Compared with endurance exercise, resistance exercise consists of much higher intensity contractions (e.g., usually 70% to 80% of one repetition maximum) repeated at high frequencies (referring again to motoneuron firing frequency) that can only be sustained for short durations due to the fatiguing nature of the exercise. Therefore, the pattern of cytoplasmic [Ca2+] oscillations and changes in [AMP]/[ATP] and ROS production with skeletal muscle contractions is different between endurance and resistance exercise. For example, endurance exercise likely results in extended periods of moderately elevated [Ca2+], while resistance exercise would generate short cycles of significantly higher intracellular [Ca2+] in skeletal muscle (Chin 2010). Therefore, differences in the magnitude and pattern of these primary signals generated by endurance and resistance exercise will result in the activation of different signaling pathways and different gene-expression and protein-synthesis responses in skeletal muscle. A number of studies have shown that resistance exercise results in increased rates of muscle-protein synthesis, about two- to fivefold after exercise for periods up to 48 h before declining to baseline values. Although acute resistance and endurance-type exercise result in a similar global anabolic response in untrained skeletal muscle, increases in muscle mass (i.e., hypertrophy) and strength are significantly greater following chronic resistance training compared with chronic endurance training. This means that chronic resistance training, but not endurance training, increases the rate of muscle-protein synthesis to levels above the rate of protein degradation.
Activation and differentiation of muscle satellite cells into new muscle cells that fuse with existing muscle fibers can also contribute significantly to the hypertrophic response to resistance exercise. Muscle fibers are large multinucleated cells. They are also postmitotic cells in that they no longer have the ability to divide and reproduce themselves. Additionally, in order to significantly increase in size or hypertrophy, muscles have to add more nuclei, since a nucleus is only able to supply mRNA for new protein synthesis to a limited amount of cytoplasm in its vicinity. Hence, a specific ratio of nuclei to cytoplasm must be maintained. In order to do this, skeletal muscles have small specialized satellite cells,or myogenic precursor cells, located at the periphery of their outer membranes. When stimulated by resistance training or by exercise-induced muscle damage, these specialized cells will be activated and induced to create daughter cells or proliferate. These new satellite cells will then fuse with the existing muscle fibers and add their nuclei to the cells to support greater protein synthesis and increase muscle hypertrophy or repair.
It is well known that the primary factor determining the hypertrophic response to contractile activity is the load across the muscle or the mechanical stretch/strain imposed on the muscle fibers, which is higher in resistance exercise than in endurance exercise. Ultimately, these mechanical/force signals are transduced by signaling pathways that regulate transcriptional and translational processes and satellite-cell activation.
For several years, researchers have focused on the role of insulin-like growth factor (IGF)-1 as a primary signaling molecule that mediates skeletal-muscle growth in response to resistance exercise, given that resistance exercise stimulates the secretion of IGF-1. IGF-1 is known to induce muscle hypertrophy by binding to its receptor on the muscle-cell surface and activating the classical growth factor pathway (see figure 3.27). IGF-1 binding to the receptor activates phosphatidylinositol 3-kinase (PI3K), which leads to the activation of Akt, a serine/threonine protein kinase. Akt phosphorylates and inactivates tuberosclerosis complex (TSC2), resulting in the activation of the ras homologous protein enriched in brain (Rheb) and mammalian target of rapamycin (mTOR). mTOR phosphorylates and suppresses the eukaryotic initiation factor 4E binding protein (4E-BP1) to blunt 4E-BP1 inhibition of translation-initiation cap-binding protein eIF4E (see figure 3.21). mTOR also phosphorylates the 70KDa ribosomal S6 protein kinase (p70S6K1), resulting in an increase in protein synthesis.
It is well accepted that mTOR signaling plays a dominant role in the adaptive response to resistance training; however, researchers have questioned the importance of IGF-1 signaling in mediating this response, based on evidence from several studies employing pharmacological and knockout-mouse approaches to systematically manipulate the IGF-1-PI3K-Akt pathway (Philp, Hamilton, and Baar 2011). It appears that the IGF-1 signaling pathway is not required for mTOR activation or increased protein synthesis that is induced by resistance-type exercise. It is possible that mechanical signals working through stretch-activated membrane channels, for example, might be able to activate mTOR and its downstream targets. However, this hypothesis needs to be examined experimentally (Philp, Hamilton, and Baar 2011).