Biomechanical Performance Factors in the Track and Field Sprint Start: A Systematic Review

How to optimise your athlete’s block start.

In sprint events, the block start performance can be fundamental to the outcome of a race. This systematic review aimed to identify biomechanical factors of critical importance to the block start and subsequent first two steps performance.

Several biomechanical determinants of success for sprinters were identified, including:

1. In the “Set” position, an anthropometry-driven block setting facilitating the hip extension and a rear leg contribution should be encouraged.

2. At the push-off, a rapid extension of both hips and greater force production seems to be important.

3. After block exiting, shorter flight times and greater propulsive forces are the main features of best sprinters.

This systematic review explored important findings and recommendations that may be relevant for researchers and coaches.

Sprint Start Analysis

This paper systematically reviews the kinematic and kinetic biomechanical variables of the block start and initial sprint acceleration phase that influence performance and distinguish sprinters of different levels.

To better understand the key factors in the sprint start, the findings from the reviewed studies are organised into three areas:

1. The “Set” position
2. The Push-Off phase
3. The first two steps of initial acceleration

The "Set" Position - Setting Up Success

The “Set” position is the first performance key factor in the block start performance because it depends on block settings and the body posture assumed by sprinters.

Is there one optimal “Set” position which should be adopted by sprinters?

The answer seems to be no...

The researched studies showed that it is not an important differentiating factor of performance, since it does not present any correlation with best 100m times or normalized block power. However, there are some interesting aspects that sprinters should look out for in a more effective “Set” position. The ideal “Set” position depends on the individual anthropometric features, strength, and morphologic characteristics and motor abilities.

1.1. Block Settings

The "Set" position depends largely on the anteroposterior block distance, which defines the type of start used. There are three types of block starts based on inter-block spacing: bunched—less than 0.30 m; medium—0.30 to 0.50 m; and elongated—greater than 0.50 m.

Studies that reported block spacing based on the individual sprinter’s preferences suggest distances between 23.5 ± 1.9 cm (for female sprinters; PB100: 11.97 ± 2.6 s) and 32 ± 5 cm (for male sprinters; PB100m: 10.79 ± 0.21). This indicates that most sprinters adopt distances within or very close to the bunched start type, favouring centre of mass (CM) positioning closer to the starting line. Research has demonstrated that elongated start settings increase block velocity (i.e., horizontal CM velocity at block clearing), but are linked to increased pushing time on the blocks, leading to significantly worse performance at 5 and 10 m compared to the bunched start.

Further findings show that the medium start offers the best compromise between pushing time and the force exerted on the blocks, allowing for better times at 10 m. Additionally, recent studies have highlighted that anthropometry-driven block setting based on the sprinter’s leg length plays a significant role in block start performance, leading to postural adaptations that provide several kinematic and kinetic advantages. Adjusting inter-block spacing based on the relative lengths of the sprinter’s trunk and lower limbs can increase force and impulse on the rear leg, resulting in higher total normalized average horizontal external power (NAHEP), a key indicator of starting block performance.

Other factors to consider in the "Set" position include feet plate obliquity and the amount of pre-tension exerted on the blocks prior to the start. The inclination of the blocks relative to the track affects the initial lengths of the plantar flexor muscle-tendon units (MTUs) and influences muscle mechanics and external force parameters during the block phase. Faster sprinters tend to produce peak torque at longer MTU lengths, and adopting a more crouched position may allow them to generate greater force during the block phase.

Research indicates that reducing footplate inclinations (from 65° to 40°) increases block velocity and enhances peak joint moments and powers, especially at the ankle. Reducing the inclination of the front block alone (from 70° to 30°) can also increase block velocity without extending push-off phase duration. However, another study found that a steeper rear foot position (65°) resulted in greater mean rear block horizontal force. Differences in the location of the centre of pressure (COP) and footplate surface length may explain this discrepancy, as a higher and more posterior COP on the starting block surface typically enhances sprint start performance.

Conversely, a pre-tensioned start does not appear to provide a performance advantage over a conventional start, as any increase in propulsive force from the lower limbs is offset by an increase in the back force exerted through the hands during the same period.

1.2. Sprinter Body Posture

Apart from block configuration, the choice of the sprinter’s body posture also determines the effectiveness of the “Set” position on the subsequent block push-off phase. The horizontal distance between the starting line and the vertical projection of the centre of mass (CM) to the ground in the “Set” position (XCM) is a factor that differentiates sprinters with different performance levels. Faster sprinters tend to move their CM closer to the starting line and closer to the ground.

