Tendon Biomechanics in Sprinting: Properties, Stiffness and Theoretical Models
Tendon stiffness is often presented in sprint training as a quality to be increased, on the assumption that a stiffer tendon returns more energy, shortens ground contact and therefore produces faster running. The assumption has a mechanical basis, and it has shaped a good part of the plyometric, isometric and heavy resistance work prescribed to sprinters. The difficulty is that studies comparing sprinters with other populations do not show a consistent difference in tendon stiffness, and the studies relating tendon stiffness to sprint times report correlations that are small, inconsistent in direction or limited to particular tendons. Part of the inconsistency is methodological, since stiffness is usually measured during slow isometric ramp contractions, within a given force range, and in a structure that often includes the aponeurosis as well as the free tendon.
A tendon in a sprinting athlete is also part of a muscle-tendon unit, attached to a lever system of a particular geometry and controlled by a nervous system that adjusts activation from one step to the next. What the tendon does at any moment depends on its own properties, on the length and velocity at which the muscle fascicles work, on the moment arm through which force is transmitted, and on how quickly the load is applied. A single stiffness value, taken without these factors, describes only part of the picture, and training choices based on it alone may be incomplete. This article looks at tendon biomechanics in sprinting, from the structure of the tissue to the models used to explain its function. Several of the conclusions are still under discussion and should not be considered settled.
Tendon Structure and Mechanical Properties
Tendons connect muscle to bone and transmit the force produced by muscle fibres to the skeleton. They consist largely of type I collagen organised in a hierarchy, from fibrils to fibres to fascicles, aligned with the main direction of force and embedded in an extracellular matrix maintained by resident cells. The tissue remodels in response to loading, and it appears to be more dynamic than was assumed for much of the twentieth century (Roberts and Azizi, 2011). Mechanically, this organisation produces a material that is strong in tension, relatively compliant at low loads and viscoelastic, so that its response depends on how fast the load is applied.
Two levels of description are used, and the distinction matters when studies are compared. Structural properties refer to the tendon as a whole and come from the relationship between force and elongation. Stiffness, in newtons per millimetre, is the slope of that relationship and depends on both the material and the dimensions, since a tendon with a larger cross-sectional area is stiffer than a thinner one of the same material, and a longer tendon is more compliant than a shorter one. Material properties refer to the tissue itself and come from stress (force per unit area) and strain (elongation relative to resting length), whose slope is the Young's modulus. Two tendons may therefore have the same stiffness for different reasons, one by being larger and the other by being made of a stiffer material, and the two cases may respond differently to the same training.
The force-elongation curve has a toe region at low force, in which collagen fibres align and the tendon elongates easily, followed by an approximately linear region of relatively constant stiffness and, eventually, damage and failure. Hysteresis is the proportion of the energy stored during loading that is not returned during unloading. In vivo values for the human Achilles tendon during one-legged hopping have been reported with an interquartile range of 145 to 231 N/mm for stiffness, 0.67 to 1.07 GPa for the elastic modulus and 17 to 35% for hysteresis, with an average peak strain of 8.3% (Lichtwark and Wilson, 2005). During treadmill running at 2.5 and 3.5 m/s, maximum Achilles tendon strain was lower, between 4.0 and 4.9% (Kharazi et al., 2021).
These values depend on the method. Most studies in sprinters use ultrasonography during isometric ramp contractions on a dynamometer and estimate tendon force from joint torque and moment arm, so the result describes tendon and aponeurosis together, at slow loading rates, which limits comparison between studies.
The Role of Tendons in Human Movement and Performance
The best known function of a tendon is as a spring in series with the muscle, stretching and storing elastic strain energy when force is applied and releasing it when force decreases. A review of biological springs describes four functions that this action can serve: conservation of metabolic energy, amplification of muscle power output, attenuation of power input during deceleration, and rapid mechanical feedback that may contribute to stability. The authors propose that the direction and timing of energy flow between the body, the elastic element and the muscle determine which function is served in a given movement (Roberts and Azizi, 2011).
