Muscle Structure and Function
Muscles are composed of vast numbers of individual fibers of two types, type 1 and type 2. Each muscle has both fiber types. Each muscle cell has its own nerve and blood supply (capillaries). Complex neurological and biochemical process causes the muscle cell (or more accurately, fibers in the cell) to contract. There are three types of contraction: 1) concentric, where the muscle shortens, 2) eccentric, where the muscle lengthens and 3) isometric, where the muscle length is constant. From a practical view point, types one and two enable us to run.
Muscle fiber type and function: All muscles have type 1 and type 2 fibers of varying amounts. Comparing the two types, type 1 appear red due to high concentrations of myoglobin (transports and stored oxygen within the cell), have high concentration of mitochondria and are slow twitch (meaning that they take longer to contract and relax). Type 2 fibers appear white due to the low levels of myoglobin, have much few mitochondria, are fast twitch and up to 5 times more powerful. There are several subtypes of type 2 fibers. Type 2a being important in distance running. World class sprinters have high amounts of type 2 fibers where as distance runners have predominately type 1. It is of course more complicated than all of this; for example, world class distance runners have high levels of type 2a fibers (these are type 2 fibers with mitochondrial levels of type 1 fibers).
Muscle cells contain contractile elements and proteins (titin) that hold the cell together and contribute to the elastic recoil of the muscle. Type 2 muscle cells have more titin and thus more elastic recoil. It is thought that repetitive eccentric contractions such as downhill running and marathon distance races can damage this protein leading to muscle soreness. The energy plant of the muscle cell is called the mitochondria. Running ability and fitness are partly related to the total ability of the mitochondria to produce energy. The mitochondria convert food (mostly triglycerides that are broken down to free fatty acids) into adenosine triphosphate (ATP) that is used as energy by the cells. This process requires oxygen. The sarcoplasm, another set of proteins in the muscle cell, can convert glycogen and glucose to ATP in the absence of oxygen, so called anaerobic glycolysis.
So, when we want to voluntarily move, the brain sends an impulse down the spinal cord out the peripheral nerve to the muscle cells. This impulse causes calcium (stored in the sarcoplasmic reticulum) to flood the muscle filaments (within the cell). This starts a chain of events within the cell resulting in muscle fiber shortening, muscle contraction. In order for the cell to “relax”, the calcium must be returned to the sarcoplasmic reticulum and the mitochondria must supply fresh ATP. This process is repeated many times in one “contraction”. The total number of cells activated in a contraction determines how powerful the muscle contracts. From the runner’s standpoint, the rate that this process takes place determines the speed that an athlete can run. With this in mind, we can speculate that some muscle cramping can result from a failure to reabsorb calcium, failure to deliver ATP or from excessive, non-voluntary, neural stimulation. The later being the likely cause of running related muscle cramps. Other causes include electrolyte imbalances and dehydration.
VO2Max
Is a reflection of an athlete’s maximum rate of work or said another way, the maximum rate of oxygen use at peak work rate. Likely limits the upper end of sustainable running. VO2Max does NOT predict speed or time in for a given event. It is roughly a function of cardiac output (CO), hemoglobin (Hgb), Hgb saturation (the amount of oxygen attached to Hgb (this is more of a factor at higher altitudes)) and oxygen extraction from the Hgb. At rest, only about 25% of oxygen attached to hemoglobin is released to muscles as the blood passes through the capillaries. As work load increases, the overall extraction percent increases to a point. It is this maximum oxygen delivery (at max CO) to muscle tissue that limits the amount of work the muscle can perform without becoming acidotic due to anaerobic metabolism. The problem with looking only at VO2Max is that it is an indirect measure of athletic potential because it ignores the velocity at VO2Max (efficiency) and long term resistance to fatigue.
VO2 Max and Factors that affect maximum work: VO2 Max decreases about 9% per decade after 25 (if training is discontinued). Males have a higher VO2Max than females. Training can increase VO2Max 5 to 15% on average. Some elite athletes have improved 25% with training. Ventilation does not normally limit VO2Max as indicated by no fall in paO2 (oxygen in the blood) during exercise.
Central Governor Model
Theory that proposes that exercise is limited by a central protective mechanism that prevents the heart from increasing its workload beyond a certain point in order to protect the organism, us, from hurting ourselves.
Muscle Power Model
Predicts that changes in exercise performance may result from increased muscle contractile function caused by biochemical adaptations in the muscle that increase force or speed (or both) of contractions (sarcomere shortening). This is per muscle fiber and is independent of neural recruitment, that is, more work (stronger contraction) by using more fibers.
