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The Science of Performance |
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Changes in Muscle Fiber Activation With Training Part 1 – Easy Training
Introduction What adaptations occur in your muscle fibers as a result of endurance training? The Muscle Power model of performance asserts that the three primary muscle characteristics of force, contraction speed, and fatigue resistance exert the greatest influence on endurance performance. If the Muscle Power model is accurate then several questions arise. First, when endurance training results in improved performance are adaptations occurring at the muscle fibers level that result in the improved performance and can we measure those changes? Second, do different types of training result in different muscle fiber adaptations? Lastly, if different training methods produce different adaptations which training methods are most effective in producing particularly desired changes? For example, if your personal goal is to improve your resistance to fatigue which training methods best accomplish that goal? Perhaps not surprising there has been little research seeking answers to these and similar questions. Due to the near universal acceptance of the cardiovascular/anaerobic model for the past 50 years exercise scientists have focused most of their research efforts on adaptations within the cardiovascular system. However the increasing number and validity of the challenges to the cardiovascular/anaerobic model have caused some scientists to consider other possible explanations for endurance performance limitations. Included in the list of possibilities is the role of muscle fiber in endurance performance. Recently scientists have begun using electromyography (EMG) and magnetic resonance imaging (MRI) to measure changes in muscle fiber activation during exercise of different intensities and, more important to our discussion, changes in fiber activation with training. Let’s take a closer look at some of these exciting research studies and see what knowledge we can glean from them. Research It is well known that repeated exposure to endurance exercise results in numerous cardiovascular changes, such as increased mitochondrial and capillary density, improved VO2peak, decreased heart rate, and decreased blood lactate levels. Additionally it is also known that force sensation is reduced in trained muscles at any exercise intensity. What is not well researched though is what, if any, neuromuscular changes may occur with endurance training. Three Canadian researchers decided to tackle this topic by conducting research to examine “how the processes of muscle activation and dynamic force sensation…adapt to endurance training”. (1) To begin they recruited 12 healthy, non-exercising male college students. All of the subjects were tested for various physical characteristics, including VO2peak, lactate levels at various exercise intensities, muscle torque, muscle fiber activation at different exercise intensities (via EMG), and force sensation during exercise. Subjects completed two cycling bouts as part of their testing procedure - a short, 10 minute, high intensity interval cycling bout and a 20 minute, low intensity cycling bout conducted at a steady power output equal to 70% of pre-training VO2peak. Multiple physiological and muscular data was collected during both exercise bouts. Once initial testing was completed six of the subjects were assigned to an exercise group and six served as controls. The exercise group engaged in 8 weeks of single-leg endurance training on a cycle ergometer. Training consisted of 30 minute bouts of single-leg cycling at an intensity of 60% pre-training VO2peak, 3 times per week. This intensity level, though considered to be an easy effort, is known to be of sufficient intensity to cause improvement in beginner trainees like these subjects. The untrained leg served as a within-subject control, making it possible to determine any central adaptations that might occur. The control group maintained their normal lifestyle during the 8 week training period. At the completion of the 8 week training period all subjects were re-tested following the same format as when first tested. Results As would be expected the subjects experienced an improvement in the standard cardiovascular measures of fitness. VO2peak during exercise with the trained leg increased 18% following training. Heart rate and lactate levels were significantly lower following training, decreasing by 7% and 42% respectively. VO2peak, heart rate and lactate also improved significantly while exercising with the untrained leg (recall that the subjects trained only one leg). VO2peak improved 6%, heart rate decreased by 2.4% and lactate decreased by 22% when the subjects exercised the untrained leg. There was no change in maximum strength or muscle fiber activation during a test of maximum strength. There was also no change in muscle fiber activation or force sensation during the short, high intensity cycling bout. There were significant changes in muscle fiber activation and force sensation during the long cycling bout. Prior to training the subjects experienced a 4-fold increase in force sensation by the end of the 20 minute cycling bout with either leg, meaning that their perception of effort was 4 times greater at the end of the test than at the beginning of the test even though the resistance remained constant during the entire 20 minute exercise. After 8 weeks of training the subjects experienced only a 75% increase in force sensation by the end of the 20 minute cycling bout with the trained leg. This equates to a 70% decrease in force sensation pre to post training. In addition to the changes in force sensation for the trained leg the subjects experienced a 2.6-fold force sensation increase during cycling with the untrained leg post-training as compared to a 4-fold increase pre-training. Despite not training this leg the sensation of force required to maintain power output with the untrained leg was significantly decreased pre to post training (see figures 1 & 2). Figure 1: Magnitude estimations of force sensation during 20-min of single leg cycling at 70% pre-training VO2peak, trained leg*