A second contribution concerns the conditions in which the muscle operates. When part of the length change of the muscle-tendon unit is taken up by the tendon, the fascicles shorten less and more slowly, and since muscle force depends on contraction velocity, a lower shortening velocity at peak force allows more force for a given activation. In the human medial gastrocnemius, the change from walking to running shifts fascicle shortening velocity at peak force to much slower values, which has been associated with a large increase in peak force and power output (Farris and Sawicki, 2012). Modelling suggests that fascicle length and tendon compliance may be matched to maximise efficiency in a given gait (Lichtwark and Wilson, 2008).
The magnitude of the energy contribution has been estimated in several tasks. In one-legged hopping, elastic recoil of the Achilles tendon returned an average of 38 J, about 16% of the total mechanical work of the hop (Lichtwark and Wilson, 2005). During treadmill running, tendon energy recoil in propulsion was between 7.8 and 11.3 J (Kharazi et al., 2021). The amount returned also depends on the moment arm, and shorter Achilles tendon moment arms have been associated with greater elastic energy storage at running and sprinting speeds (Foster et al., 2021).
Performance outcomes are harder to attribute to the tendon alone, because tendon properties change together with muscle strength. A 14-week resistance programme for the plantar flexors that increased tendon-aponeurosis stiffness by about 16% and strength by about 7% was followed by a reduction of about 4% in the energy cost of running at 3.0 and 3.5 m/s, without changes in tendon elongation during stance (Albracht and Arampatzis, 2013). The speeds were submaximal, so transfer to sprinting is uncertain. The tendon is also where high forces are concentrated, which links these mechanics to tendon injury. An example is the hamstring tendon during terminal swing, when peak force and peak musculotendon length occur together (Schache et al., 2010).
Tendon Biomechanics During Sprint Running
Musculoskeletal modelling combined with gait data indicates that, as running speed increased from jogging at about 2 m/s to sprinting at 8 m/s or more, the contribution of tendon elastic strain energy to the positive work of the muscle-tendon unit rose from 53 to 74% in the soleus and from 62 to 75% in the gastrocnemius (Lai et al., 2014). Greater activation and relatively isometric fibre behaviour made this possible, so that most of the length change of the unit was taken up by stretch and recoil of the tendon. The authors concluded that, towards maximum speed, the plantar flexors keep favouring storage and return of tendon elastic energy over fibre work.
Acceleration differs from steady running. In a maximal sprint, fascicle work decreased across successive contacts and was greater in the first contact than in steady running at 5 m/s, while differences in tendon strain energy were negligible, so that tendon elastic energy appeared as relevant at the start as at the end (Lai et al., 2016). In the first stance after a block start, adult sprinters showed more muscle-tendon unit shortening and higher shortening velocities in the plantar flexors and rectus femoris than young sprinters, a pattern considered favourable for elastic storage and release, without a difference in ankle joint stiffness (Aeles et al., 2018).
Contact times at maximum velocity are approximately 100 to 120 ms, within the range conventionally considered a fast stretch-shortening cycle (Komi, 2000). In sprinters running at 70 to 100% of maximum speed, average ankle joint stiffness stayed near 7 N·m/deg while knee joint stiffness increased from 17 to 24 N·m/deg, which was interpreted as the limb's spring-like behaviour being regulated mainly at the knee, with the ankle constrained by tendon stiffness in the triceps surae (Kuitunen, Komi and Kyröläinen, 2002). Ankle stiffness is higher in sprinting than in submaximal running (Stefanyshyn and Nigg, 1998), and improvements in maximal speed over a macrocycle have been accompanied by increases in vertical and ankle joint stiffness (Nagahara et al., 2017), which suggests that it varies with task and training.
One limitation should be stated. Direct in vivo measurements of tendon strain at maximum sprinting speed appear to be scarce. At 2.5 and 3.5 m/s the maximum strain measured with ultrasound was considered insufficient as a stimulus for tendon adaptation (Kharazi et al., 2021), whereas an electromyography-informed model in trained runners estimated free tendon strain that rose with speed, reaching a peak of 13.7% at 5 m/s (Devaprakash et al., 2022). The two studies differ in method and tendon segment, and values at sprinting speeds remain extrapolations.
