Physical development can include a change in height, children tend to get taller at a steady pace growing about 2 inches every year. Children typically will gain about 5 to 7 pounds a year as there is an increase in nutrition intake. This marks the beginning of the child's growth spurt, which occurs prior to puberty. The average age in which a growth spurt occurs in girls is 9, however, boys experience a growth spurt during the age of 11 this can explain why girls during this age tend to be taller than boys. During middle childhood, children’s muscle strength, motor skills, and stamina start to increase, making it possible for children to participate in physical activities for long periods of time without getting tired. Examples of gross motor skills that become more complex during middle childhood can include riding a bike, faster running, swimming, and involvement in physical activities such as sports. Kids demonstrate improved balance, endurance and coordination during physical activities, as they are gradually building muscle strength. Children during this age are able to do things on their own such as feeding themselves and changing their own clothes without the help of their parents. Fine motor skill development during middle childhood, specifically in 5-8-year-olds starts to increase. This can include writing all letters of the alphabet in both upper and lower case, cutting out complex shapes that have angles and curves, as well as being able to tie their own shoelaces.