The Role of Stiffness, Part 2: Task Demands, Trade-offs and Training Progressions

The first part of this series treated stiffness as a mechanical property: a measurable resistance to deformation, expressed at tissue, joint and system levels, and modulated by loading rate, running velocity, surface and fatigue. That framing is necessary but not sufficient for practice. Knowing what stiffness is does not indicate what a particular athlete requires, because the appropriate value depends on the mechanical problem being solved. Sprinting and deceleration place demands on the same structures, yet the mechanical objectives are close to opposite, and this is where most of the confusion originates.

Sprinting: A High-Stiffness Problem

At maximal velocity, ground contact times fall to approximately 100 to 120 milliseconds, and in trained sprinters often below that. Within such a window there is very little opportunity to develop force through a large range of motion, so the mechanical solution shifts toward limiting deformation and preserving elastic return. Relatively high system stiffness serves this purpose by restricting the descent of the centre of mass, maintaining a favourable tibial angle and joint alignment through contact, and reducing the proportion of stored energy lost to non-productive deformation.

The ankle is central to this arrangement. If the ankle yields into excessive dorsiflexion during the eccentric phase, effective leg stiffness declines, contact time lengthens and the propulsive phase becomes less reactive. Longitudinal data support the practical relevance of this: improvements in maximal sprinting velocity across a training macrocycle have been associated with increases in vertical and ankle joint stiffness, while leg and knee stiffness remained largely unchanged (Nagahara et al., 2017). Dynamic ankle stiffness is also higher during sprinting than during submaximal running, which suggests that it is regulated according to task demand rather than being a fixed individual characteristic (Stefanyshyn and Nigg, 1998).

Rate of force development becomes correspondingly important, since the athlete must reach a high force output within a very short interval. This is not primarily a question of maximal strength. It reflects the capacity for rapid neuromuscular activation and effective intermuscular coordination, and it interacts with the mechanical properties of the tendon in ways that maximal force testing does not capture. In observational terms, an athlete with insufficient stiffness for the task tends to appear yielding at contact, with a visible descent of the centre of mass and a propulsive phase that lacks reactivity. An athlete with appropriate stiffness presents a shorter, more abrupt contact and a more direct transmission of force through the kinetic chain.

Deceleration and Change of Direction: A Different Mechanical Problem

In braking actions and changes of direction the objective is not to maximise the return of elastic energy but to absorb shock and redistribute load in a controlled manner. This is a substantially different task, and the mechanical profile reflects it. Horizontal decelerations are characterised by high impact peaks and high loading rates, with the greatest magnitudes occurring during the early stance phase, and the forces involved can be considerably larger than those observed during the initial steps of a maximal acceleration (Harper et al., 2022). Detailed modelling of these actions has shown external ground reaction forces in the horizontal direction reaching several times those recorded during accelerated or constant-speed running, with correspondingly high joint moments and contact forces at the ankle, knee and hip (Verheul et al., 2024).

Under these conditions, stiffness that is too high becomes counterproductive. It reduces the capacity to attenuate load, concentrates peak forces on specific structures, and tends to be associated with braking strategies that use short, shallow contacts and unfavourable penetration angles. Somewhat lower and more modulable stiffness permits a greater controlled range of motion, more effective energy dissipation, and a more even distribution of force across the limb.

The complexity increases when the deceleration precedes a change of direction. The biomechanical demands of directional change depend jointly on approach velocity and cutting angle, and faster, sharper changes elevate knee joint loading while also being the manoeuvres most closely associated with successful performance (Dos'Santos et al., 2018). This creates a genuine conflict rather than a simple prescription, and the extent to which it can be mediated appears to depend on the athlete's physical capacity, particularly the ability to produce force rapidly and to control it. Recent work suggests that deceleration strategies emphasising greater horizontally oriented braking force can improve deceleration effectiveness without a proportional increase in knee loading surrogates, which points toward technical orientation of force as a partially independent factor from magnitude alone (Lin et al., 2025; Li et al., 2025).

There is also a cumulative dimension to consider. High-intensity horizontal decelerations occur frequently in team sports, and the loading conditions they impose can produce tissue damage and neuromuscular fatigue, which in turn reduce coordinative proficiency and the capacity to dissipate braking loads skilfully. Repeated exposure to these cycles over extended periods, if not managed appropriately, has been proposed as a mechanism by which damage accumulates and contributes to the chronic presentations sometimes described in terms of mechanical fatigue failure (McBurnie et al., 2021). This suggests that deceleration exposure should be treated as a trainable and monitorable load in its own right rather than as an incidental by-product of other work, and that reducing it entirely is unlikely to be protective, since athletes then encounter these demands in competition without prior accommodation.

