: Muscle fatigue is caused by the accumulation of metabolic waste (like lactic acid) and the depletion of energy stores during prolonged physical activity. Health and Posture
: Bones grow in length through the division of cartilage cells and in thickness through the periosteum (the outer membrane). Muscular System and Movement
: Skeletal muscles (striated) are responsible for voluntary movements, while smooth muscles control internal organs and are involuntary.