Elite (PB100: 10.27 ± 0.14 s) and well-trained (PB100: 11.31 ± 0.28 s) male sprinters showed XCM of 22.9 and 27.8 cm, respectively. Likewise, world-class (PB100: 11.10 ± 0.17 s) and elite (PB100: 11.95 ± 0.24 s) female sprinters presented XCM of 16.2 and 24.8 cm, respectively. This more crouched position is only possible due to the high explosive strength of the best sprinters, which allows them to produce higher levels of strength in the blocks and reduce the horizontal travel distance of the CM. This body position is complemented by a more advanced shoulder position, putting more tension on the arms, and allowing greater blocking speed during the subsequent phase.

In terms of sprinter joint angle configuration in the “Set” position, research has shown that horizontal CM velocity at the block take-off and along the first two steps increases significantly when the rear knee angle is set to 90° instead of 135° or 115°. A 90° rear knee angle allows for a better push-off of the rear leg than larger angles, suggesting that this position may be a strategy enabling some elite sprinters to maximise their strength capacity. A more flexed front knee may facilitate optimal joint moment production, but only in sprinters with exceptionally high levels of explosive strength.

The Push-Off Phase - Maximise Horizontal Velocity

The “block-phase” or “push-off phase” in the starting blocks begins immediately after the gunshot and is recognised as a complex motor task that influences sprint start performance. Reaction time is the initial factor in the sequence of the block phase, defined as the period from the gun signal to the first measurable change of pressure detected in the instrumented blocks.

While a sprinter’s ability to react is undeniably important, it is linked to information-processing mechanisms that do not appear to correlate with performance levels and is, therefore, beyond the scope of this review (for a detailed analysis of factors affecting response times, see Milloz and Hayes).

Once the reaction occurs, the objective of the block phase is to maximise horizontal velocity in the shortest time possible. Therefore, the motion variables during the block phase are the focus of this section.

2.1. Kinematic Analysis

The efficiency of the starting action primarily depends on the balance between horizontal start velocity (or block velocity) and block time (the time from the initial movement at the “set” position to exiting the blocks), resulting in horizontal start acceleration. Although horizontal block velocity is a key parameter for an efficient sprint start, it cannot be evaluated in isolation. An increased block velocity could arise either from an increase in the net propulsion force or from a longer push-off duration.

Top sprinters tend to achieve higher block velocity and greater block acceleration than slower athletes, as they are able to generate a greater impulse in less time and optimise force production on the blocks. However, if sprinters increase their anteroposterior force impulse by extending the block time, they may reduce their block acceleration and their performance at the 5 and 10 metre marks.

Studies comparing sprinters of different performance levels generally show higher block velocities (e.g., 3.38 ± 0.10 vs. 3.19 ± 0.19 m/s) and greater block accelerations (e.g., 9.5 vs. 8.8 m/s²) for faster sprinters. Additionally, higher performance levels are slightly associated with lower block vertical velocities and more horizontal CM projection angles (resulting from the CM’s horizontal and vertical block exit velocities).

The pattern of lower limb joints during the push-off phase (from movement onset to block exit) mostly involves extension movements, particularly in the hips and knees. The front leg typically extends through a significant range of motion (ROM) in a proximal-to-distal pattern, reaching maximum extension at the start of the flight phase (e.g., hip: 183.2°, knee: 177.4°, ankle: 133.1°). In contrast, the rear leg does not follow the same extension strategy, with the knee reaching peak angular velocity before the hip and ankle. This is likely due to the reduced ROM of the rear knee compared to the front knee, which starts in a more extended position in the “set” position (e.g., rear knee: 120.7°, front knee: 91.0°).

The movement of the ankles is complex, involving an initial dorsiflexion followed by extension, resulting in a stretch-shortening cycle of the triceps surae muscle. The rear ankle exhibits a longer duration of flexion (50% of the block phase) compared to the front ankle (20%). Experimental studies manipulating footplate inclinations have shown that block angles steeper than 65° may negatively impact plantar flexor function, as they alter the muscle-tendon lengths of the gastrocnemius and soleus.

