Federkraft – Games
Federkraft
The tendon as an energy store – hit the jump rhythm. With a training lab and physio knowledge.
Metabolic reserve
Federkraft
The tendon stores energy like a spring when you land and returns part of it on push-off. It doesn't produce energy – it recycles it and so saves muscular work. Your first jump is a squat jump from a standing start – from then on, your timing is what counts.
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Training lab
Set how well each property of your muscle-tendon system is developed – from 0 (untrained) to 100 (highly trained) – then feel what changes in the game. Four properties can be trained specifically; one is anatomically fixed.
What it is: how little the tendon lengthens under load (stiffness = force ÷ elongation). A stiff tendon transfers muscle force more directly and snaps back faster.Trained by: heavy, slow strength training and isometric holds (over weeks to months).In the game: the recoil ring grows faster, and a clean hit returns more energy – less is lost as heat (lower “hysteresis”).
What it is: how quickly the muscle builds force from rest – the rise in force per unit time (“rate of force development”, RFD). Not how much force at most, but how fast.Trained by: explosive strength and jump training.In the game: mainly determines the squat jump from standing – purely concentric, no spring. More explosive strength = higher first jump.
What it is: how well you use the brief landing phase to release the stored energy – an interplay of muscle pre-tension, stretch reflex and coordination, recognisable by a very short ground-contact time (the stretch-shortening cycle).Trained by: plyometrics, i.e. jump training.In the game: widens the time window at the ring – the perfect moment becomes easier to hit.
What it is: how much load the tendon tolerates before it is overloaded. It depends mainly on the cross-sectional area – the thickness of the tendon, i.e. the number of collagen fibres running in parallel – and on collagen quality.Trained by: slowly progressed loading over weeks to months (tendons adapt considerably more slowly than muscles).In the game: the metabolic reserve lasts longer, and missteps cost less.
What it is: the lever arm with which the Achilles tendon pulls on the heel bone – determined by the shape of the calcaneus. It sets how much load and how much stored energy the tendon takes up with each step at all.Why fixed: an anatomical given, not changeable by training. A shorter heel (smaller lever arm) loads the tendon more and stores more elastic energy.
Rehab note: With a painful tendon you start gently with isometric holds – this builds stiffness without strongly irritating it – and only later progress via heavy, slow strength training to jumps (plyometrics).
Sources (peer-reviewed, with DOI)
Kubo K, Kawakami Y, Fukunaga T (1999). Influence of elastic properties of tendon structures on jump performance in humans. J Appl Physiol 87(6):2090–2096. doi:10.1152/jappl.1999.87.6.2090
Kubo K, Yata H, Kanehisa H, Fukunaga T (2006). Effects of isometric squat training on the tendon stiffness and jump performance. Eur J Appl Physiol 96(3):305–314. doi:10.1007/s00421-005-0087-3
Squat jump vs. CMJ
Why you jump higher with a countermovement – and what that has to do with the ring.
With a countermovement (CMJ) you jump about 2–4 cm higher than from standing (squat jump). The main reason is not the tendon spring: by dipping first, the muscle builds up high tension before push-off and can produce more force over the first part of the extension. From a standing start this force has to build up during the extension – by which point half the range is already gone. The tendon’s elastic energy contributes as well, but dominates above all in repeated, fast hopping and running.
In the game: Your first jump is a squat jump from standing – weak, because nothing is pre-loaded. From then on you hop in the CMJ rhythm, and the ring is exactly the moment when you release what was pre-loaded. Wait too long = it decays.
Why the difference isn’t the same for everyone
The gap is not a fixed value but an individual ratio (Eccentric Utilization Ratio, EUR = CMJ ÷ SJ). It varies because several factors play a part:
- Concentric force development (RFD): A large difference can mean that the squat jump is weak (slow force build-up) – not that the spring is particularly good.
- Sport & training phase: The EUR differs by discipline and rises, for example, from the off-season to the pre-season.
- Tendon stiffness: Stiffer tendons favour fast force build-up and raise the squat jump – which makes the difference smaller.
- Technique & measurement method: Whether calculated from jump height or power – and how cleanly the squat jump is really performed without a countermovement.
