(2) Muscle cells and examples of how joints and muscle systems work

Muscle cells and muscles

Muscle cells form soft tissue found in most animals, they are relatively long and must be able to contract quickly.

Muscle cells have a striped appearance and contain protein filaments of actin and myosin that can slide past one another.

This adaptation produces a contraction or extension that changes both the length and the shape of the cell and this is how tissue made of these cells can act as muscle.

The contraction can be reversed and allows muscle tissue cells to function in such a way as to produce force and motion in opposite directions.

Muscle cells contain lots of mitochondria to supply the larger amounts of energy from respiration needed to work the muscles.

Muscle tissue is under voluntary control, and the fibres join up (in development) to give strength and co-ordinated movement.

Antagonistic muscles work in pairs that work against each other, but in unison

One muscle will contract and shorten, while the other one of the pair relaxes and lengthen.

One muscle will pull a bone one way and the other muscle can pull the same bone in the opposite direction.

Ligaments

Ligaments are a soft, but tough fibrous tissues, that connect bone one to another bone at a joint.

Tendons

Muscles are attached to bones with tendons - strong bands of fibrous material.

When a muscle contracts, a force is applied to the bone it is connected to, causes the bone to move, in many cases it turns on a pivot point at a joint e.g. knee or elbow joint (see diagram and moment calculations below).

Muscles are found in pairs acting on a joint, pivoted in the case of knee, arm and pelvic hip joints, but they can't control bone movement without the tendon connection.

Cartilage

Cartilage tissue protects the ends of bones. It resists compressive forces and enhances bone resilience.

Synovial fluid

Synovial fluid is a thick viscous liquid that cushions the ends of the bones and reduces friction when you move your joints - it stops hard bone surfaces rubbing against each other, minimising wear painful friction effects too.

The knee joint

The knee joint consists of the connection between the relatively thick femur bone and the tibia bones.

Note the protecting cartilage surface and the lubricating synovial fluid.

The bone connecting ligaments, tendons and muscles are not shown.

The arm-elbow joint

The elbow joint consists of the connection between the humerus  bone and the ulna bones.

A good example of antagonistic muscles working in pairs, the tendons are shown too.

The bone connecting ligaments and cartilage are not shown.

When the bicep muscles (biceps) contract (shorten), the tricep muscles (triceps) relax (lengthen), hence you can raise your arm as the bone is lifted, maybe lifting a weight at the same time.

When bicep muscles relax, the tricep muscles contract, hence you can lower and straighten your arm.

In terms of the 'physics' of the situation the elbow joint is the pivot point of the structure and you can use the 'principle of moments' to calculate the forces involved e.g. the force needed to raise a weight.

The 'moment equation' is: moment (Nm) = force (N) x distance (m)

Therefore: force generated (N) = moment (Nm) ÷ distance from pivot point (m)

See the skeleton-3.htm

The pelvic hip joint

The pelvic joint consists of the connection between the relatively thick pelvis bone and the femur bone.

Note the protecting cartilage surface and the lubricating synovial fluid.

The bone connecting ligaments and tendons are not shown.

If the cartilage is damaged from 'wear and tear' and breaks down, you experience pain and inflammation.

If the situation is very serious, you can have a hip replacement operation.

Other antagonistic muscles

The hamstrings and quadriceps in the legs are also antagonistic muscles.