The acceleration phase is the opening portion of a sprint in which a runner builds speed from a standstill or slow pace up toward top velocity. In a typical 100-meter dash, this phase covers roughly the first 30 to 50 meters and is where races are often won or lost, because the ability to generate horizontal force in those early steps determines how quickly a sprinter reaches peak speed. What makes this phase biomechanically distinct from full-speed running is everything from body posture and ground-contact angles to which muscles are doing the heaviest work and how the nervous system coordinates it all.
How Horizontal Force Drives Early Speed
During the acceleration phase, the single most important mechanical factor is horizontal force applied to the ground. A study of elite sprinters found that net horizontal ground reaction force impulse per unit body mass was strongly correlated with 40-meter mean speed, with the relationship driven primarily by how much force sprinters pushed backward into the track rather than how little braking force they produced on each step. In other words, the fastest accelerators were those who “pushed more,” not necessarily those who “braked less.”1PubMed. Acceleration capability in elite sprinters and ground impulse: Push more, brake less? This finding challenges the intuitive assumption that minimizing the backward-directed braking impulse at touchdown is a primary performance lever. Braking impulse during early acceleration is actually negligible, and propulsive impulse is what separates fast starters from slower ones.2PubMed. The relationship between maximal jump-squat power and sprint acceleration in athletes
The block start reinforces this picture. Sprinters with faster personal-best times generate larger relative horizontal impulses during the push phase in starting blocks, and they do so despite spending the same amount of time or even less time in contact with the blocks. The difference comes down to higher average and peak forces and faster rates of force development.3PubMed Central. The Biomechanics of the Track and Field Sprint Start: A Narrative Review – Section: External Kinetics So the acceleration phase is not about staying on the ground longer to accumulate force. It is about producing more force in the same or shorter window of contact.
Body Lean and Posture Changes Step by Step
If you have ever watched a sprinter leave the blocks, the aggressive forward lean is immediately obvious. That posture is not just for show. A forward trunk angle allows the runner to direct ground reaction forces more horizontally, which is exactly what early acceleration demands. Research examining the transition from a running pace into a full sprint found that trunk flexion increased during the earliest acceleration steps, and knee-joint excursion was significantly lower during loading and propulsion compared to the run-in strides. Greater acceleration performance was predicted by a combination of trunk and neck angles across the first three steps, with trunk flexion during the second step’s propulsion phase playing a key role.4PubMed Central. Transition from upright to greater forward lean posture predicts faster acceleration during the run-to-sprint transition
As sprinters progress through the acceleration phase, they gradually rise toward an upright posture. This transition is not smooth and linear. Large step-to-step changes in touchdown kinematics occur during the initial acceleration phase, and these changes are much more dramatic than what happens during the later transition toward maximum velocity. At toe-off, kinematic changes are more consistent across both the initial acceleration and transition phases before flattening out at top speed.5PubMed. Phase analysis in maximal sprinting: an investigation of step-to-step technical changes between the initial acceleration, transition and maximal velocity phases Think of it as a rapid recalibration: the body is essentially solving a new motor problem with every step during early acceleration, settling into a more stable pattern only as the runner approaches top speed.
When Acceleration Ends and Top Speed Begins
There is no single step where a runner flips a switch from accelerating to sprinting at full speed. Instead, researchers have identified discriminable transition points marked by specific kinematic events. At the first transition, the foot begins contacting the ground in front of the body’s center of gravity, the knee starts to flex during the support phase, and step frequency stops increasing. At a second, later transition, body postures stop changing and hip-joint movement intensity begins to slightly decrease.6Biology Open. Kinematics of transition during human accelerated sprinting – Section: RESULTS These markers give coaches and scientists a way to identify where acceleration ends for a given athlete, which matters for targeted training.
Foot kinematics follow a similar pattern. Maximal angular velocities at the ankle increase through the first six or seven strides and then stabilize. The time from touchdown to peak dorsiflexion velocity stays roughly constant across steps, but the time from peak dorsiflexion velocity to toe-off drops significantly over the first six strides, reflecting a shortening propulsive phase as the runner gains speed.7PubMed Central. Relationship between Step-by-Step Foot Kinematics and Sprint Performance – Section: Results So while the whole body is visibly rising and the stride is lengthening, the fine-grained ankle mechanics are also converging toward a steady state by about step six or seven.
Step Length, Step Rate, and Sex Differences
A common question is whether faster acceleration comes from taking longer steps or from turning them over more quickly. Among high-level sprinters, step rate stays remarkably constant across the initial acceleration, transition, and maximum-speed phases. For men, it hovered around 4.4 steps per second across all three phases, and for women around 4.3. The phases did not differ significantly within either sex.8The Journal of Strength & Conditioning Research. The Contribution of Step Characteristics to Sprint Running Performance in High-Level Male and Female Athletes This means the primary driver of speed gain during acceleration is increasing step length, not step frequency.