Tendon Stiffness and Compliance
Stiffness is the ratio between the change in force and the resulting change in length, and compliance is its inverse, so a more compliant tendon elongates more for a given force. The terms are applied at several levels, including the tendon, the muscle-tendon unit, individual joints, the leg treated as a spring and the whole body. These quantities are related but not interchangeable, and their reliability differs between measures (Brughelli and Cronin, 2008; Maloney and Fletcher, 2021). When a sprinter is described as stiff, the term may refer to different structures, and what it means for training depends on which one we refer to.
For a linear spring, the energy stored at a given force is inversely proportional to stiffness, so a more compliant tendon stores more energy for the same force, whereas for a given elongation a stiffer tendon stores more. Which case applies depends on whether the muscle controls force or length at that moment, and in a stretch-shortening action both change. Stiffness also influences how quickly force reaches the skeleton and how much of the length change the fibres must accommodate. A stiffer tendon transmits force more rapidly, which may help when contact is short, while a more compliant one stretches more and allows the fibres to work closer to isometric conditions. No value is therefore expected to be advantageous in every task.
Modelling supports this. In a Hill-type model of the medial gastrocnemius, the series elastic stiffness that maximised efficiency was equal to or slightly higher than that of the average Achilles tendon in running, and the range of values giving high efficiency was much narrower in running than in walking (Lichtwark and Wilson, 2007). A later analysis found near-optimal efficiency across fascicle lengths of 45 to 70 mm and stiffness values of 150 to 500 N/mm, although efficient walking needed a more compliant tendon than running (Lichtwark and Wilson, 2008). Both studies used speeds up to 3.9 m/s and took efficiency as the criterion. It has not been established whether their conclusions hold for sprinting, where short contact times and power output matter more than efficiency. A predictive simulation of maximum-speed sprinting offers a complementary perspective, since a 10% increase in tendon compliance raised the predicted maximum speed by 0.3%, compared with 5.9% for a 10% increase in muscle strength and 4.7% for muscle fascicle length (Lin and Pandy, 2022). Within that model, sprint speed was therefore much less sensitive to tendon compliance than to muscle properties.
Stiffness is also related to the capacity to produce force. In 66 men, tendon stiffness correlated strongly with the maximal tendon force reached (r = 0.817), which led the authors to suggest that stronger individuals may widen the margin of tolerated loading through a stiffer tendon and aponeurosis (Arampatzis et al., 2007b). Higher stiffness in a strong athlete may therefore reflect adaptation to higher force and does not necessarily show that stiffness explains performance. The moment arm also matters, since a shorter arm requires more tendon force for a given ankle moment, while a longer one improves the mechanical advantage of the triceps surae and increases fibre shortening velocity (Foster et al., 2021; Lee and Piazza, 2009).
Differences Between Sprinters and Non-Sprinters
The comparison has been examined for more than twenty years, and the results are less consistent than stiffness-based reasoning would predict. In the earliest study of the lower limbs, the compliance of the medial gastrocnemius tendon structures at high force did not differ between ten sprinters and fourteen controls, whereas the vastus lateralis tendon structures of sprinters were more compliant at low forces, and within sprinters the vastus lateralis compliance was negatively correlated with 100-m time (Kubo et al., 2000). Fast sprinters likewise showed greater vastus lateralis tendon and aponeurosis elongation at a given force than slower sprinters, with similar gastrocnemius values, and maximal elongation correlated with 100-m time (r = −0.567) (Stafilidis and Arampatzis, 2007). Later work reported lower knee extensor tendon stiffness in sprinters, with plantar flexor stiffness similar to that of untrained men (Kubo et al., 2011). Two further studies found no difference in plantar flexor tendon stiffness or hysteresis, measured with ramp and ballistic contractions, although active stiffness of the medial gastrocnemius muscle at high angular velocities (500 and 600 deg/s) was greater in sprinters (Kubo et al., 2017; Kubo et al., 2020).
