ATP (Adenosin Triphosphate) is the source of energy for muscle contraction. The head of each myosin molecule contains an enzyme called myosin ATPase.
In the presence of this enzyme along with $\mathrm{Ca}^{2+}$ then, and $\mathrm{Mg}^{2+}$ ions the ATP molecule breaks down into ADP and inorganic phosphate, thus releasing energy in the head of myosin.
$$\text { ATP } \longrightarrow \text { ADP }+P_i+\text { Energy }$$
Energy from ATP causes energised myosin to cross bridges and to bind with actin and in this way initiates muscle contraction.
Pectoral girdle Each half of the pectoral girdle consist of a clavicle and a scapula. The dorsal flat, triangular body of scapula has a slightly elevated ridge called the spine that, projects a flat expanded process called the acromion and the clavicle articulating with it.
Below the acromion their is a depression called the glenoid cavity which articulates with the head of the humerous to form the shoulder joint. Pelvic girdle consist of two coxal bones. each formed by the fusion of three bones, ilium, ischium and pubis. It articulates with femur through a cavity called acetabulum forming thigh joint.
For the muscle fibre to contract, the binding site on thin filaments must be uncovered. This occurs when $\mathrm{Ca}^{2+}$ bind to another set of regulatory proteins, called troponin complex which control the position of tropomyosin on the thin filament.
The calcium binding rearranges the tropomyosin, troponin complex, exposing the myosin-binding sites on the thin filament. When $\mathrm{Ca}^{2+}$ is present in the cytosol, the thin and thick filament slide part each other resulting in muscle contraction.
Similarly, when the $\mathrm{Ca}^{2+}$ concentration falls, the binding sites get covered and contraction stops. In case of tetany there occur low calcium levels in body fluid due to diminished function of parathyroid gland. This gland is mainly involved in the secretion of parathyroid hormone which is associated in regulating calcium levels in blood. Tetany results in periodic painful muscular spasm (wild contraction) and tremors.
Slipped disc is a medical condition in which spine is affected due to wear and tear in the outer fibrous ring (anulus fibrosus) of an intervertebral disc, allowing the soft, central portion to bulge out beyond the damaged outer rings.
These intervertebral disc are present between the bodies of adjacent vertebrae from the second cervical vertebra to the sacrum. This discs form strong joints that permit various movements of vertebral column and absorbs vertical shock.
The cause of slip disc can be due to general wear and tear of intervertebral disc during performing various jobs, that require constant sitting and squatting. The slipped disc or herniation occurs in two regions of body, i.e., cervical disc and lumber disc.
Slip disc in lower back lumber disc, lead to sharp pain in one part of leg due to sciatica, (disturbance in sciatic nerve), hip and cause numbness in other lower parts of the body.
Slip disc in neck region (cervical disc) leads to pain while moving neck near or over the shoulder bone or pain occurs while moving forearm, or fingers. It also causes numbness in shoulder, elbow, forearm and finger area. Hence, slipped disc affect the upper and lower body parts, thus, influencing life style and health.