Peak angular velocities at both hips occur due to a combination of flexion-extension, abduction-adduction, and internal-external rotation, emphasising the importance of a 3D analysis of the sprint start. While there is a consistent pattern in joint angular velocity sequences during the block phase among sprinters, the lack of comparative data between athletes of different levels prevents pinpointing the critical technical aspects for success. However, rapid hip extension should be a focal point in technique, as peak angular velocities and the range of rear hip extension are positively correlated with block power.

Although upper body kinematics during the push-off phase has been minimally studied, some important observations have been made. The movement of the upper limbs varies more among sprinters compared to the lower limbs. Nevertheless, a 3D movement pattern is recognised for shoulders and trunk, involving flexion-extension, abduction-adduction, and internal-external rotation, while the elbows extend and pronate. The velocity of the rear shoulder is generally slightly higher than other joints, but the peak angular velocities at upper limb joints are comparable to those in the lower limbs, particularly at the knees and front ankle.

No current evidence links different upper limb kinematic patterns to specific block phase performance indicators. Further research is needed to establish relevant recommendations for athletes and coaches.

2.2. Kinetic Analysis

According to Newton’s second law of motion, horizontal CM acceleration requires net propulsive forces to be applied to the athlete’s body in the sprinting direction. Therefore, as mentioned earlier, the horizontal force impulse, consisting of the mean horizontal force and push-off time, determines the horizontal velocity at block exit. The relationship between these factors indicates that applying a greater amount of horizontal force is a critical performance element, as increasing block time conflicts with the 100 m sprint criterion of achieving the shortest time possible. As a result, elite sprinters produce greater average forces, higher rates of force development, and larger net and horizontal block impulses compared to their slower counterparts.

Graham-Smith and Colyer, in their comparison of senior and junior athletes, found that sprinters with faster personal bests over 100 m (senior athletes) exhibit higher relative horizontal force during the initial block phase and during the transition from bilateral to unilateral pushing. This emphasises the importance of force generation against the blocks for effective execution of the block start, prompting researchers to delve deeper into the kinetic determinants of this crucial sprint phase.

Bezodis and colleagues sought to identify the most accurate and practical measure of push-off performance suitable for field use. Their findings suggested that the NAHEP (Net Anteroposterior Horizontal External Power) is the most appropriate measure, as it reflects how much sprinters can increase their velocity and the time taken to achieve this, while accounting for variations in athlete morphology.

Further research has highlighted that the magnitude of force applied to both blocks and their optimal orientation are major performance determinants. Studies indicate that the force generated by the front leg is crucial for forward propulsion, with faster sprinters producing higher force impulses in the front block than slower sprinters (e.g., 221.3 ± 15.8 N·s vs. 178.3 ± 13.1 N·s for faster and slower sprinters, respectively). Colyer and Graham-Smith also noted that higher force production in the front block during the transition phase (when the rear foot leaves the block, accounting for 54% of the block push) and a more horizontally oriented front block force vector (81–92%) are key differentiating factors in performance.

However, other evidence suggests that the force magnitudes from the rear block are the most predictive kinetic features of block power and sprint performance. For instance, Coh and Peharec found that faster sprinters (PB100m = 10.66 ± 0.18 s) generated greater total forces against the rear block compared to slower sprinters (PB100m = 11.00 ± 0.06 s). A longer rear leg push duration, relative to the total push-off phase, is positively associated with higher block power and is observed in sprinters with faster PB100m times.

Studies on the Centre of Pressure (COP) on the starting block surface show that COP location may also impact initial sprint performance. Better performance at the sprint start appears to be associated with a higher and more rearward COP during the force production phase. Therefore, athletes and coaches should consider positioning the calcaneus on the block (posterior location) as it may enhance 10 m times and/or horizontal external power for some individuals.

Forces under the hands have been less frequently studied, with mixed results. While some studies highlight a primary support function, others suggest that top athletes generate less negative horizontal impulse under their hands compared to slower athletes. As a result, the role of hand kinetics during the push-off phase remains unclear, warranting further investigation.

Beyond external kinetic analyses, internal kinetics (joint kinetics) provide deeper insight into segment motions responsible for CM acceleration. Recent research indicates that 55% of the variance in NAHEP among sprinters with a PB100m of 10.67 s is mainly attributable to the rear ankle joint moment (23%), front hip joint moment (15%), and front knee joint power (15%). The remaining variance is distributed among other lower limb joint kinetic variables.