Other groups reported differences. In a comparison of 28 sprinters, 28 endurance runners and 10 men not active in sport, only sprinters had higher normalised stiffness of the triceps surae tendon and aponeurosis, and the authors concluded that properties do not show a graded response to activity intensity and that strain amplitude or frequency may need to exceed a threshold (Arampatzis et al., 2007b). Young international sprinters had greater triceps surae strength and tendon stiffness than older sprinters and young recreationally active adults, whereas older sprinters did not differ from controls (Epro et al., 2021). Among sprinters, higher passive plantar flexor stiffness was associated with faster 100-m times, with a small correlation (r = −0.334) in 50 athletes (Takahashi et al., 2018). In 15 prepubescent sprinters, higher plantar flexor tendon stiffness was also associated with faster 100-m times (r = −0.58) (Enomoto, Oda and Kaga, 2019). In 15 sprinters, however, 30-m time correlated positively with Achilles tendon stiffness (r = 0.50, 95% confidence interval 0.01 to 0.81), meaning that stiffer tendons went with slower times in that small sample (Li et al., 2025).
Comparisons with other athletes add context. Normalised tendon stiffness differed only in ski jumpers, who had higher values than controls (Wiesinger et al., 2016). Patellar tendon stiffness and modulus were similar across sprinters, jumpers, gymnasts and karate athletes, while tendon stress differed and correlated with knee extension torque and jump power (Dranevicius et al., 2026). Morphology may differ more than stiffness, since magnetic resonance imaging of 40 sprinters, 40 endurance runners and 40 untrained men showed larger relative Achilles tendon cross-sectional area in endurance runners and larger patellar tendon area in sprinters (Ueno et al., 2018). Achilles tendon length and cross-sectional area did not correlate with 100-m time in 48 sprinters (Tomita et al., 2021). In 18 sprinters, however, muscle-tendon parameters, including Achilles tendon dimensions, correlated more strongly with peak power in a 20-m sprint (R = 0.81 to 0.92) than with average velocity (R = 0.41 to 0.61) (Monte and Zamparo, 2019). Sprinters also showed greater early explosive plantar flexor torque and more synchronous gastrocnemius activation than active controls, without differences in muscle-tendon unit stiffness, which suggests a neural contribution (Crotty, Furlong and Harrison, 2024).
Foot and ankle geometry gives a more consistent but not unanimous picture. Collegiate sprinters had Achilles tendon moment arms 25% smaller than non-sprinters, fascicles 11% longer and longer toes, and simulation suggested these favour forward impulse (Lee and Piazza, 2009), a finding confirmed by magnetic resonance imaging (Baxter et al., 2012). In contrast, sprint performance was unrelated to moment arm, foot geometry or fascicle arrangement among 18 sprinters with personal bests between 10.30 and 10.70 s (Karamanidis et al., 2011), and a three-dimensional study reported longer moment arms in sprint runners than in distance runners (51.8 versus 48.7 mm) without a correlation with personal best (Kovács et al., 2025).
There are several possible reasons for these differences between studies. Cross-sectional designs cannot separate the effect of training from selection, samples are small, the performance range within sprinters is narrow, and methods differ in the tendon examined, the force range, the normalisation and the loading rate. Isometric ramps at slow rates may also differ from loading in sprinting, as differences in muscle stiffness appeared only at high angular velocities (Kubo et al., 2020). Overall, tendon stiffness alone does not consistently distinguish sprinters, and the characteristics associated with performance may depend as much on muscle properties, activation and limb geometry, and on how these combine.
The Models Used to Explain Tendon Behaviour
Several models are used to interpret tendon function in sprinting, and the energy-based framework of Roberts and Azizi (2011) provides a way to organise them. In this framework the role of an elastic structure depends on the direction and timing of energy flow between the body, the spring and the muscle. When the energy of the body loads the spring and the spring returns it, the role is energy conservation. When the muscle loads the spring slowly and the spring releases the energy over a shorter time, the role is power amplification. When the spring takes up energy during deceleration, the muscle can dissipate it over a longer period, which is power attenuation. The same tendon may therefore serve different roles in different phases of a movement, and in sprinting the stance phases at maximal velocity and the first contacts of an acceleration may call for different interpretations. The models below describe different aspects of this system.
The elastic energy storage and return model, which corresponds to the energy conservation role, treats the tendon as a spring stretched in early stance that recoils in propulsion, reducing fibre work. It is supported by the rising share of muscle-tendon unit work attributable to tendon strain energy with speed (Lai et al., 2014) and in the first contact of a sprint (Lai et al., 2016). Its limits are that the energy values come from models, that hysteresis dissipates part of them, that in distance runners the estimated energy cost of the muscle exceeded the energy returned by the tendon (Fletcher and MacIntosh, 2015), and that it does not indicate which stiffness would be optimal.
