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Effectiveness of force application in sprint running: definition of concept and relationship with performance a
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J. B. Morin , P. Samozino & P. Edouard
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Laboratoire de Physiologie de l'Exercice (EA4338), Université de Saint-Etienne, CHU Bellevue, 42055, Saint-Etienne Cedex 2, France Available online: 24 Aug 2011
To cite this article: J. B. Morin, P. Samozino & P. Edouard (2011): Effectiveness of force application in sprint running: definition of concept and relationship with performance, Computer Methods in Biomechanics and Biomedical Engineering, 14:sup1, 173-175 To link to this article: http://dx.doi.org/10.1080/10255842.2011.594710
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Computer Methods in Biomechanics and Biomedical Engineering Vol. 14, No. S1, August 2011, 173–175
Effectiveness of force application in sprint running: definition of concept and relationship with performance J.B. Morin*, P. Samozino and P. Edouard Laboratoire de Physiologie de l’Exercice (EA4338), Universite´ de Saint-Etienne, CHU Bellevue, 42055 Saint-Etienne Cedex 2, France Keywords: acceleration; kinetics; technique; fatigue
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1.
Introduction
The recent validation of a 3D instrumented treadmill allowed us to measure ground reaction forces (GRF) continuously over entire sprints of any duration, including the acceleration phase from a standing start (Morin et al. 2010). The motor torque function of the treadmill also allowed subjects to freely accelerate the treadmill belt, contrary to previously used pre-set constant velocity methodologies which did not allow subjects to produce typical sprint accelerations. Given the possibility to average instantaneous GRF data over one support phase, we transposed the concept of effectiveness of force application (EFA) used in pedalling mechanics (Davis and Hull 1981) to sprint running. Basically, in pedalling, effectiveness is defined as the ratio of the component of the total force perpendicular to the crank arm (the force causing the rotation of the drive) to the total force applied onto the pedal. In sprint running, where the overall motion is directed forward, we defined EFA/orientation as the ratio of the contact-averaged net horizontal force (FH) to the total force (FTot). In this study, divided into two protocols, our aims were to measure EFA during typical sprint accelerations, to investigate (i) its relationship with field sprint performance and (ii) whether it is changing with the decrease in performance and force application capabilities induced by repeated sprint fatigue.
2. Methods 2.1 Running mechanics About one week after a complete familiarization session, force data were sampled at 1000 Hz on the sprint instrumented treadmill (ADAL3D-WR, HEF Tecmachine, Andre´zieux-Bouthe´on, France) during 6-s sprints performed from a standing start. The motor torque was set at 160% of the default torque allowing for compensation of the friction on the belt due to subjects’ body weight. Data of vertical (FV), horizontal (FH), total
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[email protected] ISSN 1025-5842 print/ISSN 1476-8259 online q 2011 Taylor & Francis DOI: 10.1080/10255842.2011.594710 http://www.informaworld.com
GRF (FTot) and belt speed (S) were averaged for each contact phase, and power produced in the horizontal direction was computed as P ¼ SFH. For each step, EFA was computed as EFA ¼ FH/FTot and expressed in percentage. The objective of sprint running is not to maximise EFA (i.e. aim at reaching 100% values), as it is the case in cycling. It is rather to produce and maintain high values of EFA despite increasing speed, which allows both a high amount of FH to induce high forward acceleration, and the amount of FV necessary to lift the centre of mass and avoid falling. EFA is maximal during the first steps (net FTot is rather oriented forward) and null during the steps at maximal velocity (net FTot oriented vertically). Thus, we defined an index of force application technique (IFAT) as slope of the linear decrease in EFA with increasing speed (Figure 1).
