Studies clearly show that resistance-training induces adaptations in skeletal MHC isoform expression. These shifts predominantly involve rearrangement in the pattern of expression involving the fastMHC. Hather et al [
29] investigated the influence of resistance training on muscle fiber composition. The resultsshow that after 19 weeks of heavy resistance training, IIb fiber composition decreased significantly, where, type IIa fiber increased. In a follow up study the authors [
30] of the same study used the samples to investigate the changes in the fast twitch fiber subtypes. The resultsshow that after training, IIb MHC composition decreased and IIa MHC, in contrast, increased. These responses were essentially mirroredby alterations in fiber type distribution. The percentage of type IIbfibers decreased whereas thepercentage of type IIa fibers increased. The control group showed no changes in MHC composition orfiber type distribution. These results suggest that heavy resistancetraining alters MHC composition in human skeletal muscle. Although it is not the focus of this review it is important to note that similar adaptations to resistance training are seen at the mRNA level as well as the protein level. Willoughby and Pelsue [
31] found that after 8 weeks of heavy resistance training MHC I, and IIa mRNA was significantly greater than the control group. However the type IIx mRNA significantly decreased after training. These findings agree with the findings of the previous studies that show a shift in the protein expression of MHC isoforms. Another study analyzed the distribution of MHC isoforms, fiber type composition, and fiber size of the vastus lateralis in a group of adult sedentary men before and after 3 months of heavy-load resistance training and, subsequently, after 3 months of detraining. Following the period of resistance training, MHC IIx content decreased significantly, with a corresponding significant increase in MHC IIa. After 3 months of detraining the amount of MHC IIX reached values that were higher than before and after resistance training. The results of this study indicated that there was a shift from type IIx fiber to type IIa after 12 weeks of resistance training [
32]. The findings of Andersen [
33] mirrored the results of the two previous studies, only they used sprinters as the subject group. These results suggest that heavy resistancetraining alters MHC composition in human skeletal muscle. Jurimae et al [
34] investigated whether 12 weeks of resistance training, which increased arm girth (5%) and forearm extensor strength (39%), also altered the MHC characteristics of the triceps brachii muscle. The results indicated that resistance training changed the contractile protein profile of trained skeletal muscle. However, changes in MHC isoform composition in the first 12 weeks of training were not implicated in the development of 1RM triceps pushdown strength. The results of this study might infer that changes in MHC composition contribute to increases myofibrillar hypertrophy. For this to be true, resistance training could not only change the composition of the MHC, but it would have to increase the synthesis of the MHC. Welle and collegues [
35] investigated the synthesis of myofibrillar proteins in young and older adults after three months of resistance training. The results indicated that resistance training significantly increased lean body mass. However this study did not find any significant changes in myofibrillar protein synthesis. This study is in contrast to others. Chesley [
11] found that muscle protein synthesis was increased significantly after resistance training as did Hasten et al [
26]. Hasten investigated the changes in mixed muscle protein, myosin heavy chain, and actin proteins before and after two weeks of resistance training. They evaluated seven subjects with a mean age of 27 and seven subjects with a mean age of 80 years old. In the young subjects the MHC synthesis rate increased 88% and the mixed muscle protein increased 121%. In the older group, MHC synthesis rates increased 105% and mixed muscle protein synthesis increased 182% over baseline. Thus MHC synthesis accounted for 80% and 65% respectively in the younger and older individuals total mixed muscle protein synthesis. Balagopal et al [
5] also found a significant training effect on MHC synthesis rate. After three months of resistance training the fractional synthesis rate of MHC increased 47% and mixed muscle protein synthesis increased 56%. It is clear that much of the current literature agrees that resistance training increases the synthesis of myofibrillar proteins and that MHC accounts for a large portion of that synthesis rate.