Efficiency
So, what are the factors that affect how far and at what speed we can run? VO2Max? VO2 Max is not a very good predictor of who will run the fastest times when comparing elite athletes. The athlete with the best VO2 Max does not necessarily finish first. Athletes must also poses economic running form. We find that running speed at VO2 Max is a good predictor of success in racing. So let’s look at factors that affect running economy or efficiency.
Short rapid strides seem more economical because lead (or impact) foot is under the center of gravity. If the foot strikes out in front of the center of gravity there is a backward energy vector or braking effect. This is bad for two reasons. First we are trying to move forward so we want all energy to be spent moving use in that direction. Second, with “over striding” we create vertical movement which further waste energy and lastly, we put more stress on the muscles and bones with this high impact “braking” stride style. The vast majority of recreational runner over strides and heal strike.
It has been shown that the most efficient position for the arms is a 90 degree (or slightly less) bend at the elbow with a movement that is straight back and forth with only a small amount of swinging across the body. The arms (elbows) should be fairly close to the body as they move back and forth. Running with the elbow way out from the body usually indicates some balance issues. The upper torso should have a slight lean forward at the hips (about 10%). The chest should be proud with the shoulder comfortably pulled back and the head in a neutral position looking 10-20 yards ahead. The face, arms and hand need to remain relaxed. Posture and style are not the only things that affect running economy.
The muscles capacity to store energy
Elastic recoil of muscles and tendons account for up to 30% of the energy needed for running on flat surfaces. Type 2 muscle fibers have better elastic recoil. Elastic elements in the lower leg muscles act as a single linear spring. The tension of the spring can be varied and is independent of speed but varies with stride frequency. That is, stiffer with higher frequency and thus returning more stored energy. Genetics as well as training influence elastic recoil.
Energy Systems
Muscles are fueled by three energy systems. Two of them are anaerobic systems, they don’t require oxygen, the other aerobic or oxygen requiring. The anaerobic systems uses energy immediately available in the muscles. The aerobic using oxygen to covert energy stores into energy usable by the muscles and must be delivered to the muscles.
The two types of anaerobic systems are the alactic acid energy system and the glycolytic energy system. The alactic system does not produce the waste product lactic acid and lasts about 7 seconds before it is used up. This is the energy system used predominately by sprinters. The gycolytic enery system uses glucose in the muscles to produce energy, but does produce lactic acid waste. The two anaerobic systems provide about 90 seconds of energy. This energy system is used by middle distance runners.
The anaerobic system taps fat and glucose stores in the body and uses oxygen to create energy. The vascular system then delivers it to the muscles. This is a very efficient energy system but it can’t keep up with demand in high intensity exercise. This system can run efficiently for about 3 hours. This the energy system used in distance events.
All of these systems can be trained to last longer and run more efficiently.
Bottom Line
Where does all this leave us? We are all born with certain traits that limit our abilities. The goal of the coach (or athlete) is to maximize what we have. Training then has 4 main goals. One is to develop anatomical structures such as muscle capillaries, muscle fibers and tendons to handle the stress of running and racing. Second is to develop enzyme systems so that we are better capable of utilizing energy sources and oxygen at any given work level. We also develop buffering systems that allow us to tolerate high levels of physiologic stress. Third, training improves economy. You get some improvement in economy with long slow miles but the fast majority of improvement comes from hard, fast, stressful and often painful workouts. Lastly, we have to develop a psychological profile of a champion. We have to learn how fast we can run how far. The successful runner has very strong sense of what pace he or she can tolerate. Better athletes do not slow down much during a race. They may go out hard for a few hundred meters to establish a good position but they do not get sucked into running at a speed that is not sustainable for the entire race. The best athletes tend to negatively split a race. That is, run the second have faster that the first.
A few caveats. Children are less economical than adults. The improvements in performance in adolescents appear to be due to changes in economy, not VO2 Max. The energy cost of running increases over time and is apparent after only one hour of running at a constant speed. This is likely due to the stretch (eccentric contraction)-shortening (concentric contraction) cycle (SSC) causing muscle damage that results in failure of the contractive capacity. This leads to mechanical changes in the stride with landing occurring on a more extended leg (over striding) and more flexed knee. This results in longer foot contact, push off phase is reduced and thus prolonged all of which impair running economy. There is also a decrease in the central recruitment of fibers so called central fatigue. This is very important in longer races.