2.2 Protocol I: 100-m field performance Thirty minutes before or after (counterbalanced order) the treadmill 6-s sprint, 12 male physical education students (72.4 ^ 8.6 kg; 1.76 ^ 0.08 m; 26.2 ^ 3.6 years) familiar with sprinting performed a 100-m sprint on a standard athletics track (same starting position and outfit/shoes as on the treadmill). Their performances were recorded by a radar (ATS Stalker, 35 Hz): 100 m mean and top speed (S100 and Smax, respectively). The distance covered in 4 s (d4, in m) was also computed and studied as a practical index of acceleration capabilities related to field sport performance (e.g. rugby, soccer). Correlations were then tested between these main performance variables and the mechanical variables of force production capabilities and force application technique.
2.3 Protocol II: repeated sprint fatigue The respective changes in the previously described running mechanics (force production and force application
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Figure 1. Typical speed– EFA relationship and IFAT values for the first and last of the 20 sprints.
technique variables) were studied during a multiple-set repeated sprint series. Twelve trained male subjects including athletes (75.1 ^ 6.9 kg; 1.80 ^ 0.06 m; 25.4 ^ 4.1 years) were asked to perform four sets of five sprints (6-s sprints followed by 24-s passive rest). The sets were separated by 3 min of recovery and neuromuscular tests. Each subject had to perform until volitional fatigue, and was requested to stop after a complete set, if not performing the entire sprint series (20 sprints in total). Changes in running mechanics were compared through the percent change between averaged values for the first two and last two sprints of each individual series and t-tests. They were completed by the changes in performance variables Smax and d4. 3. Results and discussion 3.1 Protocol I: 100-m field performance Performance variables were all significantly correlated with IFAT, and with FH and Pmax. However, no significant correlation was found between the performance variables and the amount of total or vertical force applied onto the ground during the sprint (Table 1).
Table 1.
Therefore, it seems that the technical ability to effectively apply FTot onto the ground (i.e. with an important forward orientation), as quantified through IFAT, is related to 100-m field performance. This is not the case of total force production capabilities. What seems to determine field sprint performance is the way force is applied onto the ground, rather than its total amount. One limit of this protocol may be the overall lower performances observed on the treadmill (Morin and Se`ve in press) compared with the track, mainly due to the friction caused by the application of high amounts of vertical forces onto the treadmill belt at each step (Lakomy 1987; Morin and Se`ve in press). However, the treadmill used has been shown to be valid in interpreting inter-individual differences in field sprint running performance (significant and high correlations between treadmill and track performance variables; Morin and Se`ve in press), and we can reasonably assume that subjects with a good (i.e. high) IFAT on the treadmill would also have high IFAT values on the track.
3.2
Protocol II: repeated sprint fatigue
Subjects performed 16.7 ^ 4.4 sprints on an average. Their performance decreased significantly over the multiple-set sprint series (P , 0.001): d4 decreased by 13.9 ^ 8.3% and Smax by 17.2 ^ 5.7% on an average. For the running mechanics, FTot decreased from 1.51 ^ 0.11 to 1.42 ^ 0.13 BW (i.e. 2 5.81 ^ 5.76% on average; P , 0.01). The decrease in maximal power was even larger: from 21.3 ^ 2.5 to 15.3 ^ 2.5 W/kg (i.e. 2 28.2 ^ 8.7% on average; P , 0.001). The force application technique was also strongly altered between the beginning and the end of the sprint series: IFAT of 2 0.069 ^ 0.007 pre-fatigue vs. 2 0.081 ^ 0.001 postfatigue (i.e. a mean change of 19.2 ^ 20.9%). This result shows that with fatigue, subjects could no longer apply FTot forward with a high EFA, and thus they lost EFA (Figure 1). The loss of EFA with increasing speed during sprint acceleration was higher during the last sprints of the series than during the first ones.
Correlations between mechanical and performance variables.
Maximal value of EFA (%) IFAT FH (BW) FV (BW) FTot (BW) Pmax (W/kg)
37.6 ^ 4.22 2 0.071 ^ 0.01 0.322 ^ 0.056 1.62 ^ 0.14 1.65 ^ 0.14 16.5 ^ 3.18
Smax (m/s)
S100 (m/s)
d4 (m)
8.79 ^ 0.59 0.013 (0.97) 0.735 (
