Biomechanical Analysis of Ground Reaction Forces of the Sprint Start

1. Introduction

Sprinting is a complex movement requires dynamic balance, high velocities, and precise coordination of muscles. Understanding the biomechanics of sprinting is crucial for optimizing performance and reducing the risk of injury (Bergamini, 2011).

During the starting phase of a sprint, force production is critical for achieving a powerful start. To optimize force production, positioning both feet on the track can increase tension in the calf muscles, leading to a more efficient start. Additionally, centering body mass more on the legs rather than the arms can further increase pre-tension in the muscles. Then, sprinters enter the acceleration phase, where they increase stride length and stride rate. Furthermore, force production during the start and acceleration phases is crucial for achieving high velocities (Mero et al., 1992).

One key aspect of this analysis is the measurement of ground reaction forces (GRFs) using force plates. GRFs provides valuable information about the magnitude and direction of forces exerted by the athlete on the ground during each step of the sprinting cycle (Payton, Burden, & British Association Of Sport And Exercise Sciences, 2018, p. 92).

The magnitude of the Anterior - posterior GRF (A-P) during running reflects the braking forces exerted by the athlete, when the GRF opposes the direction of forward movement.

Previous study showed that a higher braking force may indicate greater energy dissipation during the braking phase, potentially leading to inefficiencies in running mechanics or increased risk of injury. Moreover, the magnitude of the A-P GRF can provide valuable information about the athlete's ability to generate and absorb forces effectively (Munro et al., 1987; Brough et al., 2021).

Additionally, the vertical GRF represents the force exerted perpendicular to the ground. It provides insights into how effectively an athlete generates propulsion and absorbs impact forces during each stride. On the other hand, excessive or imbalanced vGRF

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during early stance phase may indicate improper shock absorption and high risk of soft tissue injuries, for examples, shin splints, stress fractures, or tendon overuse injuries.

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Furthermore, monitoring vGRF can identify the risk of injury and guide interventions to improve biomechanics and strength to reduce injury risk (Nagahara et al., 2019).

Medial Lateral (ML) GRF plays a crucial role in maintaining stability and balance during running. Changes in ML GRF impulse and moments indicate the adaptive strategies individuals employ to respond to disruptions and maintain steady locomotion. For example, individuals responded to medial disruptions with an increased ankle inversion moment, which correlated with lateral shifts in their foot center of pressure. Conversely, lateral disruptions led to a decreased ankle inversion moment and medial shifts in foot center of pressure (Rawal et al., 2021).

Interestingly, force plate contact time, horizontal impulse, and net change in horizontal velocity during sprinting also provide insights into the efficiency of force application and ground reaction forces during sprinting, aiding in injury prevention and performance enhancement. Athletes with shorter contact times tend to have more rapid turnover, which is associated with greater speed and efficiency in sprinting (Colyer et al., 2018), while a higher horizontal impulse indicates that the athlete is generating more force to transfer into forward motion (Kawamori et al., 2013). Net change in horizontal velocity also offers a precise measure of acceleration and deceleration, crucial for optimizing sprinting mechanics and overall performance (Cross et al., 2017).

In this assignment, we will conduct a biomechanical analysis of GRFs during the start of the sprint using force plate data.

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2. Methods

A 19-year-old male participant, with a height of 178.3 cm and a weight of 75.8 kg, was recruited. The participant was positioned with both legs push against the blocks, the front leg focusing on propulsion and the rear leg generating vertical force. Acceleration from starting blocks is crucial for sprinters, reaching a significant portion of his maximum speed within a short time (Bezodis et al., 2019).

The starting blocks were set up by a Kistler 9281EA force plate, ensuring that the participant's first step would step on the force plate (see Fig.1).

Figure 1. Participant setup during the experiment.

Before the data collection, the participant performed two trials of sprint to familiariz e himself with the main task. Data collection involved recording the participant's first two steps using a Panasonic HC V770 camera at a frequency of 50 Hz. Simultaneously, the force plate recorded data at 1000 Hz. After synchronizing the camera and the force plate, and placing a paper behind the scene with the number of which trial it was to prevent confusion when selecting the right video, a time was established for the point of first contact using Silicon coach Live and various biomechanical parameters during sprinting were collected by BioWare software.

