Exercise 3.5
Compare your findings with those of your classmates and calculate the class average. Discuss why do bone and muscle mass differ between individuals, and how do they contribute to the overall body weight? The musculoskeletal system in action Activity : Let us observe Move different parts of your body and observe what movement(s) it can make. Body Complete parts rotation Elbow No Shoulder Knee Neck Fingers Toes Wrist You may have noticed that some parts of our body can move easily in many directions, while others move only in a single direction. Why does this happen? The difference is due to the types of joints present. A joint is a junction between two or more bones. Joints allow movement but they cannot move the bones on their own. So, what actually causes the bones to move?
Not cross-checked
NCERT prints no numerical answer for this exercise, so this working has not been cross-checked against the book.
Bones and muscles together make up roughly half of the body's weight — bone about \(\displaystyle 12\!-\!15\% \) for all adults, and muscle about \(\displaystyle 40\!-\!50\% \) in adult males and \(\displaystyle 30\!-\!40\% \) in adult females.
Worked example for a student of \(\displaystyle 50 \) kg, taking bone \(\displaystyle = 13.5\% \) and muscle \(\displaystyle = 35\% \):
bone mass \(\displaystyle = 50 \times \dfrac{13.5}{100} = 6.75 \) kg
muscle mass \(\displaystyle = 50 \times \dfrac{35}{100} = 17.5 \) kg
together \(\displaystyle = 6.75 + 17.5 = 24.25 \) kg, i.e. \(\displaystyle \dfrac{24.25}{50} \times 100 = 48.5\% \) of the total body weight
Why they differ between individuals — the chapter names three reasons: age, gender and body composition; it adds that the percentages may also vary by ethnicity or body type.
Level of physical activity and nutrition matter too: the chapter says correct posture, proper nutrition, regular exercise and yoga keep bones strong and muscles fit, so two people of the same weight can carry very different bone and muscle mass.
How they contribute to body weight — the skeleton is the rigid framework and the muscles are the tissue that pulls on it, so between them they account for the single largest share of body weight; fat, organs and body fluids make up the rest.
Expect the class average to sit near these figures, with the boys' muscle percentage higher than the girls' and the bone percentage similar for everyone.