196 Anatomy & Physiology Essentials Copyright Goodheart-Willcox Co., Inc. Adolescents and Adults Peak development of motor control is acquired during adolescence. With the onset of puberty, strength development accelerates in boys, while girls continue to develop strength at approximately the same rate as during the preadolescent years. In the absence of adult strength training, peak strength is attained at approximately age 20 in women and between the ages of 20 and 30 in men. SELF CHECK 1. Describe the process of myogenesis in the developing fetus. 2. Why do children acquire gross motor skills before fine motor skills? Aging and Sarcopenia As people age, they experience an associated loss of muscle mass and strength known as sarcopenia. In the absence of regular resistance exercise, skeletal muscle mass and strength begin to decline in a linear fashion in a person’s 30s, with up to 50% of muscle mass lost by age 70. Reductions in both the number of muscle fibers and muscle fiber diameter contribute to this decline. The primary loss is in the Type II fibers, which undergo a remodeling process that results in their loss of innervation. Sarcopenia is brought on by a complex host of factors, including hormonal changes, decline in neuromuscular functionality, and fatty infiltration of muscle, all exacerbated by physical inactivity. Many of these changes occur at the cellular and molecular levels. The changes include reductions in muscle fiber activation, excitation contraction coupling, actin- myosin cross-bridge interaction, energy production, and capacity for repair and regeneration. Changes in the central nervous system drive, dysfunction of the peripheral nerves, and alterations in neuromuscular junction structure and function also contribute. Chronic inflammation and low protein intake can also promote loss of muscle. Interventions to address sarcopenia include exercise and proper nutrition. Adults of any age can increase muscle fiber diameter and strength by resistance training with just a few repetitions of heavy loads on a regular basis over time (Figure 6.26). Dietary strategies for maintaining muscle mass include adequate intake of protein and vitamin D. wavebreakmedia/Shutterstock.com Figure 6.26 Regular resistance exercise can maintain or increase muscle mass at any age. SECTION 6.4 REVIEW Mini-Glossary myogenesis process of muscle cell formation myoblasts embryonic stem cells that become muscle cells myocytes mature muscle cells myotubes multinucleated cells derived from myoblasts that form the precursors to myocytes sarcopenia age-related loss of muscle mass and strength Review Questions 1. Briefly describe the development of cells into myocytes in the embryo and fetus. 2. Why are the muscular and nervous systems often referred to as the neuromuscular system? 3. Why can babies raise their heads before they can sit? 4. At what point in development do boys tend to become stronger than girls? 5. What causes sarcopenia? 6. What can people do to slow the effects of sarcopenia?