The primary goal of the initial steps of a 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
[22][38]. Spatiotemporal parameters have shown that the sprinter’s step length increases regularly during the acceleration phase, while step frequency is almost instantaneously leveled to the maximum possible
[22]. Typically, the step frequency reaches the maximal values very quickly (80% at the first step and about 90% after the third step)
[22], achieving around 4 Hz immediately after block exit
[39][38]. The length of the first steps is more variable between sprinters, ranging from 0.82 to 1.068 m (senior females)
[1][29] or 0.85 to 1.371 m (senior males)
[1][7] on the first step, and from 1.06 to 1.30 m (senior females)
[1][13] or 1.053 to 2.10 m (senior males)
[7][27] on the second step. Despite this variability, step length tends to be longer in faster sprinters, particularly in the first step (e.g., 1.371 ± 0.090 vs. 1.208 ± 0.087 m
[7]; 1.30 ± 0.51 vs. 1.06 ± 0.60 m
[5]; 1.135 ± 0.025 vs. 0.968 ± 0.162 m
[29]), exhibiting an increase of about 14 cm for every 1 s less in PB100m
[29]. This may be a consequence of the lower vertical velocity of the CM at the block clearing shown by faster sprinters, allowing them to travel a longer distance despite shorter flight times
[29]. Indeed, the kinematics of faster sprinters is also characterized by a tendency to assume long ground contact times in the first two steps (e.g., mean first contact duration for Diamond League sprinters is 0.210 s for males and 0.225 s for females, which is greater than those of lower-level Italian junior sprinters: 0.176 and 0.166 s, respectively), associated to short flight times (0.045 and 0.064 s, for the first flight of world-class and elite male sprinters, respectively)
[29]. This strategy allows the high-level sprinters to optimize the time during which propulsive force can be generated, minimizing the time spent in flight where force cannot be generated. Combined with this, best sprinters have their CM projected further forward
[7] at the first touchdown, putting the foot behind the vertical projection of the CM
[3], and minimizing the braking phase. At the takeoff of the first and second steps, the CM horizontal position is also greater in elite than well-trained sprinters
[7]. This means that the CM resultant and horizontal velocity in the first two steps are generally greater in high-level sprinters
[7][15].
Lower limb joints pattern during the first two steps is associated with a proximal-to-distal sequence of the hip, knee, and ankle of the stance leg
[4][9][40]. During both first and second steps, the ankle joint undergoes dorsiflexion during the first half of stance (e.g., 17 ± 3° and 18 ± 3° for the first and second steps, respectively
[40]) and subsequently a plantarflexion movement (e.g., 45 ± 6° and 44 ± 5° for the first and second steps, respectively
[40]).
The hip performs extension for the entire stances, the knee extends until the final 5% of stances, and the ankle is dorsi-flexed during the first half of stances before the plantar flexing action
[6]. After leaving the rear block, there is a small increase in ankle joint dorsiflexion during the swing phase, preceding the plantarflexion that occurs just before touchdown
[6]. Although the ankle plantar-flexes slightly at the end of the flight, the ankle is in a dorsi-flexed position at initial contact (e.g., first stance: 70.6 ± 5.8° and second stance: 72.4 ± 7.1°
[6]). During both first and second steps, the ankle joint dorsi-flexes during the first half of stance (e.g., 17 ± 3° and 18 ± 3° for the first and second steps, respectively
[40]) and subsequently performs a plantarflexion movement (e.g., 45 ± 6° and 44 ± 5° for the first and second stance, respectively
[40]). Note that a reduction in the range of dorsiflexion during early stance, requiring high plantar flexor moments, has already been associated with increases in first stance power
[41]. Maximal plantarflexion occurs immediately following takeoff reaching, for example, 111.3° at the first stance and 107.1° at the second stance
[6]. The extension of both knees occurs just after the block exit and reaches its maximum at the beginning of the flight phase, with larger extension in the front compared with the rear leg (e.g., rear: 134.9 ± 11.2°; front: 177.4 ± 5.2°)
[6]. From a flexed position at initial contact, the knee extensors generate power to induce extension throughout stance and to attain maximal extension at takeoff, achieving peak extension angles of around 160–170° (not full extension; e.g., first stance: 165.2 ± 20.6°; second stance: 163.6 ± 17.7°
[6]). This extension action of the knee during stances on its own may play a role in the rise of the CM during early acceleration
[39]. The hip joints extend during block clearance to reach maximal extension during the beginning of the flight phase. During stance, the hips are in a flexed position at initial contact and continue to extend throughout stance, achieving maximal extension immediately following takeoff (e.g., first stance: 180.6 ± 20.9°; second stance: 181.1 ± 20.0°
[6]). There is also a considerable ROM in hip and pelvis rotation during stance as well as abduction. Although there are detailed descriptions of the lower limb angular kinematics during the first two stances and flight phases
[3][6], there seems to be no clear evidence about the joint kinematic features that differentiate faster from slower sprinters. Furthermore, there is also a lack of experimental data on arm actions during early acceleration and its relationship to performance descriptors, making necessary future research in this area to help identify the most important performance features.