The practical implication is that a footballer or basketball player is not well served by being uniformly stiff. The requirement is the capacity to modulate: relatively higher during acceleration and maximal-velocity running, lower and more controlled during braking and directional change. Modulation, rather than magnitude, is the trainable quality of greatest interest in multidirectional sport.

The Performance and Tissue-Tolerance Trade-Off

Within a certain range, increased stiffness is generally associated with better performance in rapid actions. Beyond that range, the same property becomes a source of mechanical cost. Higher stiffness raises peak forces and loading rates on tendinous and bony structures, and has been linked to overuse presentations including Achilles and patellar tendinopathy and bone stress injury. Insufficient stiffness carries a different set of costs: greater energy dissipation, longer ground contact times and reduced mechanical efficiency.

The difficulty in applying this is that the two costs are not observed on the same timescale. A reduction in efficiency associated with insufficient stiffness tends to be visible immediately, in longer contacts and slower times. The cost associated with excessive stiffness accumulates over weeks or months and often becomes apparent only when a tendon or bone begins to present symptoms. Decisions made on the basis of short-term performance feedback alone will therefore tend to be biased toward increasing stiffness, and some counterweight from load monitoring and clinical observation is generally required.

Framing this as a trade-off rather than as an optimisation problem with a single solution is more useful in practice. The relevant judgement is whether an athlete's current stiffness is adequate for the mechanical demands of the sport, and whether increasing it further would exceed what that athlete's tissues can currently tolerate given their clinical history, chronological and training age, competitive calendar and accumulated exposure. There is no universally ideal value. There is a value that is appropriate for a given action, in a given context, for a given athlete, at a given point in the season.

What Modulates Stiffness

Stiffness is not a stable trait, and several factors modify it on both acute and chronic timescales.

Surface characteristics produce an immediate adjustment. Runners alter leg stiffness in response to surface stiffness such that the combined stiffness of limb and surface remains approximately constant (Ferris, Louie and Farley, 1998). Part of this response appears to be automatic, which matters when interpreting data collected across different training environments.

Fatigue produces a fairly consistent effect in the opposite direction. Leg stiffness tends to decrease with accumulated running-induced fatigue, accompanied by longer contact times, shorter flight times and increased step-to-step variability (García-Pinillos et al., 2019; Darch et al., 2022). Recovery of stretch-shortening cycle function follows a bimodal pattern, with an initial decline, partial recovery within one to two hours, and a secondary decline coinciding with peak muscle soreness (Nicol, Avela and Komi, 2006). This has direct implications for how plyometric sessions are positioned within a microcycle.

Movement velocity increases effective stiffness through the viscoelastic behaviour of the tissues, which is one reason why an athlete who tolerates a given exercise at moderate speed may not tolerate the same exercise performed explosively. Chronic tendon loading state also matters: a tendon in a reactive phase behaves differently from a healthy one and responds differently to the same stimulus. Age and clinical history influence both tendon composition and adaptive capacity, and generally warrant a more conservative progression.

Progressions: Isometrics as a Foundation

Isometric work provides a controlled entry point for developing the capacity to generate and sustain tension. Three broad categories are useful to distinguish.

Long-duration isometrics, in the region of 30 to 45 seconds, are principally used to improve tolerance to sustained load and, in the presence of tendinopathy, to reduce pain. A single bout of isometric contractions has been shown to reduce patellar tendon pain immediately and for at least 45 minutes afterwards, accompanied by a reduction in cortical inhibition and an increase in maximal voluntary isometric contraction, whereas isotonic contractions produced smaller and less durable effects (Rio et al., 2015). Heavy isometrics of shorter duration, in the region of three to five seconds, are directed at increasing the capacity to generate high force in specific joint positions. Reactive or explosive isometrics emphasise the rate at which force is developed rather than its peak magnitude.

From the perspective of tendon adaptation, the loading characteristics matter more than the label. Mechanical loading produces moderate increases in tendon stiffness, mediated primarily by changes in tendon material properties rather than cross-sectional area, and protocols involving higher localised tendon strain produce greater adaptation than low-strain protocols (Lazarczuk et al., 2022). Contraction duration also appears relevant, with longer sustained contractions producing greater changes in tendon elasticity than shorter ones at comparable intensity (Kubo, Kanehisa and Fukunaga, 2001). The choice between categories therefore depends on the athlete's current state: management of a symptomatic tendon typically begins with longer-duration work, whereas performance development emphasises heavy and reactive variants.

Progressions: Plyometrics

Plyometric work connects the capacity developed through isometrics to system stiffness under dynamic conditions. A conventional progression moves through three stages.

