Linear Kinematics of Sprinting

Linear Kinematics in Sprinting

Introduction

As stated by Dintiman (1975), ‘There is no single greater concern of coaches and athletes in all sports than “how to run faster”.’ Linear kinematics is concerned with the description of motion and involves the study of linear movement through time, including velocity and acceleration (Hall, 2003). Velocity is defined as “the rate at which a body moves from one location to another” (Hay,1993) and linear acceleration is defined as ‘the rate of change in velocity’ (Hall, 2003). In sprinting, factors including step length and step frequency play a major role and previous research has found that world class sprinters have a longer step length and higher step frequency than non-elite sprinters (Kunz and Kaufmann, 1981). This suggests that one or both of these is a factor in optimal sprinting. A relationship exists between step length and step frequency over a certain distance – the longer the step length, the lower the step frequency and the shorter the step length, the higher the step frequency (Donati, 1996).  The 100m world record is held by Usain Bolt at 9.58 s and Beneke and Taylor (2010) have suggested that he has an advantage over other competitors due to his tall height and resulting longer step lengths after the initial acceleration period, meaning he had a lower step frequency than his opponents. Keogh et al (2010) related a strong man sled pull to acceleration phase sprinting and found that in the faster trials greater step lengths and frequencies were observed as well as a shorter ground contact times. This negative interaction between step length and step frequency was suggested to be due to vertical velocity of take off, height of take off and leg length (Hunter et al, 2004).

In a sprint, the athlete’s goal is to reach maximum velocity in the shortest possible time, meaning that acceleration must be as fast as possible abiding by the equation:

Change in Velocity = Acceleration * Time

The purpose of this report is to determine which parameter (stride length or stride frequency) is more responsible for the increased velocity required to produce maximum performance. If this is discovered, training can be adapted to be able to better the favoured parameter and therefore decrease sprint time.

Method

Two 19 year old sport participant males were used as the subjects. They ran four maximal effort 60 m sprints on an outdoor track following a personal warm up and one practice maximal effort sprint. Recovery time was unlimited and the personal choice of the athlete. The lane used had cones positioned every 5 m and Newtest photocells were set up at the 30 m and 60 m points.  They were video recorded using a Sony HC9 camera positioned 50 m away perpendicular to the middle of the 60 m track. The camera was set to a shutter speed of 1/600 s and manual focus was applied after checking there was no tilt or roll. The camera was adjusted to pan to cover 5 m at a time, which was judged using the cones on the track.

The video was then analysed using a video player to determine average velocities, average accelerations, step length and step frequency using the equations below. Step length and step frequency were measured only in the last 30 m. Within this report, one step is defined as the point at which the back foot leaves the ground to the point at which the opposite foot leaves the ground.

v = d/t

a = Δv/Δt

step length = average velocity/step frequency

step frequency = no. of steps/(Δt between 1^st and last step)

Results

It was found that there was not much difference between the average velocities of the two methods of measurement, whether it was calculated over 5 m intervals and video analysed or calculated using photocell times, as seen in Table 1. The photocell velocities will be used for the remainder of this report. Athlete A had a lower average velocity (7.92ms^-1) over the last 30 m than Athlete B (8.51ms^-1). Figure 1 shows that there is an initial increase in velocity up to about 2 s into the sprint, where the velocity begins to steady before decreasing in the last 5 m. Acceleration is the gradient of the line, so as velocity begins to plateau, acceleration is nearly constant. The acceleration in the last 30 m was also negative, implying that both athletes were slowing down towards the end of the race, although not by a great amount as can also be seen in Figure 2. Athlete B’s average step length was greater than Athlete A’s (2.16 m compared to 2.11 m) as well as the average step frequency also being greater (3.93 Hz compared to 3.78 Hz). The longer stride length and higher stride frequency means that Athlete B was able to cover the same distance in a shorter time.

		Athlete A					Athlete B
		1st Run	2nd Run	3rd Run	4th Run	Average	1st Run	2nd Run	3rd Run	4th Run	Average
Average Velocity (m.s-1)	Video Analysed	8.24	7.65	7.94	7.94	7.94	8.67	8.62	8.43	8.52	8.56
	Photocell	8.24	7.61	7.88	7.94	7.92	8.60	8.53	8.40	8.51	8.51
Average Acceleration (m.s2)		-0.14	-0.18	-0.07	-0.13	-0.13	-0.08	0.00	-0.08	-0.17	-0.08
Average Step Length (m)		2.12	2.09	2.07	2.11	2.10	2.19	2.16	2.14	2.16	2.16
Average Step Frequency (Hz)		3.90	3.65	3.80	3.76	3.78	3.92	3.95	3.92	3.95	3.93

Table 1: Table showing average velocities (video analysed and photocell times), average acceleration, average step length and average step frequency over the last 30 m of the sprint.