Criticism of the EUR: The metric is contested. Its link to actual sporting performance is barely established; a high value can even indicate a weakness (slow force build-up) and is not desirable per se. Experts recommend never reading the EUR in isolation, but only together with the absolute CMJ and SJ values.
Sources (peer-reviewed, with DOI)
Bobbert MF, Gerritsen KGM, Litjens MCA, Van Soest AJ (1996). Why is countermovement jump height greater than squat jump height? Med Sci Sports Exerc 28(11):1402–1412. doi:10.1097/00005768-199611000-00009
Bobbert MF, Casius LJR (2005). Is the effect of a countermovement on jump height due to active state development? Med Sci Sports Exerc 37(3):440–446. doi:10.1249/01.MSS.0000155389.34538.97
Van Hooren B, Zolotarjova J (2017). The difference between countermovement and squat jump performances: a review of underlying mechanisms with practical applications. J Strength Cond Res 31(7):2011–2020. doi:10.1519/JSC.0000000000001913
McGuigan MR, Doyle TL, Newton M, Edwards DJ, Nimphius S, Newton RU (2006). Eccentric utilization ratio: effect of sport and phase of training. J Strength Cond Res 20(4):992–995. doi:10.1519/R-19165.1
Hawkins SB, Doyle TLA, McGuigan MR (2009). The effect of different training programs on eccentric energy utilization in college-aged males. J Strength Cond Res 23(7):1996–2002. doi:10.1519/JSC.0b013e3181b3dd57
Kozinc Ž, Žitnik J, Smajla D, Šarabon N (2021). The difference between squat jump and countermovement jump in 770 male and female participants from different sports. Eur J Sport Sci (online first). doi:10.1080/17461391.2021.1936654
Kozinc Ž, Pleša J, Šarabon N (2021). Questionable utility of the eccentric utilization ratio in relation to the performance of volleyball players. Int J Environ Res Public Health 18(22):11754. doi:10.3390/ijerph182211754
Kubo K, Kawakami Y, Fukunaga T (1999). Influence of elastic properties of tendon structures on jump performance in humans. J Appl Physiol 87(6):2090–2096. doi:10.1152/jappl.1999.87.6.2090
Kubo K, Yata H, Kanehisa H, Fukunaga T (2006). Effects of isometric squat training on the tendon stiffness and jump performance. Eur J Appl Physiol 96(3):305–314. doi:10.1007/s00421-005-0087-3
Rehab & fall prevention
The same quality – developing force quickly, releasing it at the right moment, returning it elastically – matters far beyond sport.
After sports injuries
The tendon’s spring function does not come back on its own once the pain is gone – it is rebuilt in stages: from isometric holds through heavy, slow strength training to jumping and energy-storage exercises. Precisely this reactive timing – short ground-contact time – is among the criteria for return to sport. It is what you are practising here at the ring.
Fall prevention in older age
After a trip, what protects you is less maximal strength than the ability to quickly generate force (muscle power/RFD) and take a rapid step – a reactive, jump-like quality. Muscle power declines earlier and more strongly with age than strength does, and predicts falls better. Targeted reactive balance training measurably lowers the fall rate.
But with a sense of proportion: What counts is the quality, not a hard jump for every person. With an irritated tendon you begin isometrically rather than with plyometrics; older or frail people train with dosed power and reactive balance training rather than drop jumps. And how much strength versus fast force contributes to fall risk is not yet conclusively settled scientifically.
Sources (peer-reviewed, with DOI)
Krogh TP, Jensen TT, Madsen MN, Fredberg U (2022). An isometric and functionally based 4-stage progressive loading program in Achilles tendinopathy: a 12-month pilot study. Transl Sports Med 2022:6268590. doi:10.1155/2022/6268590
Kubo K, Yata H, Kanehisa H, Fukunaga T (2006). Effects of isometric squat training on the tendon stiffness and jump performance. Eur J Appl Physiol 96(3):305–314. doi:10.1007/s00421-005-0087-3
Simpkins C, Yang F (2022). Muscle power is more important than strength in preventing falls in community-dwelling older adults. J Biomech 134:111018. doi:10.1016/j.jbiomech.2022.111018
Mohammadi S, Lotfi M, Zarei H (2025). The effect of perturbation-based balance training on fall incidence, mobility, postural control, and fear of falling of the older adults: a systematic review and meta-analysis. J Appl Gerontol (online first). doi:10.1177/07334648251412655
Mind & body
Above the mechanics lies a control level: the tendon is the spring, the muscle the motor – but the nervous system is the conductor. Warm-up and mindset decide how well it works at the right moment.
