A 16-year-old boy comes to the office for an annual health maintenance visit. He has no medical conditions and feels well. The patient plays football for his high school team and states he is planning to increase his physical workouts to prepare for the upcoming season. He indicates that last season he was often fatigued during the second half of games, and he plans to focus on endurance training. Physical examination shows no abnormalities. Which of the following is most likely to increase in this patient's large skeletal muscle groups after a prolonged period of regular training?

Question

A 16-year-old boy comes to the office for an annual health maintenance visit.  He has no medical conditions and feels well.  The patient plays football for his high school team and states he is planning to increase his physical workouts to prepare for the upcoming season.  He indicates that last season he was often fatigued during the second half of games, and he plans to focus on endurance training.  Physical examination shows no abnormalities.  Which of the following is most likely to increase in this patient's large skeletal muscle groups after a prolonged period of regular training?

Quizplus · Accepted Answer

11ec160b_3d04_d208_9249_332f7c0cc3d9_MD0001_00 Skeletal muscle fibers (or myocytes) are categorized into 2 major types based on the magnitude and sustainability of contraction force they generate and their predominant method of energy production. Type I fibers (slow-twitch fibers) specialize in stamina (fatigue resistance) but can generate only relatively low force per contraction. These fibers primarily use oxidative phosphorylation (OP) to allow for sustained and efficient production of ATP. Therefore, they have a high content of lipids (fatty acids are a primary energy source for OP), myoglobin (to supply oxygen), and mitochondria (the cellular location of OP). The high myoglobin content gives these fibers a red color. Type II fibers (fast-twitch fibers) primarily specialize in rapid bursts of strength; they generate high force per contraction but quickly fatigue. These fibers produce ATP primarily via anaerobic glycolysis and have high glycogen levels. Therefore, they have low quantities of mitochondria and myoglobin (giving them a pale red or white color). Targeted physical training stimulates specific phenotypic adaptations in skeletal muscle fibers to improve skeletal muscle function. Endurance training (eg, distance running, sustained light-to-moderate weightlifting) promotes the characteristics of type I fibers (eg, increased size and quantity of mitochondria) and also increases capillary density. In contrast, resistance training (eg, heavy weightlifting) amplifies the characteristics of type II fibers while also stimulating increased muscle mass. (Choice A) Anaerobic glycolytic enzymes will decrease and aerobic oxidative enzymes will increase with endurance training. (Choice C) Muscle fibers are arranged into a motor unit, with each unit innervated by a single motor neuron and endplate. Although the size and metabolic characteristics of muscle fibers change with training, the number of muscle fibers within a motor unit or endplates per fiber does not change. (Choices D and E) Endurance training amplifies the phenotypic characteristics of existing type I fibers; however, the total quantity of muscle fibers (myocytes) or the proportion of each type of muscle fiber does not significantly change with training. Educational objective: Type I muscle fibers (slow-twitch or red fibers) primarily use oxidative phosphorylation and contain high quantities of lipids, myoglobin, and mitochondria. They specialize in sustained, low force contraction, and their function is amplified by endurance training. Type II muscle fibers (fast-twitch or white fibers) primarily use glycolysis and specialize in rapid bursts of high force contraction. Their function is amplified by resistance training. __________ References: Exercise-induced skeletal muscle remodeling and metabolic adaptation: redox signaling and role of autophagy. (http://www.ncbi.nlm.nih.gov/pubmed/24450966)

A 16-Year-Old Boy Comes to the Office for an Annual