Figure 1:  Velocity – time graph to show average velocity over 5 m intervals over the last 30 m, of each of the 4 runs, for Athlete A and Athlete B.

Figure 2: Acceleration – time graph to show average acceleration over 5 m intervals over the last 30 m, of each of the 4 runs, for Athlete A and Athlete B.

Discussion

The average step frequency for both athletes is very similar for each of the four runs, so the average for each athlete will be used in the discussion. In relation to the purpose of the study, which was to determine which factor, step length or step frequency, was more important in sprinting, it was found that Athlete B completed the 60 m sprint in a shorter time than Athlete A, whilst having a higher step frequency and longer step length. However, the average step frequencies were very similar, Athlete A being 3.78 Hz and Athlete B being 3.93 Hz. This leads to the belief that even the small difference in step length (0.06 m) is responsible for the increased velocity of Athlete B, as velocity is equal to step length multiplied by step frequency (Maulder et al, 2008; Hay, 2003). This supports previous research of Hunter et al (2004) which suggested that there is a “negative interaction” between step length and step frequency. It is assumed from these results that step length is the determining factor of velocity in the 100 m sprint. However, inferential statistical analysis was not conducted, so it is unsure as to whether this difference in step length is significant or not.

Velocity and acceleration profiles are useful tools for coaches in order to establish which phase of the sprint can be improved, and when paired with step length and step frequency, has the potential to produce a specific training programme.

Acceleration is said to happen in the first 10m of a 100m sprint, with 36-100m being at maximum speed (Delecluse, 1997). This can be seen in Figures 1 and 2 as at about 4 s, both athletes had reached about their maximum velocity and were beginning to keep a constant velocity or decelerate very slightly. The aim of a sprinter is to reach maximal velocity as soon as possible, therefore needing a fast acceleration phase (Maulder et al, 2008). Once maximum velocity is achieved, it is then required to be maintained to complete the sprint in the fastest time possible.

Ito et al (2006) found that world class sprinters tended to take a wider step when accelerating from the blocks, before narrowing the step width when in full stride sprinting, which was recognised in the results as with every one of the four runs, both athletes maintained a similar step length for the final 30 m. This means that there is a shorter step length in the initial acceleration phase, suggesting other reasons as to how maximum velocity is reached. Weyand et al (2000) had previously suggested that faster sprinters were more successful due to the force applied on the ground as opposed to how fast they moved their legs. This implies that power to produce a larger force and resultant larger step length or width is required.

Spinks et al (2007) found that both step frequency and step length can be increased by strengthening the hip and knee extensor muscles to produce more power. This can be done through resistance training, in the form of, for example, plyometric training or running on sand (Costello, 1985). Maulder et al (2008) found that step length and step frequency both decreased when resistance was applied, but when resistance was removed, they increased along with acceleration, meaning maximum velocity was reached faster, therefore maximising the sprint time.

Much of the research suggests that there is a compromise between step length and step frequency, with athletes compromising in order to save energy, thus suggesting that individual preferences can begin to play a role (Cavanagh and Williams,1982; Cavanagh and Kram, 1989). It has also been noticed that increasing the force on the ground not only increases step length, but also step frequency, further implying the difficulty to differentiate which parameter is more important (Young, 2007).

This study focused on step length and frequency, assuming that these were the parameters to alter the ultimate velocity. However, other factors involving individual differences could be the cause of the difference in step length or frequency initially. These could include muscle distribution and muscle type (Mero et al, 1981). In a later study by Mero (1985) it was found that there was a positive correlation between fast twitch fibres and maximal velocity and also between fast twitch fibres and step rate. This implies that velocity is not purely to do with step length or step frequency, but delves deeper into their own production, involving muscle fibres. Due to the fact that no muscle biopsy was conducted, there is no way of telling whether the muscle fibre types in the two participants differed at all.

However, there are potential problems with this study in that only two male participants of similar training were used. This means the results cannot be generalised to larger populations. The step length and step frequency were also only calculated in the last 30 m of the sprint, making it impossible to analyse whether there were differing step lengths and step frequencies in different phases of the sprint. Further studies could be conducted to gain step length and step frequency over the full 60 m of the sprint, enabling the differentiation of the phases of sprinting, and to see whether the step length or step frequency changes dependent on which phase the sprinter is in. This would allow training to be adapted further to determine whether step length or step frequency is important in different phases of the sprint.

In conclusion of this study, step length appears to be the factor determining velocity in a 60 m sprint. Through strength training, power can be increased and therefore step length and resulting velocity can also be increased.