Arhtritis is inflammation of joints. This happens mostly in synovial joints. Because of ageing and wear and tear the synovial fluid dries up. This leads to corrosion of cartilage. At the end the movement of joints is not as smooth as it should be. This results in general discomfort while doing normal tasks and causes swelling and pain of the affected joint.

Pectoral girdle provides fulcrum to forelimbs.

In locomotion and movement, There is a sentence which is this - Cartilage has slightly pliable matrix due to chondroitin salts.

Certain muscles have high concentration of myoglobin which gives it a red appearance. They also contain plenty of mitochondria which utilize large amounts of oxygen to produce ATP. Because of this they are also called aerobic muscles.

Malleus, incus and stapes. These are found in internal ear.

Muscle fibre is a syncitium as the sarcoplasm contains many nuclei. Myofilaments are parallelly arranged and are present in large number in muscle fibre.

Pelvic girdles provide  articulations to the bones of legs. These support and protect abdominal visceral organs and provide attachement to certain leg muscles.
Pectoral girdles provide an attachment point for numerous muscles that allows shoulder and elbow joints.

The cell is muscle.
These names refer to the following parts
Sarcolemma -Plasma membrane
Sarcoplasm -Cytoplasm
Sarcoplasmic reticulum -Endoplasmic reticulum

Still's disease is a kind of rheumatoid arthritis that occurs in younger people.

During muscle contraction, chemical energy is converted to mechanical energy when ATP is hydrolysed during cross-bridge cycling. This mechanical energy is then distributed and stored in the tissue as the muscle deforms or is used to perform external work.

Bones and cartilages constitute our skeletal system. The skeletal system is divisible into axial and appendicular. Skull, vertebral column, ribs and sternum constitute the axial skeleton. Limb bones and girdles form the appendicular skeleton. Three types of joints are formed between bones or between bone and cartilage - fibrous, cartilaginous and synovial. Synovial joints allow considerable movements and therefore, playa significant role in locomotion.

Osteoporosis is the condition in which the bone mass is decreased. This leads to general weakening of bones. Even a minor slip can result in fractures if a person is suffering from osteoporosis.
Women are at greater risk of suffering from osteoporosis because of several reasons. During child bearing major part of calcium from their body is utilized in formation of the skeleton of the foetus. Additionally during lactation there is further depletion of the calcium reserve of the body because of milk formation. A calcium rich diet can help prevent the progression of osteoporosis. Milk and eggs are good source of calcium. During pregnancy oral calcium supplement is also given to prevent osteoporosis.

Summary of steps in skeletal muscle contraction
(i) Stimulation :
(a) Nerve impulse arrives at neuromuscular junction and release of acetylcholine takes place.
(b) Spreading of action potential through muscle fibre.
(c) The release of Ca²⁺ from sarcoplasmic reticulum to cytoplasm of muscle fibre.
(ii) Contraction :
(a) ATP joins to meromyosin head that hydrolyses into ADP and Pi releasing energy, which in turn raises the head to high energy state (the head joins active site on actin, forming cross-bridge).  
(b) ADP and Pi are released. And head returns to low energy state.
(c) Actin filament is thus, pulled towards center of sarcomere. And a new ATP joins head, detaching it from actin filament.
(d) Above mentioned steps are thus, repeated many times during one contraction to draw the thin myofilaments further inward.

This condition is called as arthritis.
The possible reasons can be
(i) inflammation of the synovial membrane,
(ii) progressive erosion of articular cartilage,
(iii) infection by a microorganism,
(iv) due to metabolic defect causing excess of uric acid and urate crystals.

Structure of Contractile Proteins
i) Each actin (thin) filament is made of two 'F' (filamentous) actins helically wound to each other. Each 'F' actin is a polymer of monomeric 'G' (Globular) actins.
ii) Two filaments of another protein, tropomyosin also run close to the 'F' actins throughout its length.
iii) A complex protein Troponin is distributed at regular intervals on the tropomyosin. In the resting state a subunit of troponin masks the active binding sites for myosin on the actin filaments.

Each myosin (thick) filament is also a polymerised protein. Many monomeric proteins called Meromyosins constitute one thick filament. Each meromyosin has two important parts, a globular head with a short arm and a tail, the former being called the heavy meromyosin (HMM) and the latter, the light meromyosin (LMM).

The HMI component, i.e.; the head and short arm projects outwards at regular distance and angle from each other from the surface of a polymerised myosin filament and is known as cross arm. The globular head is an active ATPase enzyme and has binding sites for ATP and active sites for actin.

Pectoral and pelvic girdle help in the articulation of upper and lower limbs, respectively. Each girdle is made of two equal halves.
Each half of a pectoral girdle consists of clavicle and scapula. Scapula is a large triangular flat bone. There is glenoid cavity at the joint of scapula, clavicle and acromian process, which articulates with the head of humerus to form the shoulder joint.
Each half of pelvic girdle is formed by three bones, i.e., ilium, ischium and pubis. At the point of their fusion, there is a cavity called acetabulum to which the head of femur articulates.