Interestingly, men did not fully capitalize on their greater explosive strength during initial acceleration. Despite having substantially higher force-producing capacity, their normalized step length during the initial acceleration phase was only about four percent longer than women’s. The interaction between step characteristics and performance differed between men and women across phases, suggesting that coaching cues and training priorities during the acceleration phase might need to be sex-specific rather than one-size-fits-all.
The Hamstrings as the Acceleration Engine
When most people think of powerful muscles for sprinting, the quadriceps come to mind. But during the acceleration phase, the hamstrings play a surprisingly central role. Research measuring muscle activation alongside ground reaction forces found that the sprinters who produced the greatest horizontal force were those who both highly activated their hamstring muscles just before ground contact and possessed high eccentric hamstring peak torque. Specifically, the combination of biceps femoris activation during the late swing phase and the knee flexors’ eccentric strength predicted horizontal ground reaction force production.9PubMed Central. Sprint Acceleration Mechanics: The Major Role of Hamstrings in Horizontal Force Production
This makes mechanical sense. During the forward-leaning posture of early acceleration, the hamstrings act to pull the body forward over the planted foot and to pre-tension the leg before ground contact. It is this pre-activation during the swing phase, before the foot even hits the ground, that appears to set up effective force transmission once contact happens. Athletes who can eccentrically load their hamstrings at high force levels and then rapidly switch to propulsive effort have a built-in advantage during the acceleration phase.
Concentric force development matters too. Jump-squat power, which is largely a concentric expression of lower-body strength, is significantly related to 5-meter sprint time, with correlations in the range of −0.64 to −0.68 depending on the squat variation.10PubMed. The relationship between maximal jump-squat power and sprint acceleration in athletes The picture that emerges is that both eccentric preparedness and concentric explosiveness contribute to acceleration performance, but through different mechanisms at different moments in the stride cycle.
Rate of Force Development and What Type Matters
It is not enough to be strong; how quickly you can produce force matters for acceleration. But the type of muscle action involved shapes which aspect of performance benefits. Research examining concentric, eccentric, and isometric rate of force development found that faster 10-yard sprint times correlated only with concentric rate of force development. Eccentric rate of force development, by contrast, was linked to change-of-direction ability rather than straight-line acceleration. Isometric rate of force development showed no correlation with any of the sprint or agility measures tested.11PubMed Central. Muscle contraction type-specific association of acceleration and deceleration performance with rates of force development – Section: Results
This is a useful distinction for training. If the goal is pure acceleration improvement, exercises emphasizing concentric explosiveness (think jump squats, power cleans, or explosive hip thrusts) likely transfer more directly than isometric holds or slow eccentrics. Eccentric training still matters for injury prevention and for sports that require rapid deceleration and direction changes, but the direct carryover to straight-ahead acceleration appears to run through the concentric pathway.
Training the Acceleration Phase with Resisted Sprints
One of the most studied training methods for improving acceleration is resisted sprinting, particularly sled towing or sled pulling. The logic is straightforward: adding resistance to a sprint forces the athlete to produce more horizontal force, specifically targeting the demands of the acceleration phase. But the choice of load matters a great deal.
A study of male high-school athletes found that changes in the force-velocity profile after a sled-pull training program were specific to the load used. Moderate to heavy loads produced the largest improvements in maximal horizontal force, with large effect sizes. However, the heavy-load group saw a significant decrease in maximum velocity, while the unresisted group improved their top speed. Maximum power output increased across all resisted groups.12The Journal of Strength & Conditioning Research. Influence of Resisted Sled-Pull Training on the Sprint Force-Velocity Profile of Male High-School Athletes The takeaway is that heavier sleds build force capacity at low speeds (the start of the acceleration phase), but can erode top-end speed if that is not trained separately.
Research on soccer players using very heavy sled loads confirmed this pattern. Very heavy resisted sprints clearly increased maximal horizontal force production and mechanical effectiveness compared to unloaded sprint training, with effect sizes around 0.80 for force and 0.95 for effectiveness of horizontal force application.13International Journal of Sports Physiology and Performance. Very-Heavy Sled Training for Improving Horizontal-Force Output in Soccer Players – Section: Results “Mechanical effectiveness” here refers to the proportion of total force that is directed horizontally rather than vertically, which is precisely the quality that distinguishes great accelerators from average ones.
