Nathan Miller and Dr. Allen Parcell, Health and Human Performance
Research has given us an increased understanding of skeletal muscle. We have learned that skeletal muscle function is influenced by neural, biochemical, molecular, and biomechanical factors. As our understanding of these factors increases so does our ability to facilitate positive adaptations in muscle function.
Human skeletal muscle consists of three fiber types—or isoforms—as defined by their speed of contraction. They are called types I, IIa, and IIx. The myosin heavy chain (MHC) protein in the muscle fiber dictates these characteristics. Single muscle fibers in humans may possess one or more types of myosin heavy chain. Those with multiple isoforms are known as hybrid fibers. Hybrids have been found more often in untrained and elderly populations, and occur less frequently in people who participate in exercise training.
Hybrid fibers affect single fiber contractile characteristics, but it is still unknown how they influence the function of whole muscle. Many studies have been done to better understand their role. MHC alterations have been studied using resistance training, which consists of high force, low velocity movements. No information, however, has been documented on MHC alterations as a result of intense, high velocity sprint training. In order to increase our understanding on the subject we conducted a sprint training study. Our hypothesis was that there would be a reduction in the hybrid containing type I and type IIa (written as MHC I/IIa) accompanied by an increase in MHC IIa content. Also, we believed that the amount of MHC IIa/IIx hybrid isoform would be maintained due to the intense nature of the exercise.
Ten subjects were chosen to participate in the study. They were all college-age males that were not involved in an exercise program at the time. The protocol of the experiment was explained to them and they were informed of the risks and benefits of participating in the study. Each signed a written consent form before testing began.
The first aspect of our study involved performing a biopsy on each subject taking a small portion of the vastus lateralis (thigh) muscle. This is done by numbing the area, making an incision, and inserting a biopsy needle that slices and gathers the muscle. The tissue sample was then frozen to be used later for pre-training analysis. This analysis provided us with the initial data for the experiment.
One week after the biopsy the subjects began the eight week sprint training program on the stationary bicycle. Two exercise sessions were performed each of the first two weeks of training and was increased to three sessions per week thereafter. At the first session the subjects performed two sprints. Each sprint lasted 15 seconds and was followed by a five-minute recovery interval. The following session they performed three sprints. Beginning in the second week they did four sprints. The number of sprints per session then gradually increased so they did 6 sprints during each session the final two weeks. By the end of the eight weeks the final number of sessions totaled 22. During each sprint session we kept track of the subject’s total work performed so that we could monitor their process and make sure that they were sprinting at full speed each time.
When the eight weeks were completed we once again performed a biopsy of the same muscle. It was taken as close to the original biopsy site as possible. The tissue from this biopsy was used for our post-training analysis.
A number of steps were required to analyze the pre- and post-training muscle samples. First, we put a portion of the sample in a relaxing solution and dissected the individual fibers from the sample. The fibers were then put into a buffer solution that denatured the proteins to help with the separation of the individual myosin heavy chain proteins. Finally, a polyacrylamide gel electrophoresis was carried out. To do this we made a gel which contained grooves, or lanes, in which we put the buffer solution containing the muscle fiber. This gel was then exposed to an electrical current which caused the proteins to separate into different bands on the gel according to their size. According to their final location on the gel we could differentiate between the respective types (I, IIa, and IIx). By doing this same procedure for the pre-training and the posttraining samples we were able to count each type and to see where changes occurred.
Our results did not prove to be exactly as we had hypothesized. Our assumption was that there would be a reduction in the MHC I/IIa isoform accompanied by an increase in MHC IIa content. Also, we believed that the amount of MHC IIa/IIx isoform would be maintained. We found statistically significant decreases in the number of I/IIx isoforms and also in the number of IIx isoforms. We also saw—as hypothesized—an increase in the IIa isoforms. This increase, though, was not enough to be considered statistically significant.
Our goal with this study was not to make a colossal breakthrough in the world of sports medicine. We wanted to simply increase the overall pool of knowledge about the muscles in our bodies. We discovered that sprint training does have an effect on the myosin heavy chain isoforms of the muscle by increasing the number of IIa isoforms and by decreasing the number of IIx isoforms and I/IIx hybrids. The results from this study can be used as a reference to further studies done on myosin heavy chain, and will help us to better target specific modifications with certain types of exercise.