Leg stiffness topic is now very well detailed and known as a probable predictor of the capacity of the neuromuscular system to store and give back elastic energy as well as optimize the stretch shortening cycle. Elite runners have greater values of leg stiffness, elite jumpers as well...
A recent approach proposes to use an indirect index linked to the leg stiffness: the reactive strength index, which is the ratio of the height jumped on the contact time.
Curiously, we found that we can change the value of leg stiffness of athletes without changing their RSI.
http://www.coachesinfo.com/index.php?option=com_content&view=article&id=146:rugby-rsi&catid=47:rugby-general&Itemid=77
Guillaume,
I think leg stiffness can be difficult to define, and therefore also difficult to interpret. I'm not sure how you measured leg stiffness, but in any case, I can easily modify my leg stiffness through co-contraction without necessarily affecting the outcome of a task. This could perhaps be part of the reason you aren't seeing differences in performance with changes in leg stiffness.
I'm not sure I understand the RSI measure, but it appears that it will suffer from floor- and ceiling- effects, given that one cannot have a contact time of zero, and jump height cannot be infinite. Therefore, even if I try to change my RSI, the results of the equation are mathematically limited to a certain range of values.
Without knowing more details, those are my guesses as to why you might get the results you did. I hope that is helpful.
Sean
thank you sean. Effectively, two strategies can occur to have a high value of RSI: jumping high with a medium contact time ou jumping medium with a short contact time. But with the second strategy, the leg stiffness value increases dramatically because the peak of the ground reaction force increases in very high proportion whereas the leg shortening decreases.
I will post some data soon.
Dynanometry can be used to assay stiffness of a leg muscle specifically (see some of the Lauer and Barbe papers - Pierce 1st author).
In my rat model of repetitive strain injury (upper extremity, so there are likely to be inherent differences here, but..), I see increased collage as a type of fibrosis within the muscle with 6 weeks of performance of a high repetition high force task (Abdelmagid et al and Barbe paper.. While in my model that contributes to reduced muscle strength, so does increased muscle inflammatory cytokines. Increased muscle cytokines are known to contribute to decreased muscle strength as a type of muscle hyperalgesia or myalgia (reflexive muscle pain driving nociceptor firing). Depending on what is happening in the muscle being examined, decreased strength can be linked to either or both increased collagen and inflammatory cytokines. While stiffness as measured by dynanometry may only be a consequence of increased collagen matrix.
Guillaume,
Mechanically, those two results that you describe make perfect sense. By making my ground reaction impulse short (i.e. high force in a short time), I can reduce my contact time with the ground. However, since my contact time is less, I have less time to accelerate my centre of mass, and my jump height is reduced. Vice-versa if I lengthen my ground reaction impulse.
A "stiffness" analogy might get a bit muddled here since the body doesn't truly behave like a spring. A spring does work to convert energy into elastic potential that is stored, and then returns some/most of the energy when it releases. The body is doing work during the whole movement, but very little is conserved (i.e. not stored). If it were, you would easily be able to jump back up to the height from which you dropped, even at large drop heights.
Perhaps I am missing something but I don't see anything odd about your description, at least from a mechanical perspective. Data would be really interesting to look at, thanks.
We measure leg stiffness during locomotion on curves of ground reaction force VS displacement of the centre of mass. There is a good linear fit but leg stiffness appears to be tuned by the nervous system and by gait adaptation to each situation.
thierry, i agree and the linearity of the force-compression curve is rarely respected. Human behave like a mass sprint onlyduring sprinting (after the push-off phase) and jumping on place at a selected range of frequency.
the reactive stength index did not neccessitate this condition .
Dear Guillaume,
I think that it depends on how you test leg/vertical stiffness, because it is frequency-dependent (Hobara et al., 2011). It means that if you test RSI and leg/vertical stiffness during a hopping task (for example) in a frequency of 2 Hz, there will be a direct correlation between leg/vertical stiffness and RSI, since the key determinants of both index are flight and contact times (matematically, Morin et al., 2005) and if you change one of these parameters the other one will be affected in the opposite direction (e.g.: if you increase contact time, the flight time will decrease, and leg/vertical stiffness and RSI decrease also) . However, if you test RSI during a maximal repetitive jumping task, it will not be restricted by the time imposed by a metronome, and in this condition, you can change flight or contact time independently, thus you can affect RSI and leg stiffness in a different way.
However, from these observations I would propose another question: is leg/vertical stiffness directly related to the maximal performance during locomotor activities (running, hopping,...)??
Cheers!
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