Extensive plyometrics involves low to moderate intensity jumping at higher volumes, with moderate contact times, and emphasis on coordination, rhythm and the quality of the movement pattern. The purpose at this stage is to build tolerance and control rather than to maximise output. Intensive plyometrics involves higher-intensity jumps and bounds with shorter contact times, placing greater demand on stiffness and rate of force development; volumes decrease while the load per contact increases. The shock method, using drop jumps from meaningful heights with very short contact times, represents the highest demand on both tendinous and neural structures and is correspondingly the most costly stage to recover from.

The sequence is not arbitrary. It develops the capacity to absorb and manage load before increasing the demand for stiffness and rapid stretch-shortening cycle transition, and exposes the athlete to near-maximal stimuli only once the system has demonstrated it can accommodate them. The most common errors in practice involve introducing intensive or shock work before an adequate base of isometric and extensive plyometric work exists, and failing to adjust volume and frequency in relation to the competitive calendar and current tendon status.

Transfer between these qualities is not automatic. A systematic review of training interventions targeting deceleration performance in team-sport athletes found little consistent effect on deceleration-specific outcomes, with substantial methodological heterogeneity across studies (Marvin et al., 2024). This does not argue against structured progression, but it does suggest caution about assuming that improvements in one quality will appear in another without task-specific exposure.

Monitoring

Direct measurement of stiffness is available in some environments but not most. Several accessible proxies provide reasonable insight into how the system is responding. Ground contact time during jumping and running tasks reflects the interaction between stiffness and rate of force development. The reactive strength index, calculated from jump height and contact time, captures fast stretch-shortening cycle function and has been used both as a monitoring variable and as a means of prescribing plyometric intensity (Flanagan and Comyns, 2008). Drop jump reactive strength index has also been shown to relate moderately to maximal horizontal deceleration ability, with stronger associations to the early deceleration sub-phase, which suggests it carries information relevant beyond vertical tasks (Harper et al., 2021).

Alongside these, the qualitative appearance of contacts, the athlete's own perception of stiffness and reactivity, and the behaviour of any symptomatic tendon all contribute to the interpretation. A change in any single variable is rarely conclusive; a consistent pattern across several is more informative.

A Decision Framework

The topics covered in both parts of this series can be reduced to a sequence of questions rather than a prescription. The first concerns the task: what are the principal mechanical demands of the sport and of the athlete's position within it, and what balance between elastic return and load absorption do they require? The second concerns the athlete: what does their clinical history indicate, what does the quality of their ground contacts suggest, and what do contact times and reactive strength values show? The third concerns the objective: whether the aim is to increase stiffness, to maintain it, or to improve the capacity to modulate it across different actions. The fourth concerns the means: which progression is appropriate, and how should volume, frequency and intensity be adjusted to the current point in the season and the current state of the tissues.

The end point of this reasoning is not an athlete who is as stiff as possible. It is an athlete who can regulate stiffness according to the demands of the action, the context in which it occurs and the moment of the season, in a way that supports performance while remaining within the limits of what their tissues can tolerate.

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Frequently Asked Questions

Q: Should a team-sport athlete train for higher or lower stiffness?

Neither in isolation. Multidirectional sports require relatively high stiffness during acceleration and high-speed running and lower, more controlled stiffness during braking and directional change. The trainable quality of interest is the capacity to modulate between these states rather than the absolute value in either direction. Programming that develops only one end of the range tends to leave the athlete less capable in the other.

Q: How long does it take to change stiffness through training?

It depends on which level is being addressed. Neuromuscular and coordinative changes affecting system stiffness can appear within weeks. Adaptation of tendon material properties develops more slowly, generally over months, and the magnitude of change is moderate. Acute fluctuations driven by fatigue and surface can be larger than the chronic adaptation, which is one reason why single measurements are difficult to interpret without context.

Q: When is it appropriate to introduce the shock method?

Only after an athlete has demonstrated tolerance to progressively more demanding isometric and plyometric work, and when the competitive calendar allows adequate recovery. Drop jumps from meaningful heights impose the highest tendinous and neural cost of the common plyometric methods, and the recovery pattern following intensive stretch-shortening cycle work is not linear. Introducing this stage prematurely, or scheduling it in congested periods, tends to produce accumulated load without the intended adaptation.

Q: Can isometric work be used with a symptomatic tendon?

Yes, and it is often where management begins. Longer-duration isometric contractions have been shown to reduce patellar tendon pain immediately and for a period afterwards, with an accompanying increase in maximal voluntary contraction, which allows loading to continue during periods when other modalities are poorly tolerated. Prescription should still be individualised, and the response monitored rather than assumed.

Q: What can be monitored without force plates?

Ground contact time, the reactive strength index derived from jump height and contact time, the qualitative appearance of ground contacts, the athlete's perception of reactivity, and the behaviour of any symptomatic tendon. None of these measures stiffness directly, but taken together over time they provide a reasonable indication of whether the system is adapting favourably or accumulating load faster than it can accommodate.

