Elsevier

Journal of Biomechanics

Volume 44, Issue 15, 13 October 2011, Pages 2719-2723
Journal of Biomechanics

Effect of fatigue on force production and force application technique during repeated sprints

https://doi.org/10.1016/j.jbiomech.2011.07.020Get rights and content

Abstract

We investigated the changes in the technical ability of force application/orientation against the ground vs. the physical capability of total force production after a multiple-set repeated sprints series. Twelve male physical education students familiar with sprint running performed four sets of five 6-s sprints (24 s of passive rest between sprints, 3 min between sets). Sprints were performed from a standing start on an instrumented treadmill, allowing the computation of vertical (FV), net horizontal (FH) and total (FTot) ground reaction forces for each step. Furthermore, the ratio of forces was calculated as RF=FHFTot−1, and the index of force application technique (DRF) representing the decrement in RF with increase in speed was computed as the slope of the linear RF-speed relationship. Changes between pre- (first two sprints) and post-fatigue (last two sprints) were tested using paired t-tests. Performance decreased significantly (e.g. top speed decreased by 15.7±5.4%; P<0.001), and all the mechanical variables tested significantly changed. FH showed the largest decrease, compared to FV and FTot. DRF significantly decreased (P<0.001, effect size=1.20), and the individual magnitudes of change of DRF were significantly more important than those of FTot (19.2±20.9 vs. 5.81±5.76%, respectively; P<0.01). During a multiple-set repeated sprint series, both the total force production capability and the technical ability to apply force effectively against the ground are altered, the latter to a larger extent than the former.

Introduction

Although top running speed and single all-out sprint effort are the basis of the main track and field event (100 m), the ability to repeat shorter sprints is of importance in many sports such as soccer or rugby. Therefore, repeated sprint ability (RSA) has been a recent area of investigations, and the focus of many studies in the past 15 years or so (for reviews, see Glaister, 2005, Spencer et al., 2005), which almost exclusively focused on the physiological features of RSA, contributing to a detailed knowledge of this type of exercise. Comparatively, the biomechanical aspects of running RSA have almost never been explored.

Indeed, except for the mechanical output variables (typically work, velocity or power) measured as indicators of the performance decrement during cycling or running sprints series (e.g. Balsom et al., 1994, Gaitanos et al., 1993, Hughes et al., 2006, Mendez-Villanueva et al., 2008, Serpiello et al., 2011, Spencer et al., 2008), no study focused on how the orientation of the total force produced by lower limbs changes over a series of repeated sprints (RS). Morin et al. (2006) reported changes in running kinematics and spring–mass parameters over four consecutive field 100 m, but their field measurements at each step of the sprints did not include data of ground reaction forces (GRF) amplitude or orientation. More recently, Girard et al. (2011) reported changes in sprinting kinetics, kinematics and spring–mass characteristics over a series of 12 40 m sprints, and showed that positive peaks of horizontal GRF and horizontal positive and net impulses decreased, but peak vertical GRF did not change with fatigue. However, in their study, data were measured using a 5 m force plate yielding measurements of 2–4 steps in the 5–10 m (odd-numbered trials) or 30–35 m (even-numbered trials consisting in sprinting back to the starting point) zones.

In contrast to this limited number of steps analyzed over each sprint, the recent validation of an instrumented sprint treadmill (Morin et al., 2010) makes continuous measurements of instantaneous horizontal and vertical GRF as well as the running speed (S) possible over an entire sprint, whatever its duration.

On the basis of these GRF measurements, we recently proposed the computation of the ratio of support-averaged net horizontal and vertical forces (RF=FHFTot−1) as an indicator of the overall technical ability of force application and orientation against the ground, independently from the amount of total force applied (Morin et al., in press). Further, RF decreasing linearly with the increase in speed over a sprint acceleration from null to top speed, an index of force application technique (DRF) was computed as the slope of the linear RF-speed relationship (Morin et al., in press). Thus for a given sprint, the higher DRF, the more RF is maintained at high values despite increasing speed, and the more forward-oriented FTot. In turn, higher values of FH are applied against the ground for a same given amount of FTot produced by the lower limbs. This DRF index was highly and significantly correlated with field 100-m performance, while FTot computed over the entire acceleration phase was not (Morin et al., in press). Therefore, we proposed DRF as an index of the overall force application technique during an accelerated run, in contrast to the amount of total force applied against the supporting ground FTot, which represents the overall physical capability of force production. In this study, it was shown that these two mechanical variables were not correlated, and that (at least for the population tested) the orientation of the total force applied against the ground at each step seemed more important than its amount.

The aim of the present study was therefore to compare the fatigue-induced changes in the technical ability of force application/orientation against the ground (especially represented by the DRF index) to those in the physical capability of force production (especially the amount of total force applied against the ground per unit body weight (BW)). The physical capability of total force production was expected to decrease with fatigue and performance decrement induced by RS, and since force application technique has recently been related to single sprint performance (Morin et al., in press), we hypothesized that this variable would also decrease over a series of RS. The comparison of the respective extents of these decreases, if observed, was the main focus of this study.

Section snippets

Subjects and experimental protocol

Twelve male subjects (body mass (mean±SD) 75.1±6.9 kg; height 1.80±0.06 m; age 25.4±4.1years) volunteered to participate in this study. They were all physical education students and physically active, and had all practiced physical activities including sprints (e.g. soccer, basketball) in the 6 months preceding the study. Written informed consent was obtained from the subjects, and the study was approved by the institutional ethics review board of the Faculty of Sport Sciences, and conducted

Results

Subjects performed (mean±SD) 16.7±4.4 sprints. Among the 12 subjects, seven performed 20 sprints (4 sets), two performed 15 sprints (3 sets) and three completed 10 sprints (2 sets). Performance decreased significantly over the RS series, as shown in Table 1. This overall decrease was consistent with intra-set decreases in performance (Table 2). For instance, S-max decreased by ∼8–10% on an average over each of the four sets of 5 sprints (Table 2), for an overall mean decrease of 17.2±5.7% (P

Discussion

The present results show that, along with the expected significant and large decrease in performance, RS induced both a significant decrease in the capability to produce total force and a significant and even larger decrease in the ability to apply it with a forward orientation during acceleration. The magnitude of these individual changes in DRF was significantly (P<0.01) larger than for FTot. Finally, this study reported similar ranges of decrease in performance than previous studies using

Conflict of interest

We declare that we have no conflict of interest.

Acknowledgments

We are grateful to Gauthier Perez for his assistance in data collection, and to the subjects of this study for having performed maximal efforts until volitional fatigue.

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