When researchers examined what happens to sprint mechanics under load in real time rather than after a training block, they found that increasing sled load reduced maximum velocity and increased the force ratio (more force directed horizontally), but did not significantly change maximum force or maximum power output during the sprint itself. The decrease in the force ratio over the acceleration phase was steeper at heavier loads, meaning the athlete lost their ability to maintain horizontal force orientation more quickly.14PLOS ONE. Effect of weighted sled towing on sprinting effectiveness, power and force-velocity relationship – Section: Results Coaches can use this information to select loads that target specific deficits: lighter loads for athletes who need to maintain force application at higher speeds, heavier loads for those who need more raw horizontal force off the line.
Why Hamstring Injuries Cluster in the Acceleration Phase
Hamstring strains are among the most common injuries in sprinting sports, and they disproportionately occur during the acceleration phase rather than at top speed. A biomechanical study of the biceps femoris long head, semitendinosus, and semimembranosus muscles across an entire overground sprint acceleration found that the biceps femoris long head exhibited the largest peaks in muscle-tendon strain compared to the other two hamstring muscles at every step. Its overall strain actually decreased as the number of steps increased and the sprinter approached maximum speed. Hip flexion angle was found to be a strong predictor of the peak muscle-tendon strain, with pelvic orientation in the sagittal plane having the greatest influence.15PubMed. Biceps femoris muscle-tendon strain during an entire overground sprint acceleration: a biomechanical explanation for hamstring injuries in the acceleration phase
This aligns with the earlier point about the hamstrings’ dominant role in horizontal force production during acceleration. The same muscle that is doing the most mechanical work is also experiencing the highest strain, particularly in the early steps when the trunk is deeply flexed forward and the hip joint is in extreme flexion. As the runner rises toward an upright posture in later steps, hip flexion decreases and so does the peak strain on the biceps femoris. The practical implication for athletes and coaches is that hamstring conditioning for sprinting should account for the extreme lengthened positions that occur during the first several steps out of the blocks, not just the high-speed mechanics of top-end running.
Accelerating on a Curve
Most sprint research examines straight-line running, but many sports require acceleration on curved paths. Curving changes the demands on each leg differently. In soccer players sprinting through curves, the outside leg showed higher activation in the biceps femoris and gluteus medius, while the inside leg showed higher activation in the semitendinosus and adductor muscles. The inside leg was more affected overall by the switch from a straight sprint to a curved one.16PubMed. Curve Sprinting in Soccer: Kinematic and Neuromuscular Analysis
For youth soccer players, tighter curve radii reduced both top speed and the overall acceleration-speed profile, but the effect was not uniform across the sprint. Early acceleration was relatively preserved even on tight curves; the biggest losses showed up in late acceleration and top-speed attainment. Acceleration performance was also side-dependent, meaning athletes were measurably worse when sprinting around curves on their non-dominant side, with lower acceleration capacity and top speed on the weaker side.17PubMed. Curvilinear sprint acceleration-speed profile in youth soccer players is constrained by tighter radii and sprinting side dominance For field-sport athletes, this suggests that training curved acceleration on both sides could address an asymmetry that straight-line sprint drills never expose.
How Footwear Stiffness Affects the First Steps
Sprint spikes are not all created equal, and the stiffness of the shoe’s sole plate can influence acceleration mechanics. Research testing different levels of longitudinal bending stiffness during early acceleration found that changes in performance across shoe conditions were related to altered contact times, ground force application, and overall body orientation, rather than to changes in energy absorption at the toe joint. A stiffer sole acted as a gearing mechanism, increasing the lever arms at the toe joint and ankle, which may improve the effectiveness of horizontal force application during the push-off.18Footwear Science. Biomechanical response to altered footwear longitudinal bending stiffness in the early acceleration phase of sprinting
The finding that sole stiffness works through a gearing mechanism rather than through energy storage at the toe joint is worth noting. It means the benefit is not about the shoe “returning energy” the way a distance-running super shoe does. Instead, a stiffer plate changes the geometry of force application at the foot, potentially helping the athlete direct more of their effort horizontally. For sprinters choosing footwear, this suggests that sole stiffness should be matched to the athlete’s foot strength and preferred acceleration posture rather than simply selecting the stiffest option available.
Where You Focus Your Attention
Coaching cues during acceleration often involve telling athletes to “push the ground away” (an external focus) or “extend your hip” (an internal focus). Research on sprint performance found that both an external-focus cue and a control condition with no specific attentional instruction produced significantly faster 10-meter sprint times than an internal-focus cue. There was no significant difference between external focus and no focus instruction at all, and no differences in ground force variables across conditions.19PubMed Central. Experience level influences the effect of attentional focus on sprint performance The practical message for coaches is fairly clear: directing an athlete’s attention inward to specific body parts during the acceleration phase can slow them down, while either focusing attention externally or simply letting the athlete run without overthinking produces better results. For experienced athletes especially, less internal cueing appears to be more.