The nervous system – the conductor
The stiffness of the leg on landing is not a pure material property but is actively controlled. The central nervous system regulates it via three levers: the pre-tension of the muscles before ground contact (feedforward), reflex activity, and the programmed activity after contact.
Crucially, this happens anticipatorily: the brain sets the spring in advance – according to the expected landing – and adapts the stiffness to the surface. An anticipatory co-contraction of the muscles increases joint stability on impact.
This is why “reactive strength” is trainable at all: a large part of the improvement from jump training is neural – timing, pre-tension, coordination. That is exactly what you practise here at the ring. And for fall prevention the same sensorimotor speed counts: detecting a perturbation and triggering a protective step in a flash.
Warm-up – preparing motor and conductor
Warming up acts on several levels at once – temperature, metabolism, nerves and mind, including a short-term performance potentiation. The warmth speeds up contraction and nerve conduction, lowers the internal resistance of muscles and joints, and lets metabolic reactions run faster. A pre-loading stimulus (a heavy or plyometric exercise) can briefly boost the subsequent explosive performance.
Especially important in practice is injury prevention: structured neuromuscular warm-up programmes markedly reduce injury risk. In the FIFA 11+ programme, reductions of around 30–46 % were seen compared with traditional warm-ups – mediated by better neuromuscular control, trunk and hip stability, and landing mechanics.
Nuance: For explosive and reactive tasks, dynamic warm-up is preferable to long static stretching – the latter can even briefly dampen explosive performance.
Mind – whether and how the conductor starts
In rehab the mind has a say: positive psychological responses are linked to a higher rate of return to sport; conversely, fear of re-injury can lead people not to return at all. “Psychological readiness” is therefore a return criterion in its own right – not just physical healing.
In older people, fear of falling plays an analogous role: it changes movement behaviour (stiffening, cautious gait, less activity → deconditioning) and can even raise the risk. The good news: reactive balance training demonstrably improves not only balance but also fear of falling.
And quite practically: motivation and adherence carry the rehab – which is exactly where a playful presentation like this game comes in.
The overall picture: spring (tendon), motor (muscle), conductor (nervous system) – prepared by the warm-up, released or restrained by the mind. All three can be influenced; that is precisely the holistic remit of physiotherapy.
Sources (peer-reviewed, with DOI)
Taube W, Leukel C, Gollhofer A (2012). How neurons make us jump: the neural control of stretch-shortening cycle movements. Exerc Sport Sci Rev 40(2):106–115. doi:10.1097/JES.0b013e31824138da
McGowan CJ, Pyne DB, Thompson KG, Rattray B (2015). Warm-up strategies for sport and exercise: mechanisms and applications. Sports Med 45(11):1523–1546. doi:10.1007/s40279-015-0376-x
Al Attar WSA, Alshehri MA (2019). A meta-analysis of meta-analyses of the effectiveness of FIFA injury prevention programs in soccer. Scand J Med Sci Sports 29(12):1846–1855. doi:10.1111/sms.13535
Ardern CL, Taylor NF, Feller JA, Webster KE (2013). A systematic review of the psychological factors associated with returning to sport following injury. Br J Sports Med 47(17):1120–1126. doi:10.1136/bjsports-2012-091203
Mohammadi S, Lotfi M, Zarei H (2025). The effect of perturbation-based balance training on fall incidence, mobility, postural control, and fear of falling of the older adults: a systematic review and meta-analysis. J Appl Gerontol (online first). doi:10.1177/07334648251412655
Why this is so: When a tendon is pre-stretched just before the muscle contracts, it stores elastic energy and returns part of it on push-off – the stretch-shortening cycle. Importantly: the tendon produces no energy, it recycles it and saves muscular work. What matters is the short ground-contact time – wait too long and the stored energy is lost, and the muscle has to do the work actively (and expensively). Tendon stiffness, reactive strength and load capacity can be specifically trained; heel geometry, by contrast, is innate. This is how the Achilles tendon helps you run more economically.