The muscle-tendon decoupling model focuses on the consequences of tendon stretch for the muscle. Relatively isometric fibres at high speeds (Lai et al., 2014) and slower fascicle velocities at peak force in running than in walking (Farris and Sawicki, 2012) suggest that a main contribution of the tendon is to keep the muscle in a favourable region of its force-velocity and force-length relationships. It is consistent with the efficiency models (Lichtwark and Wilson, 2007, 2008) but is mainly supported for walking and running at moderate speeds.
The power amplification model, the second role in the framework, proposes that energy stored over a longer time and released over a shorter one raises power above what the muscle could produce directly (Roberts and Azizi, 2011). The shortening pattern after the block start (Aeles et al., 2018) and the greater fascicle work in the first contact (Lai et al., 2016) are compatible with such a role in acceleration, although in steady running the evidence points mainly to energy conservation and direct demonstration in sprinting is lacking.
The optimal stiffness, or tuning, model proposes that tendon stiffness is matched to muscle properties and to the task, with a range of near-optimal values that is narrower in running than in walking (Lichtwark and Wilson, 2007, 2008). A gain in running economy after stiffness and strength increased (Albracht and Arampatzis, 2013) is compatible with starting values below the optimum in some individuals, while the lack of a consistent difference between sprinters and other groups may indicate that values vary little around a suitable range. An 18% increase in stiffness after isometric training reduced tendon recoil during running by about 30% while fascicle shortening was preserved, which the authors considered inconsistent with simple in-series models (Werkhausen et al., 2019). An optimum for sprint speed has not been derived, and the optimal level of stiffness for running performance remains to be identified (Masson, Millet and Kerhervé, 2026).
The load-tolerance model proposes that tendon properties follow the demands placed on the tissue. Stiffness relates to maximal muscle force (Arampatzis et al., 2007b), and adaptation depends on a strain stimulus above a threshold, as the leg exercised at high Achilles tendon strain (4.55%) showed higher stiffness and modulus after 14 weeks while the leg exercised at low strain (2.85%) did not (Arampatzis et al., 2007a). Longer strain duration per contraction produced a greater response than higher frequency (Arampatzis et al., 2010), and meta-analytic evidence indicates larger effects in interventions of twelve weeks or more (Bohm et al., 2015). The model may explain why sprinters' tendons often resemble those of untrained people when habitual strain stays below the threshold.
A geometric model completes the set, in which moment arm and foot proportions determine how muscle and tendon force relate to joint moment and how fast the fibres must shorten (Lee and Piazza, 2009; Foster et al., 2021). The inconsistent evidence in sprinters (Karamanidis et al., 2011; Kovács et al., 2025) suggests that geometry is not an independent determinant and that its effect depends on the muscle acting through it.
These models are not mutually exclusive. The energy model describes what the tendon does at the level of work, the decoupling model what it does for the muscle, the tuning model which properties would suit those functions, the load-tolerance model how they arise, and the geometric model how limb structure scales them. A sprinter's tendon is likely to be described by several at once.
Implications for Assessment and Training
Most of this evidence comes from cross-sectional studies, small samples and submaximal tasks, so the implications are tentative. For assessment, a stiffness value reflects the level, force range, loading rate and normalisation used, so combining tendon measures with muscle strength, rate of force development, contact time and reactive strength may describe the athlete better than any one alone.
For training, higher stiffness is not consistently associated with faster sprinting, and modelling suggests a range of suitable values. The aim may therefore be described as a tendon able to tolerate the forces the muscles generate, with properties appropriate to the task. In some athletes this may mean increasing stiffness, in others maintaining it. Intervention data indicate that adaptation depends on strain above habitual values and on loading duration per contraction (Arampatzis et al., 2007a; Arampatzis et al., 2010), with larger effects in programmes of twelve weeks or more (Bohm et al., 2015; Lazarczuk et al., 2022). This is consistent with combining heavy and sustained loading with reactive and plyometric work that exposes the tendon to the loading rates of sprinting. Because the tendon strain reached in sprint training is uncertain, monitoring the individual response may be more informative than a single stiffness measurement.
