Dr.Hab., ProfessorG.I. Popov¹
PhD, Associate ProfessorV.S. Markaryan^1
1Russian State University of Physical Education, Sports, Youth and Tourism (SCOLIPE), Moscow

Keywords: athlete, running, accelerometry, shock impulse, shock wave, wave peak, shock absorption.

Background. Every ground contact in a running/ walking process is associated with a shock commonly referred to as the footstrike. The shock waves and impulses going through the body may be detrimental to the musculoskeletal apparatus, with the exposure to the mechanical traumas being particularly high in a few modern sport disciplines. Despite the fact that the sport-specific ground contacts have been long and comprehensively studied [1], many of the relevant issues are still ranked among the top priority topics by the sport science [3, 5, 2, 6-8]. The shock wave absorption in the running process is generally secured by (a) the runner’s musculoskeletal system as such; (b) ground surface qualities; and (c) footwear; with all the tree components combined in what may be defined as the integrated ground contacting process.

Objective of the study was to analyze the shock wave propagation and absorption processes in the athlete’s body in the sequence of ground contacts.

Methods and structure of the study. The study analyzes the footstrike-generated shock wave processes based on the treadmill tests of sport footwear. The athletes were tested with the Brule@Kyer triple-axis accelerometers rated at 400MHz and fixed on the ankles and body mass centers (BMC). Due to the body contacts of the accelerometers being still too loose regardless of how tight were the rubber fasteners, the fasted accelerometers were tested to actually operate at 270Hz and 190Hz frequencies in the lower limb and body points, respectively.

Sampled for the study purposes were 13 mid-distance Class I-III runners. Given on Figure 1 hereunder are the typical records of the longitudinal/ shock waves in the treadmill tests. In the test data processing process, we applied the following parameters:

∆t–time gaps between the ankle and BMC acceleration peaks;

∆f– frequency range computed as provided by the wave packet sizing theory: ∆f ·∆t ~ 1;

v– mean wave travel speed as a ratio of the inter-accelerometer distance to ∆t; and

f– frequency rate as a value opposite to the shock wave period on the ankle-area records: see Table 1 hereunder.

Figure 1. Samples of the longitudinal wave records for the ankle area (solid line) versus the BMC area (dashed line): (a) 5m/s fast running (b) 5+m/s fast running

Figure 2. Samples of the 5m/s (a) and 5+ms running records of the longitudinal acceleration rates, with the ankle-area line shown above the BMC-area line

Study results and discussion. Shock acceleration processes for some athletes, as demonstrated by the Table 1 hereunder, are described by a single-peak shock wave curve and for the others by a wavelike curve.

Table 1. Longitudinal shock wave parameters of the running process

The faster is the run speed, the more expressed is the ∆t sagging trend, with the growing speed of the wave peak travel over the body. It should be mentioned that low run speeds are characterized by the single-peak wave curves; and the faster is the run speed, the more often we observe the shock wave frequency growing with the run speed. It was found that at 5m/s run speed the shock wave curve takes a wavelike shape. This is the reason why the above Table 1 gives both the frequency range ∆f and frequency rate f for the 5m/s run speed. Furthermore it was found by the experiments that the typical frequency rate (f) of a longitudinal wave varies within the range of 35-60Hz; mean wave travel speed (v) varies within the range of 20-50m/s; and the wave length (λ) varies within the range of 0.6-1.1m.

Unlike the above, records of the lateral bending vibrations never show single peaks, with their propagation process being characterized by the wavelike curves. Frequencies of the lateral vibrations (f_lv) for the run speeds indicated in the above Table vary within the range of 50-65Hz; with the relevant mean travel speeds (u)  and lateral wave lengths (Λ) close to the above ranges for the longitudinal waves.

Energy flow rate (Umov’s vector) for the waves is known to be proportional to the squared wave amplitude and squared wave frequency; with the longitudinal wave amplitudes being 1.5-2 higher than the lateral ones, and f <f_lv. Therefore, as far as the surface-footwear system is concerned, it may be beneficial to mitigate the longitudinal waves with their amplitudes being reduced by the high absorption qualities of the surface and footwear; and with the lateral waves mitigated by the frequency-reduction efforts to step up the absorption/ hysteresis properties. It is not always necessary to take the both wave type mitigation efforts at a time since the longitudinal and lateral waves are known [1] to be in a non-symmetric relationship. Generally, a longitudinal deformation wave serves as a parametric energy source (booster) for the lateral wave, whilst the lateral wave provides a power source for the longitudinal one.

The longitudinal/ lateral wave propagation qualities of the contact surfaces may be changed by the contact surface types and foot striking techniques being combined so as to mitigate the shock waves of both types. It was found that the sport footwear and running techniques should be designed to secure that the shock waves are distributed in the body in such a way that the longitudinal/ lateral shock wave center/ knot is kept within the body element needed to be protected from the shocks. For example, the shock waves reaching the runner’s head may be mitigated when the wave geometrics are managed so as to keep the 0,5n/Λ index equal to the body length, with the odd number n>1. Optimal frequency rates for the surface-footwear system may be calculated using the following ratio for lateral waves: ƒ= u/Λ.

Practical calculations using the above formulae show that the optimal vibration frequencies within the contact surface system should vary within the range of 50-60Hz. It may be stated that the biomechanically beneficial distribution of the shock waves in the runner’s body may be achieved when the contract surfaces secure the surface contacts being harmonized with the athletes’ musculoskeletal system performance.

Conclusion. Efforts to improve the running process comfort and speed with an emphasis on the athlete-footwear-surface system kinematics are recommended to give a special priority to the longitudinal/ lateral shock waves generation processes being harmonized with the movement biomechanics. Since the above physical mechanisms are largely individual for every athlete, the initiatives to attain the optimal energy benefits by varying the contact surface qualities and running technique biomechanics shall give a special attention to the mechanical qualities of the sport footwear as the most variable parameter of the system by sole, insole, foot taping material etc. being prudently selected on an individual basis.

References

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Corresponding author: vstepmark@mail.ru

Abstract

Modern sport science gives a high priority to analysis of the positive and negative aspects of the sport-specific shocks and vibrations to control the mechanical aspects of the ground contacting process. The study analyzes the footsrike-generated shock wave processes based on the treadmill tests of sport footwear. The athletes were tested with the Brule@Kyer triple-axis accelerometers fixed on the ankles and body mass centers (BMC) to obtain the following test data: time gaps between the ankle and BMC acceleration peaks; and the frequency rates of the shock waves going via the test points. The study data and analyses showed that the shock wave is normally a single-peak-shaped at low (under 5m/s) run speeds and transverse/ longitudinal-wave-shaped at high speeds. It was found that the sport footwear and running techniques should be designed to secure that the shock waves are distributed in the body so that the shock wave center/ knot is kept within the body element needed to be protected from the shocks.