In the rear block, horizontal force generation is influenced by the rear hip extensor moment, rear hip extensor power, and significant ankle joint plantarflexion moment, with minimal knee joint contribution. In the front block, a proximal-to-distal pattern of peak joint power is observed, reflecting a strategy commonly adopted in power-demanding activities, with the main periods of positive extensor power at the front ankle and knee occurring after the rear foot leaves the block.

A study involving 12 sprinters from the University of Tokyo team (PB100m: 10.78 ± 0.19 s) showed that the peak lumbosacral extension moment was significantly larger than any other lumbosacral and lower-limb moment, correlating positively with starting performance. This peak value occurred during the double-stance phase, where both hip joints exerted extension moments. These findings align with those of Slawinski and Bonnefoy, who reported that the lower limbs and head–trunk segments are the main contributors to the total body’s kinetic energy. The upper limbs contribute 22% to total body kinetic energy, indicating their non-negligible role in the push-off phase.

The First Two Strides: Transition from Block to Strides

The primary objective of the first steps in sprinting is to generate a high horizontal velocity. However, the transition between the block start and the first steps presents a unique biomechanical challenge: integrating temporal and spatial acyclic movements into a cyclic action. The efficiency of this transition depends on the biomechanical demands of the initial stances following block clearance, which differ significantly from subsequent stances during acceleration.

The sprinter's aim is to produce maximal forward acceleration during the transition from the start block into sprint running, while also generating enough upward acceleration to move from a flexed position in the blocks to a more extended posture. This requires specific technical (kinematic) and dynamic (kinetic) skills to successfully manage the transition, and these skills are the focus of this section.

3.1. Kinematic Analysis

The primary goal of the initial steps in sprint running is to generate a high horizontal sprint velocity, which results from the product of the length and frequency of the sprinter’s steps. Spatiotemporal parameters reveal that the sprinter’s step length increases progressively during the acceleration phase, while step frequency rapidly reaches its maximum. Typically, step frequency hits about 80% of its maximum at the first step and approximately 90% after the third step, achieving around 4 Hz immediately following block exit.

The length of the first steps varies among sprinters, with senior females ranging from 0.82 to 1.068 m and senior males from 0.85 to 1.371 m on the first step. On the second step, senior females range from 1.06 to 1.30 m, while senior males range from 1.053 to 2.10 m. Despite this variability, faster sprinters tend to have longer steps, particularly in the first step, showing an increase of about 14 cm for every 1 second decrease in their PB100m times. This may be attributed to the lower vertical velocity of the CM (centre of mass) at block clearance, allowing them to cover a longer distance despite shorter flight times.

The kinematics of faster sprinters are also characterised by longer ground contact times during the first two steps. For example, Diamond League sprinters have a mean first contact duration of 0.210 s for males and 0.225 s for females, which is longer than those of lower-level Italian junior sprinters (0.176 and 0.166 s, respectively). This strategy allows elite sprinters to optimise the duration of force application, reducing time spent in the air where force cannot be generated. Additionally, elite sprinters project their CM further forward at the first touchdown, placing the foot behind the CM’s vertical projection and minimising the braking phase. At takeoff, the CM’s horizontal position is greater in elite than well-trained sprinters, meaning their CM’s resultant and horizontal velocities are generally higher in the first two steps.

Lower limb joint movement during the first two steps follows a proximal-to-distal sequence involving the hip, knee, and ankle of the stance leg. During the initial and second steps, the ankle joint experiences dorsiflexion in the first half of stance, followed by plantarflexion. For example, dorsiflexion angles are around 17° and 18° for the first and second steps, respectively, while plantarflexion angles reach 45° and 44°. A reduction in the dorsiflexion range during early stance, requiring high plantar flexor moments, has been linked with increased power in the first stance.

Maximal plantarflexion occurs immediately following takeoff, reaching angles such as 111.3° in the first stance and 107.1° in the second. Knee extension follows after the block exit and peaks at the start of the flight phase, with greater extension in the front compared to the rear leg. The hip joints also extend during block clearance and reach maximal extension during the early flight phase. A substantial range of motion in hip and pelvis rotation, as well as abduction, is present during stance.

Although there is detailed information on lower limb angular kinematics during the first two stances and flight phases, there is no definitive evidence on the joint kinematic features that differentiate faster from slower sprinters. Additionally, there is a lack of experimental data regarding arm actions during early acceleration and their relationship with performance, highlighting the need for future research in this area.

