Several studies have shown that muscle growth differs along the length of the muscle fiber. For example, a previous study found that muscle growth in the triceps was greater with the area that was closer to where tension was placed on the muscle and less in the proximal muscle fibers where tension was less prevalent. Some researchers have proposed that the differences in muscular hypertrophy at different points along the length of the muscle could arise because of differences in muscle-firing activity along the length of the muscle during the resistance exercise. In other words, the researchers suspected that there is greater muscle firing at the muscle sites where more growth is occurring.
Other researchers have suggested that the differences in muscle oxygenation (i.e., metabolic stress) between muscle fibers could be the determining factor for muscle growth. Researchers wanted to examine whether there were differences in muscle tissue oxygen saturation and EMG activity in the distal and middle regions of the vastus lateralis (quadriceps) during heavy resistance exercise. They examined EMG and tissue oxygenation in the mid and proximal area of the quadricep afer a heavy resistance protocol of four sets of eight reps with the right leg at 80% of 1RM, with a 90-second rest period between sets. During the resistance exercise, neuromuscular activities and muscle oxygenation status at the middle and distal regions (50% and 70% of the thigh length, respectively) of the vastus lateralis were measured by using electromyography and near-infrared spectroscopy, respectively. At the end of the study, they found that the EMG activities during exercise were similar, but tissue oxygenation was different between regions during the contractions. They found that tissue oxygenation values in the distal region (further away) were significantly lower than those in the middle region. These results suggest a possibility that the regional difference in muscle oxygenation, not in neuromuscular activity during fatiguing heavy resistance exercise, is responsible for the regional difference in hypertrophy within a muscle. Increased blood flow to the muscles saturates the muscle fibers with nutrients and also anabolic hormones, which may enhance muscle growth. This may be the reason that the area of the muscle that had more tissue saturation had greater increases in muscle growth compared to the further away regions, which had less tissue saturation. The study examined only one exercise, which reinforces the need to use multiple exercises from multiple angles to get the maximum benefits for increasing muscle hypertrophy for a muscle group. They found that tissue oxygenation values in the distal region (further away) were significantly lower than those in the middle region. - FLEX