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NCERT Solutions for Class 9th Science Chapter 3 .4 The Musculoskeletal System — Activity 3.4: Let us investigate

Book page 393 Updated on2026-09-08

Q1.
What percentage of total body weight comes from bones and muscles?
Answer

Bones make up about 12 – 15 per cent of an adult's body weight; muscle makes up about 40 – 50 per cent in adult males and about 30 – 40 per cent in adult females. Together, well over half your weight is musculoskeletal.

Worked example for a student weighing 50 kg (use your own reading from the weighing scale in step 1):

Weight of a part = total body weight × (percentage ÷ 100)

Bone (12 – 15%): lower estimate = 50 kg × 12/100 = 6.0 kg
               upper estimate = 50 kg × 15/100 = 7.5 kg

Muscle, adult male (40 – 50%) = 50 kg × 40/100 to 50 kg × 50/100 = 20 kg to 25 kg
Muscle, adult female (30 – 40%) = 50 kg × 30/100 to 50 kg × 40/100 = 15 kg to 20 kg

Bone + muscle (male figures) = 6.0 kg + 20 kg = 26 kg  to  7.5 kg + 25 kg = 32.5 kg
As a fraction of body weight = 26 ÷ 50 = 0.52  to  32.5 ÷ 50 = 0.65, i.e. 52% – 65% of total body weight
Check it yourself: to get the class average, add every student's estimated bone weight and divide by the number of students —
class average bone weight = (sum of all bone weights in kg) ÷ (number of students). Do the same for muscle.
Why the answer is a range and not a single number: the percentages are averages for a population. Your own value depends on age, gender, body composition and even ethnicity, so the honest answer is an interval, not one figure. That is also why step 2 asks you to look up values for an Indian body type rather than use a number from anywhere in the world.
Q2.
Discuss why do bone and muscle mass differ between individuals, and how do they contribute to the overall body weight?
Answer

Why they differ. Bone and muscle are living tissues that are constantly being built up and broken down, and the balance depends on the person:

  • Age — bone mass builds up through childhood and adolescence, peaks in early adulthood, and slowly declines later.
  • Gender — adult males on average carry more muscle (40 – 50%) than adult females (30 – 40%).
  • Physical activity — muscles and bones respond to load. A kabaddi player or a farmer who lifts weight daily builds denser bone and larger muscle than someone who sits all day.
  • Nutrition — protein is needed to build muscle; calcium, phosphorus and vitamin D are needed for the bone matrix. A poor diet lowers both.
  • Body size and composition — a taller, heavier frame carries a bigger skeleton; a person with more fat has a lower percentage of muscle even at the same weight.
  • Health — illness, immobility and hormonal changes reduce both.

How they contribute to body weight. Bone is dense because its matrix is loaded with calcium and phosphorus compounds; muscle is bulky because it is the largest single tissue in the body. Adding the two:

Bone (12 – 15%) + muscle (30 – 50%) = roughly 42% to 65% of total body weight
The rest is fat, skin, blood, organs and body fluids.
Why this matters: a change in weight is not the same thing as a change in health. Two students of the same weight can have very different amounts of muscle and bone. Regular exercise, correct posture, proper nutrition and yoga increase the muscle and bone share — which keeps bones strong, joints flexible and the body protected from stiffness — even if the reading on the weighing scale hardly moves.
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