sportsphysio, on 12 June 2012 - 11:26 PM, said:
Normally a fan of your posts but I don't know if you've shed any light on the subject. Your assertion that hamstrings bends the knee at toe-off isn't on the money and the comment about "free energy" as opposed to muscle energy is inaccurate as well. The stored energy in tendons can only be utilised in the presence of muscle energy (that is, the muscle must remain active to allow the tendons to store energy. No muscle force = no stored energy).
I will attempt to answer both BT’s & your query in the one answer (as all are somewhat interrelated). Firstly, nowhere did I say/imply that
“hamstrings bends the knee at toe-off
”... however, I did state that Hamstrings are responsible for flexing the knee. This should have been picked up on via the following central statement...
BEN-HUR, on 11 June 2012 - 10:22 PM, said:
However, effective degree of hip & knee flexion is not so much the result of contracting flexor muscles (i.e. Psoas, Hamstring groups) but occurs from the degree of forward movement inertia force.
That previous sentence (above quote) is a loaded statement as it goes towards answering all the queries i.e. the issue of muscle activation, the issue of inertia (or inertial) force & what I stated as... “so called free energy
” (“so called” in reference to what this type of energy is sometimes referred as in discussions of this nature [i.e. “free” recoil energy as opposed to activated muscle energy] – I personally feel a more appropriate term would be recoil/elastic energy
). I must say that appropriate terminology is important in discussions of this nature to offset the potential for confusion/misinterpretation... which can also be affected when one is in a rush to complete a post.
To clarify further; at the propulsion phase - the foot, leg & hip use recoil/elastic energy
stored in the muscles & tendons during the absorption phase to propel the body forward. It's important to use most of this recoil energy to push the body forward & not upward. EMG research has found that the least amount of muscle activity noted at during “toe-off” [TO] (see diagram below) & greatest amount of muscle activity during “initial foot contact” [IC] (this also goes to explaining appropriate “leg stiffness” theory via increased muscle activation preparing for ground contact – another interesting issue in itself), mid-support, & foot descent (the issue of foot descent being the crux of the thread topic I would think in relation to reducing impact force/noise). The Tibialis Anterior muscle is active for the greatest total time with the Hamstrings being the second most activated muscle (see below).
It is also interesting to note here that studies have also shown that more conditioned (i.e. elite) runners’ exhibit lower muscular activity than less conditioned runners. This means they are more efficient in using the energy they produce. The above diagram is interesting as it shows that there is little activation in the calf muscles during late support phase which means that runners do not (or should that be... should not) "push off" from the foot via calf activation (albeit different I would think running up an increasing incline/hill)... instead the large Achilles tendon acts as a spring to return elastic energy
to the ground as the foot leaves the ground. The Plantar Fascia under the foot also helps with the spring like loading & unloading to propel you & subsequent leg forward
. Together, the tendons of the foot & the Achilles tendon provide at least half of the elastic energy used to propel the body forward. Here is where the “forward movement inertia (or inertial) force
” comes into play. This “inertial force” is the effective force required to rebound/recoil
the leg/foot forward back to the body in which creates varying degrees of flexion (at joints) of the lower limb based on the mass & acceleration (hence why heavier shoes may have potential adverse impact here on muscles [i.e. Hamstrings] in controlling this moment of inertia).
Initial leg swing & shortening the lever arm for greater economy is sped up by activation of the Rectus Femoris (in part via the use of that recoil motion/elastic energy). Towards the end of swing phase, contact is pulled closer to the body by the Hamstrings (which was first activated at mid-swing phase), thus slowing the forward swing of the lower limb just prior to ground contact (as well as Glute activation) which start to pull the thigh back prior to ground contact. However, if we have weak Hamstrings or inefficient Hamstring activation, they may be less able to efficiently pull the lower leg back prior to ground contact. This is how/where overstriding can/could take place. Add on to all of this a “heavy” pair of shoes at the end of the foot that increases the moment of inertia of the lower leg as it rotates at the knee which can exacerbate further problems. As we swing that foot forward with a heavier shoe, it may be harder for the Hamstrings to effectively do their job to slow the leg down & prepare it for a more optimal lighter landing... hence greater impact force generated (i.e. potential greater contact/shoe noise)... hence also landing in a potentially more over extended position ahead of COG or COM (centre of Gravity/Mass) – hence potential heavier/exacerbated heel striking – hence greater eccentric loading forces – hence potential greater chance of injury. Hence the importance of performing exercises to strengthen the likes of Hamstrings/Gluteus Max. etc. in association with routines such as plyometrics & vertical (upright two leg & one leg) core routines. Hence also the potential benefits in incorporating some element of barefoot running/training into one’s running schedule... all of which help entrench better technique/form & neuromuscular activation/strength.
I hope this helps clarify the issues raised & also help understand the “why” behind some of the problems faced by some runner’s injuries & technique/form woes. However, I can’t help but resonate with the following picture when all is said & done. The interesting conundrum here is that sometimes things appear overly complicated (humans like to complicate things) yet the answers always tends to lie in getting back to the basics – same applies to nutrition --> natural/plant based food... & primarily raw (whoops, another controversial & potentially complicated topic).
Edited by BEN-HUR, 13 June 2012 - 07:46 AM.