PDA

View Full Version : Re: Another response to James Dowling (elastic energy utilization)



James Dowling
10-22-1998, 03:54 AM
On Fri, 16 Oct 1998, Jeff McBride wrote:
>
> Very little difference was found between the amount of work
> performed in the concentric phase between a static (concentric only) and
> countermovement squat if the static squat was preceded by an isometric
> preload (Walshe et al. 1998). Therefore, more work isn't achieved in a
> countermovement squat over a static squat if a pre-force is added. Cavagna
> et al. (1985) reported storage of elastic energy after both stretch and
> isometric contraction. So where is the increased work coming from if
> cross-bridge interaction is not a factor? This would only leave the elastic
> structures within the muscle tissue itself (titin)...

The isometric preload is achieved through increased cross-bridge
interaction. This is precisely the sort of experimental observation that
caused G.J. Van Ingen Schenau, A.E. Chapman, myself and others to
re-examine the role of elasticity in human movement. Consider two
identical motor cars that are to travel over a fixed distance (range of
motion) and one starts with a higher level of activation (greater force)
than the other. Unless the other car can achieve the same activation as
the first in zero elapsed time, the first will perform more work and be in
a state of greater kinetic energy by the time the distance is covered.
There is a mathematical proof of this too. I think you would agree that
the first car did not perform more work because of stored elastic energy.

> Do you have additional
> evidence (other than your computer model of the elbow joint) that more work
> can be performed during the concentric phase of a stretch-shorten cycle
> WITHOUT the usage of stored elastic energy from the visco-elastic components
> of the muscle?

Of course not. As I stated earlier, it is impossible to eliminate
visco-elasticity from laboratory experiments of biological tissues.
Studies which have tried to reduce series elasticity by removing the
tendon have been unable to eliminate the elasticity of the aponeurosis
and other series elements such as your beloved titin. A Master's thesis
in my lab by Charles Lynch showed enhanced work of a stretch-shorten cycle
over a quick release movement which was higher than a concentric movement.
This work can be found in the Proceedings of the Second North American
Congress on Biomechanics, Chicago Ill. pp.195-196, 1992. We believe that
this experiment demonstrated that there was evidence of enhanced work
due to recovered strain energy but the EMG showed that the activation
differences also accounted for some of the enhanced work. We do not
believe that our work is definitive but it seems highly unlikely that the
differences in these three movements were due entirely to either
elasticity or to activation.

> If a muscle fibre produces more work after stretch with no filament
> overlap (no cross-bridge interaction) (Morgan 1990) and can also produce
> more work, as you have stated, with no elasticity then I am at a loss.
> Where is the ability to do additional work coming from?

The reason that you are at a loss is because you are comparing apples with
oranges. No one has shown more work with neither cross-bridge interaction
or elasticity. Morgan's results are evidence for elasticity and
particularly for titin but people do not move without considerable
cross-bridge interaction and as soon as you have activation, you have the
possibility of the effect that I mentioned above. You and Cavagna may
"not think it is unreasonable to speculate that the difference between
changes in positive energy during a countermovement jump (or drop jump)
and static jump could be due purely to stored elastic energy" but I think
it foolhardy to ignore such strong evidence of additional factors.

Jim Dowling, Ph.D.

Department of Kinesiology
McMaster University
Hamilton, Ontario L8S 4K1
CANADA

-------------------------------------------------------------------
To unsubscribe send UNSUBSCRIBE BIOMCH-L to LISTSERV@nic.surfnet.nl
For information and archives: http://www.bme.ccf.org/isb/biomch-l
-------------------------------------------------------------------

-------------------------------------------------------------------
To unsubscribe send UNSUBSCRIBE BIOMCH-L to LISTSERV@nic.surfnet.nl
For information and archives: http://www.lri.ccf.org/isb/biomch-l
-------------------------------------------------------------------