Policentrismo: Funciones asociadas a subcentros de empleo

The current thesis is the first research to report such comprehensive external kinetics for sprint velocities >9.0m/s. Due to the lack of literature regarding the kinetics at such high velocities there is little knowledge regarding how the kinetics of sprinting can improve performance. Once such method that has been adopted by authors is the correlation of key kinetic variables to sprint velocity.

There was a moderate positive correlation (r=0.484) between horizontal velocity and average Fy (relative to BW), indicating Fy increased as velocity increases which is comparable to that reported by the literature. Nummela et al. (2007) reported that average Fy increased linearly with velocity from 5m/s to maximal, and average Fy (relative to BW) was significantly correlated to maximal velocity (0.56). However this was with a sample of endurance runners and therefore ‘maximal’ ranged from 7.20m/s to 9.40m/s which are velocities slower than the current study. Kuitunen et al. (2002) also reported an increase in Fy as running velocity increased from 70-100% in a sample of male sprinters, with 100% representing 9.73m/s. Yet it is important to note that the focus of the aforementioned research articles is how force production changes as an athlete increases their speed and thus represents a much larger range of velocities, whereas the current study looks at a range of maximal velocities. Subsequently the appearance of increasing horizontal force when running at higher velocities is actually an indicator of coping with the reduction in ground contact time. At maximal velocity the horizontal velocity should be constant, and thus the net horizontal impulse should be zero. The horizontal impulse is a combination of a negative (braking) phase followed by a positive (propulsive) phase. Therefore in order to maintain a net horizontal impulse of zero the propulsive impulse must be sufficient to overcome the braking impulse. The limiting factor to attaining a greater maximum velocity is the point where contact time is so short that all effort must be directed vertically in order to overcome gravity, and therefore cannot produce any horizontal impulse in order to increase velocity (Goodwin, 2011). The aim must be to decrease the braking impulse, and subsequently the propulsive impulse necessary to overcome it so that contact time can be minimised. There was a moderate positive correlation between sprint velocity and net horizontal impulse (r=0.488), yet when divided into

the respective components the magnitude of the braking impulse had a strong positive correlation to velocity (r=0.620) whereas the propulsive impulse had a weak positive correlation to velocity (r=0.012). This disagrees with the findings of Kyrolainen et al. (1999) who reported that the average Fy in the propulsive phase was more influential on overall velocity than Fy in the braking phase. Similarly Nummela et al. (2007) found that the average Fy of the propulsive phase was significantly correlated to horizontal velocity, whilst the average Fy of the braking phase was not. However these authors only investigated the average Fy with no consideration to the temporal components of the force application and the time over which it was applied. Current theories believe the braking impulse is a negative entity, yet the positive correlation between braking impulse and velocity actually indicates higher velocities are accompanied by higher braking impulses. This is opposite to what might be expected as the braking impulse will cause a decrease in horizontal velocity and thus is disadvantageous. However Mero and Komi (1986) found the average resultant braking force increased from 1314N when running at 4.95m/s to 2257N under supramaximal conditions. Kuitunen et al. (2002) also reported an increase in the peak braking force with an increase in speed. Cavagna, Komarek, and Mazzolen (1971) proposed that the braking force could be involved in the storage of elastic energy and therefore may have advantageous properties. Further Putnam and Kozey (1989) highlighted that it is unknown if the braking GRF is related to other mechanical properties which affect performance, such as the propulsive and vertical GRF components and/or SL and SF.

Due to the constant horizontal velocity at the maximal phase of sprinting research tends to look at the relationship between the vertical components of GRF and velocity. Current literature has only investigated velocities up to 7.0m/s and very little is known about velocities greater than this. There was a very weak correlation between average Fz and horizontal velocity (r=0.151). This coincides with Brughelli et al. (2011) who found a weak correlation of 0.13 between average vertical force (relative to body mass) and horizontal velocity. The authors fail to report the actual velocities (only reported as a percentage of maximum) and therefore it is difficult to extrapolate these results further. Nummela, Keranen, and Mikkelsson (2007) reported Fz remained constant at velocities >7.0m/s, which also coincides with the findings of Kuitunen et al. (2002) and Kyrolainen et al. (1999). In contrast Weyand et al. (2000)

used a regression analysis to conclude that the average vertical force was 1.26 times greater for an individual sprinting at 11.1m/s in comparison to an individual at 6.2m/s. However the test was conducted on a treadmill which has been shown to affect the kinetics of the ground contact phase in comparison to over ground running (Wank et al., 1998). Most importantly the range of velocities is much larger than in the current study, and thus regressions are likely to be stronger. Whilst they show how to increase from slower velocities, the research does not discuss changes in kinetics at the higher end of the velocity spectrum (>9.0m/s). There was a comparable weak correlation between peak vertical Fz (relative to bodyweight) and maximal horizontal velocity (r=0.120). Both Mero and Komi (1986) and Nilsson and Thorstensson (1989) found no increase in peak Fz with increased velocity. Kuitunen et al. (2002) reported peak Fz was consistent as sprinters increased their speed from 70% (7.00m/s) to 100% (9.73m/s) velocity. These maximal speeds are comparable to the current thesis and thus a similar relationship may be expected. The weak correlations between the both average and peak Fz and horizontal velocity can be attributed to the lack of temporal consideration. The change in vertical velocity is proportional to the vertical impulse, and thus the time over which the vertical force is applied must be taken into consideration. A slower velocity might be associated with a greater average Fz, but if this is achieved as a result of a longer ground contact time this is disadvantageous to sprint performance as step is negatively affected.

There was a weak positive relationship between maximal horizontal velocity and relative vertical impulse (r=0.138). Whilst the greatest horizontal velocity (11.26m/s) corresponded with the greatest vertical impulse (1.25Ns/kg), the slowest horizontal velocity recorded (9.43m/s) had a similar vertical impulse of 1.24Ns/kg. The weak relationship between vertical impulse and velocity is in contrast to the conclusions made by Weyand et al. (2000) that faster running speeds are achieved by the amount of force applied to the ground as opposed to how rapidly the limbs are repositioned in the air, however the negation of vertical impulse limits these conclusions. The role of vertical impulse at maximal velocity is unclear and the weak correlation to horizontal velocity can be attributed to the need for an optimum level based on the individual relationships between SL and SF. Vertical impulse is necessary to provide the vertical lift necessary to reposition the limbs in the swing phase and to increase the likelihood of a negative foot speed at the next ground contact. However too much vertical

motion would increase the flight time, and subsequently decrease the SF. The findings in Chapter 3 indicated a strong relationship between SF and horizontal velocity, with the aim of maximising SF. Furthermore Salo et al. (2011) found SF to be individually reliant due to its interrelationship with SL. Therefore it is proposed that the weak relationship between vertical impulse and velocity is due to individual differences in SF. There was a strong correlation (r=0.573) between vertical impulse and SF thus proving this theory. Athletes with a high vertical impulse had a lower SF, but this coincided with a higher SL. Subsequently the individual variation in the SL/SF relationship leads to a weak correlation between vertical impulse and horizontal velocity.