Figure 2: Magnitude estimations of force sensation during 20-min of single leg cycling at 70% pre-training VO2peak, untrained leg*
There were also significant changes in muscle fiber activation with training. Prior to training an increasing volume of muscle fiber was required to maintain a constant power output. Post training the trained leg was able to supply the required power throughout without having to recruit additional muscle fibers or by increasing the firing rate of the fibers that had already been recruited. In the untrained leg there was no significant difference in muscle fiber activation pre to post training (see figures 3 & 4). Figure 3: Electrical activation required to maintain a constant power output during 20 min single leg cycling at 70% of pre-training VO2peak, trained leg.*
Figure 4: Electrical activation required to maintain a constant power output during 20 min single leg cycling at 70% of pre-training VO2peak, untrained leg.*
Discussion Recall that the subjects trained 3 times per week, 30 minutes per session, at 60% of pre-training VO2peak for 8 weeks. This level of intensity is considered to be an easy level of intensity for trained subjects, but is known to cause improvements in untrained subjects. There are several significant findings from this research related to muscle fiber activation. The first significant finding is that the volume of muscle fiber required to initiate exercise decreased only slightly with training and did not reach a level of significance. The level of muscle fiber required to initiate exercise was basically the same pre and post training (see fig. 3). Despite fitness improvements evidenced by improvements in VO2peak, blood lactate levels, heart rate, force sensation, and end exercise muscle fiber activation these improvements did not also result in significantly fewer muscle fibers being activated at the beginning of an exercise bout. Additionally there was no change in maximum voluntary contraction force (MVC) or muscle fiber activation during the short, high intensity exercise bout. Since the same mass of muscle fibers was required to initiate the same absolute power output pre and post training and there was no increase in MVC or fiber activation during the high intensity cycling bout the conclusion is that training did not significantly strengthen the muscle fibers. If the muscle fibers had become stronger then fewer muscle fibers would have been required to reach the same absolute power output. The second finding from this study related to muscle fiber activation is that post training fewer muscle fibers were required to maintain power output than pre-training. In fact, muscle fiber activation stayed nearly level throughout the entire 20 minute exercise bout (see fig. 3). During exercise progressive fatigue of the active muscle fibers requires additional muscle fibers to be recruited in order to maintain power output. This was clearly evident pre-training as muscle fiber activation increased steadily throughout the 20 minute exercise bout (fig. 3 pre-training line). Post-training however, muscle fiber activation remained stable throughout exercise (fig. 3 post-training line). The conclusion is the muscle fibers adapted to training by increasing their resistance to fatigue. Whether these changes are due to an increase in the oxidative capacity of the muscle fibers (as the researchers suggested) or are due to other changes in the fibers is not known because changes within the fibers themselves were not evaluated. In either case this research demonstrates that the result of easy endurance training is muscle fibers more resistant to fatigue. The third finding from this research is the changes in the untrained leg. Note that the force sensation, heart rate, VO2peak, and lactate levels all improved in the untrained leg despite this leg not being trained. The fact that improvements were observed might be explained by central adaptations in the body. Scientists often classify changes as either being central or peripheral. Central adaptations are changes in shared bodily systems, such as heart, lungs, circulation, etc. Peripheral adaptations are changes that occur in systems that are only used in certain exercises. In this research peripheral systems are the muscles of the leg exposed to exercise. Since only one leg was trained during this study any fitness improvements of the untrained leg could logically be attributed to central rather than peripheral changes. However, if central adaptations explain improvements in the untrained leg why was there also not a corresponding improvement (i.e. decrease) in muscle fiber activation in the untrained leg? If central systems are limiting exercise in some way and improvements in the central system result in improved fitness wouldn’t fewer muscle fibers have to be recruited to initiate and maintain performance? In this case the answer is no. Fitness measures improved and force sensation decreased significantly but muscle fiber activation levels did not change. There is a disassociation between force sensation, measures of fitness, and muscle fiber activation for the untrained leg. As the researchers put it, “It is somewhat more difficult to account for the changes in the untrained leg.” I will save for another day a more complete analysis of this phenomenon. For now I will simply suggest that changes in the central governor could easily explain these results. Summary Though the cardiovascular adaptations from endurance training are well known, neuromuscular adaptations to training are less well known. This study revealed that endurance training did not result in fewer muscle fibers being required to initiate an absolute power output but did result in a significant decrease in the rate of muscle fiber activation throughout the exercise bout. The decreased rate of muscle fiber activation explain the force sensation and the perceived difficulty of the exercise session. The implication is that endurance training causes improvement in the fatigue resistance of muscle fibers but not an increase in muscle fiber strength. Reference: 1. Cafarelli, E., Liebesman J., Kroon J Effect of endurance training on muscle activation and force sensation Can J Phsiol Pharmacol 1995 73: 1765-1773
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