3.2. Kinetic Analysis

Fast acceleration is a crucial determinant of performance in sprint running, where a high horizontal force impulse in a short time is essential to reach high horizontal velocity. The highest centre of mass (CM) acceleration during a sprint occurs during the first stances. For example, during the first stance, the acceleration is 0.36 ± 0.05 m·s⁻², and in the second stance, it is 0.23 ± 0.04 m·s⁻². The ability to generate greater absolute impulse, maximal external power, and a forward-leaning force oriented in the sagittal plane during this phase is linked to overall higher sprint performance.

Larger propulsive horizontal forces are particularly important during early acceleration, serving as a discriminating factor for superior levels of performance. Experienced male sprinters (PB100m: 10.79 ± 0.21 s) can produce propulsive horizontal forces of around 1.1 bodyweight during the first stance. However, a negative horizontal force has been reported during the first contact after the block exit, even if the foot is properly placed behind the vertical projection of the CM. During the first stance, the braking phase represents about 13% of the total stance phase, with the magnitude of the braking forces reaching up to 40% of the respective propulsive forces.

3D analysis studies highlight lower body motion outside the sagittal plane during the initial ground contact phases. During the first steps, a stance medial deviation is often observed, resulting from an impulse in the transverse plane. Although the medial impulse is the smallest of the three orthogonal stance impulses, its non-zero value can affect the motion of the CM and step width. Well-trained sprinters present similar step widths in early acceleration to those of both trained and non-trained sprinters. Additionally, manipulations of both the “set” position and first step widths have shown no effect on block-induced power, braking force, or net anteroposterior impulse, indicating that smaller step width is not a discriminator factor of superior performance levels.

At the joint level, the hip, knee, and ankle joints generate energy during stance leg extension, with the ankle joint being the main contributor to CM acceleration. The knee plays an important role during the first stance, crucial for forward and upward CM acceleration. The importance of power generation at the knee is specific to the first stance when the knee is flexed and the sprinter is leaning forward. As the sprinter progresses to the second stance, the ankle becomes more dominant since the plantar flexors are better positioned to contribute to forward progression.

During the first step, the sprinter favors immediate power generation of the knee extensors rather than preserving a stretch-shortening cycle. In contrast, a stretch-shortening mechanism is confirmed at the hip and ankle. Hip extensors reach maximal power generation near touchdown, actively pulling the body over the touchdown point. While the hip can generate large joint moments and power, it contributes minimally to propulsion and body lift during the first two stances.

Ankle plantar flexors act throughout both the first and second stances under a stretch-shortening cycle, leading to an initial phase of power absorption preceding forceful power generation at take-off. The ankle joint can generate up to four times more power than it absorbs during the first two stances. The significance of ankle stiffness during this phase remains unclear. Some studies have found a correlation between greater ankle stiffness and horizontal CM velocity at take-off, while others did not, highlighting the lack of differences between faster (senior) and slower (junior) sprinters.

Kinetic factors differentiate senior and junior athletes, with seniors producing greater horizontal power during the initial part of the first and second ground contact. For instance, senior sprinters exhibit higher power outputs compared to junior sprinters during the first and second steps. Furthermore, adult sprinters generate more joint power at the knee during the first step than young sprinters, resulting in longer step lengths and higher velocities. Conversely, younger sprinters prioritize a different technique, with the hip contributing more to total power generation while the knee contributes less.

There is no evidence of differences in ankle joint stiffness, range of dorsiflexion, or plantar flexor moment between young and adult sprinters, suggesting that the technical performance-related parameters of the first stances do not explain the better 100 m sprint times in adults compared to young sprinters.

Recommendations

Based on this review, here are some key points and tips to help athletes and coaches:

• 1. Adjust the distance between blocks based on the sprinter’s leg length. Set individualised distances for each athlete based on their comfort and by testing their initial 10m times.
• 2. Adjust footplate angles to support the sprinter’s initial foot position. Angles around 40° are best, and avoid angles steeper than 65°.
• 3. Position the heel at the back of the block. This may help improve the athlete's initial 10 m time and increase power out of the blocks.
• 4. Keep block exit times short and cue the athlete to make their first ground contact quickly. This helps generate more propulsive force.
• 5. Focus on how much force is applied to the back block. This is a key factor in getting a good start.
• 6. Encourage quick hip extension during the push-off phase. This should be a focus for both athletes and coaches.
• 7. Recognise the important roles of the hips, knees, and ankles in the early running phase. Train these areas to build strength and power for a better start.

© 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https://creativecommons.org/licenses/by/4.0/).