References

• Darch, L., Chalmers, S., Wiltshire, J., Causby, R. and Arnold, J. (2022) 'Running-induced fatigue and impact loading in runners: a systematic review and meta-analysis', Journal of Sports Sciences, 40(12), pp. 1338–1352.

• Dos'Santos, T., Thomas, C., Comfort, P. and Jones, P.A. (2018) 'The effect of angle and velocity on change of direction biomechanics: an angle-velocity trade-off', Sports Medicine, 48(10), pp. 2235–2253.

• Ferris, D.P., Louie, M. and Farley, C.T. (1998) 'Running in the real world: adjusting leg stiffness for different surfaces', Proceedings of the Royal Society B: Biological Sciences, 265(1400), pp. 989–994.

• Flanagan, E.P. and Comyns, T.M. (2008) 'The use of contact time and the reactive strength index to optimize fast stretch-shortening cycle training', Strength and Conditioning Journal, 30(5), pp. 32–38.

• García-Pinillos, F., Cartón-Llorente, A., Jaén-Carrillo, D., Delgado-Floody, P., Carrasco-Alarcón, V. and Roche-Seruendo, L.E. (2019) 'Does fatigue alter step characteristics and stiffness during running?', Gait & Posture, 76, pp. 259–263.

• Harper, D.J., Cohen, D.D., Carling, C. and Kiely, J. (2021) 'Drop jump neuromuscular performance qualities associated with maximal horizontal deceleration ability in team sport athletes', European Journal of Sport Science, 21(10), pp. 1390–1398.

• Harper, D.J., McBurnie, A.J., Santos, T.D., Eriksrud, O., Evans, M., Cohen, D.D., Rhodes, D., Carling, C. and Kiely, J. (2022) 'Biomechanical and neuromuscular performance requirements of horizontal deceleration: a review with implications for random intermittent multi-directional sports', Sports Medicine, 52(10), pp. 2321–2354.

• Kubo, K., Kanehisa, H. and Fukunaga, T. (2001) 'Effects of different duration isometric contractions on tendon elasticity in human quadriceps muscles', The Journal of Physiology, 536(2), pp. 649–655.

• Lazarczuk, S.L., Maniar, N., Opar, D.A., Duhig, S.J., Shield, A., Barrett, R.S. and Bourne, M.N. (2022) 'Mechanical, material and morphological adaptations of healthy lower limb tendons to mechanical loading: a systematic review and meta-analysis', Sports Medicine, 52(10), pp. 2405–2429.

• Li, W., Lin, J., Dos'Santos, T. and Turner, A. (2025) 'Mechanical determinants of superior horizontal deceleration performance in multidirectional sportspersons', Journal of Strength and Conditioning Research, 39(7), pp. 753–759.

• Lin, J., Li, W., Dos'Santos, T. and Turner, A. (2025) 'Is there a performance-injury conflict between maximum horizontal deceleration and surrogates of noncontact anterior cruciate ligament injury?', European Journal of Sport Science, 25(3), e12268.

• Marvin, L., Cronin, J., Hébert-Losier, K. and Uthoff, A. (2024) 'Training interventions for improved deceleration ability in adult team-based field sports athletes', International Journal of Strength and Conditioning, 4(1).

• McBurnie, A.J., Harper, D.J., Jones, P.A. and Dos'Santos, T. (2021) 'Deceleration training in team sports: another potential "vaccine" for sports-related injury?', Sports Medicine, 52(1), pp. 1–12.

• Nagahara, R., Naito, H., Miyashiro, K., Morin, J.B. and Zushi, K. (2017) 'Development of maximal speed sprinting performance with changes in vertical, leg and joint stiffness', The Journal of Sports Medicine and Physical Fitness, 57(12), pp. 1572–1578.

• Nicol, C., Avela, J. and Komi, P.V. (2006) 'The stretch-shortening cycle: a model to study naturally occurring neuromuscular fatigue', Sports Medicine, 36(11), pp. 977–999.

• Rio, E., Kidgell, D., Purdam, C., Gaida, J., Moseley, G.L., Pearce, A.J. and Cook, J. (2015) 'Isometric exercise induces analgesia and reduces inhibition in patellar tendinopathy', British Journal of Sports Medicine, 49(19), pp. 1277–1283.

• Stefanyshyn, D.J. and Nigg, B.M. (1998) 'Dynamic angular stiffness of the ankle joint during running and sprinting', Journal of Applied Biomechanics, 14(3), pp. 292–299.

• Verheul, J., Harper, D.J., Robinson, M.A. and Vanrenterghem, J. (2024) 'Forces experienced at different levels of the musculoskeletal system during horizontal decelerations', Journal of Sports Sciences, 42(9), pp. 811–822.

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).