Calcium plays a key regulatory role in muscle contraction. These ions bind to troponin causing change in its shape and position. This in turn alters the shape and position of tropomyosin. This shift exposes the active sites on the F-actin molecules and myosin cross-bridges able to bind to these active sites.
The complete process is outlined in the figure below
Role of calcium ion, is the contraction and relaxation process. The head of each myosin molecule contains an enzyme myosin ATPase. In the presence of myosin ATPase, $\mathrm{Ca}^{2+}$ and $\mathrm{Mg}^{2+}$ ions, ATP breaks down into ADP and inorganic phosphate as
$$\text { ATP } \xrightarrow[\mathrm{Ca}^{2+}, \mathrm{Mg}^{2+}]{\text { Myosin ATPase }} \mathrm{ADP}+\mathrm{P}_{\mathrm{i}}+\text { Energy }$$
Energy from ATP causes energised myosin cross-bridges to bind with actin.
The pectoral and pelvic girdle are responsible in providing support to the upper and lower body portions
| Pectoral Girdle | Pelvic Girdle |
|---|---|
| It occurs in the shoulder region, hence also called as shoulder girdle. | It occurs in the hip region, hence also called as hip girdle. |
| Pectoral girdles are divided into two parts, i.e., one clavicle and one scapula. | There is one pelvic girdle, which is formed by two, innominate bones. Each bone consist of three parts. i.e., ilium, ischium and pubis. |
| Image | Image |
| Clavicle and scapula helps in articulation of the upper limb with axial skeleton. | The innominate at the middle of its lateral surface has a deep, cup shaped acetabulum. where head of the femur articulates the two halves of the pelvic girdle and meet ventrally to form public symphysis. |
| It has no articulation with the vertebral column. | It has articulation with vertebral column. |
| Bones associated with pectoral girdle are light, as they are not subjected to much stress. | Bones associated with pelvic girdle are hard as they are subjected to much stress |
| There perform like holding, lifting | There function like running, standing, jumping |