The musculoskeletal system is the network of bones, muscles, tendons, ligaments, joints, and cartilage that gives your body its shape, holds it upright, and allows it to move. It also performs less obvious jobs: protecting your organs, storing minerals, and producing blood cells. Every movement you make, from blinking to running, depends on this system’s components working together.
Bones: More Than a Framework
An adult human has between 206 and 213 bones. That range exists because some people have extra small bones, particularly in the hands and feet, or variations in how certain bones fuse during development. These bones come in five shape categories: long bones (like the femur in your thigh), short bones (like the small bones in your wrist), flat bones (like your skull and shoulder blades), irregular bones (like vertebrae), and sesamoid bones (small, round bones embedded in tendons, with the kneecap being the most familiar).
Bones do far more than provide structure. Inside many of them sits bone marrow, a spongy tissue responsible for producing billions of new blood cells every day. Red bone marrow contains stem cells that generate red blood cells (which carry oxygen), white blood cells (which fight infection), and platelets (which control bleeding). Bone tissue also serves as the body’s main reservoir for calcium and phosphorus, releasing these minerals into the bloodstream when other organs need them.
The Three Types of Muscle
Your body contains more than 650 muscles, but not all muscle tissue works the same way. There are three distinct types, each built for a different job.
- Skeletal muscle attaches to bones and is the only type you control voluntarily. When you decide to pick up a cup or take a step, skeletal muscle does the work. Under a microscope, it has a striped (striated) appearance.
- Smooth muscle lines the walls of hollow organs like the intestines, blood vessels, and bladder. It contracts on its own without any conscious effort, pushing food through your digestive tract or regulating blood flow.
- Cardiac muscle exists only in the heart. It’s striated like skeletal muscle but involuntary like smooth muscle, contracting rhythmically for your entire life without you ever having to think about it.
When people talk about the musculoskeletal system, they’re primarily referring to skeletal muscle, since it’s the type directly connected to the skeleton and responsible for voluntary movement.
Tendons, Ligaments, and Cartilage
Muscles can’t move bones on their own. They need tendons, tough bands of fibrous tissue that anchor muscle to bone. When a muscle contracts, the tendon pulls on the bone, creating motion. Ligaments look similar but serve a different purpose: they connect bone to bone, holding joints together and keeping them stable. A simple way to remember the difference is that tendons transmit force, while ligaments provide restraint.
Cartilage is the smooth, rubbery tissue that covers the ends of bones where they meet at a joint. It acts as a cushion, absorbing shock and reducing friction so bones can glide past each other without grinding. You also have cartilage in your nose, ears, and between the vertebrae in your spine, where it provides flexible support.
How Joints Enable Movement
Joints are the points where two or more bones come together. The most mobile type is the synovial joint, which contains a small amount of lubricating fluid inside a capsule. Your body has six varieties of synovial joints, each allowing a different range and style of motion:
- Ball and socket joints at the shoulders and hips allow rotation and movement in almost every direction.
- Hinge joints at the knees and elbows open and close along a single plane, like a door hinge.
- Pivot joints in the neck let your head turn from side to side.
- Condyloid joints at the wrists and the base of the toes allow movement in two directions without rotation.
- Saddle joints at the base of each thumb give it the ability to cross over the palm, a motion unique among your fingers.
- Planar joints in the wrist bones and between the vertebrae allow small, sliding movements.
How the Brain Tells Muscles to Move
Movement starts with an electrical signal in the brain. That signal travels down the spinal cord and along specialized nerve cells called motor neurons until it reaches a connection point with skeletal muscle, known as the neuromuscular junction. When the signal arrives, tiny channels on the motor neuron open, triggering a chain reaction that crosses the gap between nerve and muscle. The muscle fiber responds by contracting. This entire process happens in milliseconds, which is why you can react to a hot stove almost instantly.
Every voluntary movement, from typing to sprinting, follows this same basic pathway. The speed and force of the contraction depend on how many muscle fibers the brain recruits and how rapidly it sends signals. Fine motor tasks like threading a needle involve small numbers of fibers contracting precisely, while lifting something heavy recruits large groups of fibers all at once.
How the System Changes With Age
Muscle mass and strength begin a gradual decline in your 30s or 40s. The rate accelerates significantly between ages 65 and 80, when you can lose as much as 8% of your muscle mass per decade. This progressive loss, called sarcopenia, makes everyday tasks harder over time and increases the risk of falls and fractures.
Bone density follows a similar trajectory. Bones reach their peak strength in early adulthood, then slowly lose mineral content as you age. Women experience a sharper drop after menopause due to declining estrogen levels. Cartilage also wears down with time and repeated use, which is why joint stiffness and osteoarthritis become more common in older adults. Tendons and ligaments lose some of their elasticity, making them more susceptible to tears.
Nutrients That Support Bones and Muscles
Three nutrients play an outsized role in keeping the musculoskeletal system healthy: calcium, vitamin D, and protein.
Adults between 19 and 50 need about 1,000 mg of calcium per day. Women over 50 and everyone over 70 need 1,200 mg. Calcium is the primary mineral in bone tissue, and when you don’t get enough from food, your body pulls it from your bones to maintain blood calcium levels, weakening them over time.
Vitamin D is essential because your body can’t absorb calcium efficiently without it. The recommended intake is 600 IU per day for adults up to age 70 and 800 IU per day after that. Your skin produces vitamin D when exposed to sunlight, but many people don’t get enough from sun alone, especially in northern climates or during winter months.
Protein provides the building blocks for muscle repair and growth. The baseline recommendation is about 46 grams per day for women and 56 grams for men, though people at risk for osteoporosis or muscle loss often benefit from higher intakes, roughly 1 to 1.2 grams per kilogram of body weight daily. For a 150-pound person, that translates to about 68 to 82 grams per day. Resistance exercise amplifies the benefit of dietary protein by stimulating muscle fibers to rebuild stronger after each session.