At this stage, we have only discussed the muscle fibres being exercised in a general sense. They’re not all in the runner’s legs. The great secret that seems to be ignored by most writers is that muscles are really organs with a nerve supply and a blood supply, like any other. All organs have a sensory feedback to the central nervous system that monitors blood chemistry constantly. Levels of oxygen, carbon dioxide, mineral balance, alkalinity and acidity are relentlessly examined, and subtle changes made to accommodate imbalances or signs of overload. The organ that also benefits greatly is the heart. Steadily, over a number of weeks, it gets a beautiful network of capillaries all through it, so that the general blood supply to the cardiac muscle is very low-pressure even with very high demands. Think of a high-pressure large diameter pipe that feeds off into hundreds of small pipes: No matter what the pressure is in the main pipe (major artery), the pressure in the small pipes (arterioles) is way less, and if they again join up throughout deep muscle beds of smaller pipes again (‘anastamosis of capillaries’) the ability of the blood to deliver fuel and oxygen deep into the muscle on an almost individual cellular basis is greatly increased. Likewise the ability of muscle cells to hand back carbon dioxide in exchange for the oxygen, and also for breakdown products of metabolism (metabolites) to be released back into the bloodstream.
With sufficient endurance training, the cardiac mitochondria proliferate too. They’re big and have multi-tasking capabilities in terms of being able to rapidly break down either long fatty acids or glucose so that this particular muscle never runs out of fuels to burn with oxygen. With many weeks of subtle high-aerobic pressure on the heart, it will develop a thicker left ventricular muscle wall to go with all the dense capillarisation, however later on in the season with near-maximal interval work, this will bring along stroke volume and power even more.
We haven’t even discussed yet that there are broadly speaking three types of muscle fibre in the human; slow twitch and two distinct subtypes of fast twitch. Each one has its own very strong association with a particular energy system and metabolic profile. Slow twitch fibres are thin endurance fibres with low power, vast volumes of mitochondria, and dense capillary supply to the fibres. They are a deep red colour due to to the presence of oxygen-bearing iron complexes (globins) that are the equivalent of the haemoglobin in the red blood cells. Same molecule: only it’s ‘affixed’ within the muscle cell near the periphery, as close as possible to incoming capillaries. The molecule is therefore now named myo-globin due to its presence within the ‘myo’ (or muscle) cell. You can see in the illustration above that the aerobically-trained muscle cross-section now has 31 capillaries highlighted, as opposed to 11 in a biopsy from exactly the same muscle taken before the endurance-training block.
Skeletal slow-twitch muscles can bang away constantly for up to 90 minutes, rapidly stripping down fats or glycogen (stored glucose chains) into glucose for delivery into the mitochondria. Slow twitch fibres have enough force to support constant speeds of up to 8.5 miles per hour (13.7 km/hr: nearly 44min 10k pace) for an athlete with normal limb lengths. Thereafter, force comes more and more from fast twitch muscles, while the slow twitch seem to switch to providing metabolic activity to supply the substrates for the fast twitch fibres, rather than force per se. It’s a fascinating division of labour overseen by the central nervous system.
Fast twitch fibres are large powerful fibres with far lower endurance characteristics, and little or no mitochondrial volume. There are various subtypes now being recognized and discussed. One subtype that becomes far more metabolically active with endurance training is the IIA; this is a big strong muscle fibre that has a good capacity to develop oxidative pathways if subjected to endurance training. The other sub-type, the IIB, is extremely powerful and innervated by huge, highly myelinated neurons. Myelin is a derivative of the much-maligned substrate cholesterol, and is laid down more and more along frequently recruited neuronal pathways by Schwann cells that wrap around the neurons, much as one would tape bike handlebars or a tennis racket. The purpose of myelin seems to be very similar to the purpose of insulation on high-voltage cables- it helps nerve conduction speed up considerably from source to destination. Fascinating recent research indicates that myelination of major pathways could be the holy grail of neuro-motor training. Somehow Schwann cells “know” when to start myelination of a much-used neuronal pathway to ensure rapid responses in the nervous system with
The IIB fibre is all power and strength, with no mitochondria and no need for oxygen-based fuel combustion. It’s just a very long cellular bag loaded with high-density muscle fibres and very few organelles. Its dominant energy system is the ‘oxygen-independent anaerobic system’. This is also called the ‘alactic anaerobic’ system because its activity is all “done and dusted” before the lactic anaerobic system starts to really kick in.
It’s also known as the creatine-phosphate system, because energy is derived extremely rapidly from the very rapid stripping down of the three-phosphate molecule ATP (adenosine triphosphate) to ADP (adenosine diphosphate), then AMP (adenosine monophosphate). Each time a phosphate is booted off in this sytem, energy is released into muscle contraction. The system has up to 15 seconds of rapid activity before it needs to recover, and the now-depleted adenosine molecule picks up new phosphate attachments from intramuscular creatine phosphate stores.
So, in effect, we have two anaerobic systems and two fast twitch fibres subtypes analogous with them. We have one aerobic system and one fibre type analogous with it.
More next post.