have all been significantly correlated with peak TA [26]. The moderate correlation between peak TA and GRF is not surprising as the GRF represents the summed acceleration of all body segments. These points withstanding, the axial component of TA has been shown to discriminate between runners with and without tibial fatigue fractures [27], and between runners injured and uninjured limb [28]. Additionally, the likelihood of the history of tibial fatigue fracture has been shown to increase by a factor of 1.4 for every 1 g increase in axial TA [29].
Previous literature reviews on the use of accelerometers in running have highlighted some of the key elements for consideration, such as the attachment method and placement location of the accelerometer, and the need for a low mass multi-axis device for increased measurement accuracy [23,24]. Despite this, the scope of these reviews did not address many of the issues and potential limitations that must also be considered when measuring TA from runners, including the influence of running velocity, technique, fatigue and surface characteristics. The objective of this review is to update current knowledge of the measurement of TA in runners and to provide recommendations for those intending on using this assessment method in research or clinical practice.
Section snippets
Methods
PubMed, Web of Science, SPORTDiscus and Google Scholar were searched to Jan 2018 using the following terms linked with the Boolean operators (‘AND’ and ‘OR’): ‘run*’, ‘tibia* acceler*’, ‘shock’, ‘inertia*’ and ‘biomech*’, with no limits. Additional relevant studies were identified using article reference lists. Titles, abstracts and full-texts of retrieved documents were sequentially reviewed to determine their relevance. Only papers published in English, that specifically measured TA during
Definition of terms
A number of terms are used interchangeably to describe different aspects of TA, including peak TA, peak shank deceleration, peak positive acceleration and tibial shock. For the purpose of this review, axial (TA-A), anterior-posterior (TA-AP), and medio-lateral tibial acceleration (TA-ML) are used where time-domain peak acceleration magnitude components from a device aligned to the long axis of the tibia are reported. Resultant tibial acceleration (TA-R) is where the peak acceleration magnitude
Device selection
Devices contain one, two or three accelerometers mounted at right angles, each reacting to the orthogonal component acting along their axis [30]. They operate relative to the Earth’s gravitational field, constantly registering 9.81 m/s/s (1 g) as a reaction to gravitational acceleration [31]. The maximum contribution of the acceleration due to gravity is 1 g (when the shank is vertical), and some accelerometers will register 9.81 m/s/s or 1 g in this position at rest, while others may read zero
Normalisation
To account for variability in absolute magnitudes between sessions, normalisation of TA data has been proposed [65]. Expressing TA-A relative to the mean observed at the slowest running velocity, provided a ‘shock ratio’, which can be useful considering the absolute values of the peak accelerations are susceptible to noise and vibration. Focusing on the relative magnitudes of acceleration measures can be informative for many applications (e.g. cushioning properties of running shoes), however to
Outcome measures
Where triaxial devices are used, TA signals can be resolved into three acceleration components. The coordinate system axes can be defined differently, but commonly the orthogonal axes are defined with respect to the tibia: TA-A, TA-AP and TA-ML. The TA-A corresponds to a line bisecting the proximal and distal ends of the tibia in both the frontal and sagittal planes. The medio-lateral axis runs perpendicular to the axial axis and parallel to a line joining the two malleoli, and the
Running velocity
The seminal work analysing the effect of running velocity report consistently increased peak TA magnitude with faster running velocities (3.5 and 4.7 m/s) across all components of TA (TA-A, TA-AP and TA-ML) from a single recreational runner, using a bone-mounted accelerometer [58]. This increase in TA-A was also reported at a series of faster running velocities (spanning 3.4 to 5.4 m/s) from 10 well-trained runners [65]. Further to this, linear regression analysis revealed that average TA-A
Running surface
Owing to their cushioning properties, treadmills typically have a lower compliance compared to tarseal or concrete running surfaces. There is evidence to suggest that TA-A measured overground can be substantially higher than running on some treadmills under comparable conditions [51,83,106], however the relationship between TA-A magnitude and surface compliance is not straightforward. Fu et al. [107] found no differences in TA-A across a wide range of surfaces running at 3.3 m/s, whereas
Conclusions and recommendations
Clinicians and researchers commonly use tibial acceleration during running as a proxy measurement for the impact forces experienced at the tibia. There is an assumption that this measure corresponds to the acceleration of the bone, and ultimately bone stress and strain, however this is yet to be proven. For users of tibial mounted accelerometers, there are several recommendations that should be adhered to in order to achieve accurate and reproducible results. Devices should be secured firmly to
Conflict of interest
Dr. Besier is a consultant for IMeasureU-Vicon and is involved in the development of inertial sensor solutions.
Funding and acknowledgements
None.
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