2.3.2. First Two Steps Kinetic Analysis
As said before, fast acceleration is a crucial determinant of performance in sprint running, where a high horizontal force impulse in a short time
[13] is essential to reach high horizontal velocity
[40]. Thus, as the highest CM acceleration during a sprint occurs during the first stances
[7][9][14] (e.g., first stance: 0.36 ± 0.05 m·s
−2; second stance: 0.23 ± 0.04 m·s
−2 [14]), the ability to generate during this phase greater absolute impulse
[7][18], maximal external power
[32][33], and a forward-leaning force oriented in the sagittal plane
[21][22][24][33] is linked to an overall higher sprint performance. Larger propulsive horizontal forces are particularly important during early acceleration, being a discriminating factor for superior levels of performance
[42]. Experienced male sprinters (PB100m: 10.79 ± 0.21 s) can produce propulsive horizontal forces of around 1.1 bodyweight during the first stance
[18]. However, a negative horizontal force has also been reported during the first contact after the block exit, even if the foot is properly placed behind the vertical projection of the CM
[18]. During the first stance, for example, the braking phase represents about 13% of the total stance phase and the magnitude of the braking forces can reach up to 40% of the respective propulsive forces
[18].
At joint level, the hip, knee, and ankle joints generate energy during stance leg extension
[6], although it appears that the ankle joint is the main contributor to CM acceleration
[14]. However, experimental and simulation studies highlight that the knee plays an important role during the first stance, being decisive for forward and upward CM acceleration
[4][6][14][15]. The importance of power generation at the knee seems to be specific for the first stance when the knee is in a more flexed position and the sprinter is leaning forward. From the second stance onwards, the knee becomes less and the ankle more dominant since the plantar flexors are in a better position to contribute to forward progression
[6]. As the knee is in a flexed position during the first step, the sprinter favors the immediate power generation of the knee extensors rather than preserving a stretch-shortening cycle
[6]. In contrast, a stretch-shortening mechanism can be confirmed at the hip and ankle
[4][6][14][15]. Hip extensors maximal power generation occurs near touchdown
[4][6] where the hip extensors actively pull the body over the touchdown point
[6]. The hip can effectively generate large joint moments and power
[14], but only contributes minimally to propulsion and body lift during the first two stances
[14]. Ankle plantar flexors act throughout both the first and second stances under a stretch-shortening cycle. There is therefore an initial phase of power absorption preceding the forceful power generation at take-off
[4][14]. As a major contributor to CM acceleration, the ankle joint can generate up to four times more power than it absorbs during the first two stances
[40]. Nevertheless, the importance of ankle stiffness during the first two stances remains unclear. While Charalambous, Irwin
[43], in a case report, found a correlation between greater ankle stiffness and greater horizontal CM velocity at take-off (
r = 0.74), Aeles, Jonkers
[9] did not, still highlighting the lack of differences between faster (senior) and slower (junior) sprinters. Future work is therefore needed to further clarify this issue. Furthermore, it remains unclear whether ankle stiffness is influenced by foot structure and function (e.g., planus, rectus cavus, clubfoot) as well as other important performance variables such as greater maximal power, a forward-leaning force oriented in the sagittal plane, or COP location during push-off.
Concerning kinetic factors differentiating senior and junior athletes, Graham-Smith, Colyer
[32] reported that, contrarily to the block phase where there are marked differences between groups, the force and power waveforms relating to the first two steps did not differ considerably across groups. Still, senior sprinters are able to produce greater horizontal power during the initial part (10–19% of the stance phase) of the first and second ground contact (first step: 25.1 ± 3.6 W·kg
−1 vs. 23.1 ± 6 W·kg
−1 and second step: 26.7 ± 3.6 W·kg
−1 vs. 24.9 ± 4.5 W·kg
−1, for senior and junior sprinters, respectively), and also exhibit a higher proportion of forces immediately after braking forces are reversed (from 9% to 15% and 25% to 29% of stance phase)
[32]. Furthermore, Debaere, Vanwanseele
[15] also highlight that adult sprinters are able to generate more joint power at the knee during the first step compared to young sprinters, inducing longer step length and therefore higher velocity
[15]. Younger sprinters tend to prioritize a different technique: the hip contributes more to total power generation, while the knee contributes far less
[15]. This indicates that younger sprinters lack the specific technical skills observed in adult sprinters, likely due to less musculature than adults
[1][9][15]. However, there is no evidence of differences in ankle joint stiffness, range of dorsiflexion, or plantar flexor moment between young and adult sprinters
[9]. This indicates that the technical performance-related parameters of the first stances are not likely to explain the better 100 m sprint times in adult compared to young sprinters
[9].
3. Conclusions
(i) the choice of an anteroposterior block distance relative to the sprinter’s leg length may be beneficial for some individuals, promoting greater block start performance (greater normalized average horizontal external power); (ii) the use of footplate inclinations that individually facilitate initial dorsiflexion should be encouraged—footplate angles around the 40° are recommended and block angles steeper than 65° should be avoided; (iii) pushing the calcaneus onto the block (posterior location) may be beneficial for some individuals, improving the 10 m time and/or horizontal external power; (iv) short block exit flight times and optimized first stance contact times should be encouraged, as they maximize the time during which propulsive force can be generated; (v) focus attention on the magnitude of force applied on the rear block, as it is considered to be a primary determinant of block clearance; (vi) rapid hip extension during the push-off phase should be a priority in sprinter focus and coach feedback; (vii) the large role played by the hips on the push-off phase and by both the knee and ankle at the early stance must be acknowledged within physical and technical training to ensure strength and power are developed effectively for the nature of the sprint start.