First, contact time was calculated by the following equation:

Contact Time = Contact period end time – Contact period start time. (1)

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Second, horizontal impulse was determined by the following equation:

Horizontal Impulse = Normalized force × Contact Time. (2)

Also, the net change in horizontal velocity has been quantified by the following equation:

Net change in horizontal velocity = Mass / Horizontal Impulse. (3)

Peak power output has been calculated using the following equations (4,5):

Time to peak force (s) = Contact period start time (s) + (Peak force (N) / Rate (Hz) (4)

Peak power output (W) = Peak force (N) × Peak velocity (m/s) (5)

Besides that, the magnitude of the ground reaction forces (GRF) in the medial-lateral (ML), vertical (V), and anterior-posterior (AP) directions was investigated using the force plate data. Finally, the resultant force has been calculated using the Pythagorean theorem:

F_r = √f_x^2 + √f_z^2

F_r is the resultant force, F_x refers to horizontal force, while F_z refers to vertical force.

These equations (Winter et al., 2009; Valamatos et al., 2022) provided a comprehensive framework for kinetic analysis.

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3. Results

Our study analysed ground reaction forces (GRFs) during running in vertical, antero-posterior (AP), and medial-lateral (ML) directions. We found the following force values: braking force (AP) at -518.1 N, vertical force at 1417 N, and ML force at 85.9 N. These results offer kinetic insights into the mechanics of sprinting (see fig.2).

Figure 2: Ground Reaction Forces (GRF) in the Anteroposterior (AP) and Mediolateral (ML) directions over time. The vertical axis represents GRF (N) while the horizontal axis represents

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Table 1: kinetic Parameters Derived from Sprinting Analysis.

Parameter Value

Contact Time 2.997 s

Horizontal Impulse 1498.5 Ns

Net change in horizontal velocity 19.74 m/s

Average acceleration 6.59 m/s^2

Peak power output 998 W

Additional parameters include contact time (2.997s), horizontal impulse (1498.5 Ns), average acceleration (6.59 m/s^2), and peak power output (998 W) (see Table 1 for details).

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4. Discussion

The analysis of ground reaction forces (GRFs) during sprinting provides insights into

kinetic aspects of human locomotion and understanding the mechanisms underlying

performance and injury risk.

Our study found that the average vertical ground reaction force (VGRF) was 1417 N,

which was notably lower than the findings of previous studies (Pataky et al., 2013; Morin et

al., 2015; Yu et al., 2021), which reported VGRF was approximately 2.5 times body weight

(BW). On the other hand, the finding of VGRF was consistent with (Salo et al., 2005), who

found the maximum value during first step was 1400 N. This difference suggests that the

participant exerted less force for forward propulsion during sprinting.

Regarding the AP force, our study found a braking force of -518.1 N, which aligns with

(Pataky et al., 2013), who reported A-P GRF was about -500 N indicating effective energy

preservation by the runner during braking phase.

The ML force observed in our study (85.9 N) was within the range (around 100 N) reported

in (Pataky et al., 2013), who found the maximum value during first step was 1400 N, suggesting postural stability during sprinting without medio-lateral

perturbations.

Our findings revealed that the contact time observed during sprinting, averaging 2.997

seconds, was significantly higher compared to (von et al., 2022), who reported a normal

contact time of 0.1 seconds. Similarly, the calculated horizontal impulse of 1498.5 Ns was

notably higher than the value reported by (Valamatos et al., 2022), who found a normal

value of 140.7 Ns. The longer contact time could be indicative of less efficient movement

patterns or biomechanical inefficiencies. Additionally, the study revealed that the peak

power output observed during sprinting was significantly lower than values reported in

(Samozino et al., 2016), who reported a normal power output of 1400 watts.

While our findings were consistent with some aspects of previous literatures, discrepancy

was noted in the vertical force, horizontal impulse, and peak power. This difference may

be influenced by factors: First, the distance between foot plates affects push phase and

total impulse. Second, inclination of foot plates affects block velocity and muscle-tendon

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mechanics. Third, sprinter's body configuration in the "set" position varies, with faster sprinters often exhibiting specific joint angles (Guissard et al., 1992; Schröder et al., 2017).

Understanding GRFs during running is crucial for optimizing performance and preventing injuries. While our study contributes to this understanding, our study was conducted on one participant, which may limit the generalizability of our findings.

5. Conclusion

In conclusion, our study provided valuable insights into the ground reaction forces (GRFs) during running, revealing notable differences in the magnitudes of vertical GRF, horizontal impulse, and peak power. whereas consistent antero-posterior (AP), and medial-lateral (ML) forces compared to some previous literature. Future research with larger sample sizes and further investigation the biomechanical aspects of running and their implications for performance and injury prevention.

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