>Now, given that high lactate numbers are not a real concern, we're back to
>"what causes endurance horses to get tired?" Next most likely culprit: muscle
>fuel depletion.
Other theories I have come across to explain muscle fatigue - in other
species, not so much horses are :
1) muscle injury - lengthening exercise causes more injury than shortening
(ie it causes more muscle disruption to make the muscles work while they are
being stretched, for instance running down hill).
Some ideas on how this muscle injury may occur :
mechanical disruption of the muscle cells themselves, causing an influx of
calcium ions, with subsequent activation of cell enzymes such as proteases
and phospholipases, which can in effect "eat" the muscle fibres.
The generation of free radicals (those same things that are incriminated in
aging and cancer etc) after exhaustive exercise when there may be periods
of relative ischaemia (lack of sufficient oxygen supply in the blood).
2) Excitaion-contraction coupling. For the muscle cell to contract it
requires an electrical stimulus to pass from the outside to the inside of
the cell. To do this, ions like sodium and potassium have to move from one
side of the cell membrane to the other. When the muscle is working hard and
using energy to make the contractions happen, there may be less energy left
to help pump sodium and potassium back to where they were ready for the next
electrical signal to do its thing. The decreasing muscle pH (due to the
accumulation of lactate) can apparently also interfere with the
sdium/potassium pump. So we are left with muscle cells that require a higher
amount of "excitation" to make them contract.
This loss of potassium from working muscles is apparently an established
fact (I may be able to dig out some references if someone wants the source
work). I find the second theory particularly interesting.
Low blood potassium, for instance, as would occur after sweating with
exercise, and after the horse had some time to recover its muscle potassium
(suck it out of the blood back to the muscle) could be the culprit in the
reduction in gut motility and in the severe colics we have occasionally
encountered one or more hours after finishing a 50 mile ride.
Comments, please?
Anne Barnes
Murdoch University
Western Australia
Dr. Anne Barnes
Lecturer, Veterinary Reproduction
School of Veterinary Studies
Murdoch University
Murdoch, W. A. 6150
Australia
Phone : (09) 360 2643
Fax : (09) 310 7495.