Research indicates that the factor that ultimately limits performance in an endurance event such as a marathon or an Ironman is the muscle’s capacity to do work whereas previous thought always linked limited performance to a weak cardiovascular system (the ability for the heart to supply blood and oxygen to muscles). But it is now known that cardiovascular fitness is just one part of an athlete’s potential and it is not the only thing that will limit their performance in a race. A USA triathlon webinar given by Krista Austin of Performance and Nutrition Coaching in 2010, discusses limitations to exercise performance
(By the way, you can find these webinars at: https://usat.confedge.com/ap/registration/home.cfm?i=register&e=f7413f5f-8e53-4c04-b8ad-51d0b3d3e63f&grp=Archive&newRegistrant - it costs $24.99 for coaches and $39.99 for the general public).
She discusses in great detail the models of performance that are in the book “The Lore of Running” by Timothy Noakes. The five models being: cardiovascular, energy supply and depletion, muscle recruitment and power, biomechanical, and psychological/motivation. But while cardiovascular fitness, energy supply, biomechanics, and motivation certainly play roles in performance, it is ultimately the ability to recruit muscle fibers and produce power that is the key. According to Austin, the athlete that is able to sustain the highest amount of power production over the longest period of time is going to win the race.
There have been a number of studies performed at altitude, during prolonged submaximal exercise, and in extreme heat that indicate it is not the heart that is limiting performance and causing fatigue but rather the muscle cell itself. It is believed that it is the muscle cell that needs to adapt and as it adapts it drives the response of the heart. As the heart pumps, it is sends blood to the brain which then receives information on the amount of oxygen supply in the blood. If not enough oxygen is reaching the brain, muscle recruitment becomes inhibited because the central nervous system is trying to protect itself. It is what Noakes refers to as our “Central Governor.” This concept was demonstrated with studies done at altitude.
At altitude, there is less oxygen available and the athlete is therefore functioning in an anaerobic state. They found that an athlete’s ability to exercise at increasingly high altitude levels decreased. Their lactate values were lower than what they could achieve at sea level and maximal heart rate and cardiac output also decreased. Under the cardiovascular model however, the heart rate should have increased to supply oxygen to the working muscles. Instead, the brain senses the limited amount of oxygen that is available and protects itself by choosing to reduce the amount of work the body is able to do thereby limiting muscle fiber recruitment. These studies used EMG (electromyography) to monitor the activity of the muscle fibers and found that recruitment decreased as altitude increased. Interestingly, when the athlete who was performing at a maximum effort was suddenly provided with oxygen, the EMG showed higher levels of muscle recruitment. Therefore, fatigue at altitude can not be attributed to a weak cardiovascular system but rather the protective mechanism of the brain that inhibits muscle fiber recruitment.
Another study done supports evidence that it is the muscle cell that drives performance. This study compared East African runners with Swedish runners during a submaximal (lower intensity, below lactate threshold) effort. There were no differences in V02 max between the groups. The results indicated that the East Africans were able to run at a higher percent of V02 max and at a faster pace for a longer period of time without getting tired. It is hypothesized that it is due to the contractile properties of the muscle and its efficient nature that enables them to go for longer periods of times. It is believed that there is some mechanism in the muscle that relays messages to the brain to help dictate the recruitment rate of the the muscle fiber to sustain exercise intensity.
Further, studies that had individuals exercise in the heat found that the body’s core temperature influences how long the body is able to sustain exercise. The thermostat within the brain senses the internal core temperature through the blood and if it gets too hot, the brain protects itself by limiting recruitment of muscle fibers. The hotter the muscle gets the less work it is capable of doing. This underlies the argument again that the cardiovascular system is not always what causes fatigue.
What caused performance limitations during heat, long submaximal efforts, and altitude was the inability to recruit muscle fibers. The brain is constantly receiving feedback from the blood and if it needs to protect itself under a threatening circumstance (such as not receiving enough oxygen) it reduces the body’s capacity to do work via the muscle.
To be continued…
(i’ll get back to that model I put up in part 1 as well).