Conclusion
Tendons contribute to sprinting by storing and returning elastic energy, by modifying the conditions in which muscle fibres operate and, in some phases, by supporting power production. Modelling estimates suggest that the share of plantar flexor work attributable to tendon strain energy rises towards sprinting speeds and remains high in the first contacts of a maximal sprint.
Comparisons between sprinters and non-sprinters do not show a consistent difference in tendon stiffness, and the variation between studies is compatible with differences in methods, populations and loading conditions. Differences in muscle properties, neural activation, limb geometry and muscle-tendon behaviour at high loading rates appear to account for a substantial part of what distinguishes sprinters. The proposed models address different questions and are best seen as complementary. Tendon stiffness is therefore more useful as one of several variables describing the muscle-tendon unit than as a target in itself. Direct measurements of tendon behaviour at maximum speed and longitudinal studies in sprinters are the main gaps, and they are likely to refine several interpretations presented here.
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Frequently Asked Questions
Q: Is a stiffer Achilles tendon associated with faster sprinting?
The evidence does not show a consistent association. Some studies report higher tendon stiffness in sprinters than in endurance runners or non-athletes (Arampatzis et al., 2007b; Epro et al., 2021), while others find no difference in plantar flexor tendon stiffness (Kubo et al., 2011; Kubo et al., 2017; Kubo et al., 2020). Within sprinters, correlations with sprint time are small and differ in direction (Takahashi et al., 2018; Li et al., 2025). Methods and loading rates differ between studies, and modelling suggests that a range of values may suit running and that sprint speed is relatively insensitive to tendon compliance (Lin and Pandy, 2022), so a higher value is not necessarily more favourable.
Q: What is the difference between tendon stiffness and tendon compliance?
Stiffness is the ratio between the change in force and the change in length, in N/mm, and compliance is its inverse, so a more compliant tendon elongates more for a given force. Stiffness depends on the material and on the dimensions of the tendon, whereas the Young's modulus describes the material after normalising for size. The terms are also applied to the muscle-tendon unit, the joint, the leg and the whole body, which are not interchangeable, so a value is best interpreted with its level of measurement, force range and loading rate.
Q: How much do tendons contribute to energy return during sprinting?
Modelling estimates indicate that the share of positive muscle-tendon unit work attributable to tendon strain energy rose from 53 to 74% in the soleus and from 62 to 75% in the gastrocnemius between jogging and sprinting at 8 m/s or more (Lai et al., 2014), and was also high in the first contact of a maximal sprint (Lai et al., 2016). These values depend on model assumptions, and direct in vivo measurements at maximum speed appear to be limited.
Q: Can tendon stiffness be modified by training?
Mechanical loading can increase tendon stiffness, with the response depending on loading magnitude and on strain duration per contraction. In a 14-week study only the leg exercised at high Achilles tendon strain (4.55%) showed higher stiffness and modulus (Arampatzis et al., 2007a), and effects are larger in interventions of twelve weeks or more (Bohm et al., 2015). Stiffness changes together with strength, and a range of values may suit running, so the aim is better described as matching tendon and muscle capacity to the demands of the sport.
Q: Does the Achilles tendon moment arm influence sprint performance?
The evidence is mixed. Sprinters were reported to have shorter moment arms and longer forefoot bones than non-sprinters (Lee and Piazza, 2009; Baxter et al., 2012), and shorter moment arms are associated with greater tendon stress and stored energy (Foster et al., 2021). Within groups of sprinters, however, moment arm did not correlate with performance (Karamanidis et al., 2011), and a recent study found longer moment arms in sprint runners than in distance runners (Kovács et al., 2025). Its effect may depend on muscle size and strength.
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Antonio Robustelli is the founder of Omniathlete. He is an international high performance consultant and sought-after speaker in the area of Sport Science and Sports Medicine, working all over the world with individual athletes (including participation in the last 5 Olympics) as well as professional teams in soccer, basketball, rugby, baseball since 24 years. Currently serving as Faculty Member and Programme Leader at the National Institute of Sports in India (SAI-NSNIS).
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