Centre of Gravity (COG)

1. Definition: The centre of gravity is the single point in a body (or object) where the entire body weight is considered to act vertically downward, regardless of the body's position or orientation.
2. Location in the Human Body: In the anatomical standing position, the COG is located approximately at the level of S2 vertebra, just anterior to it, roughly at 55-57% of a person's total height from the ground.
3. Stability and COG: The lower the COG, the greater the stability of the body. Broad-based postures (e.g., wide stance) lower the COG and enhance balance - this is clinically relevant in gait training and balance rehabilitation.
4. Dynamic Nature: The COG is not fixed; it shifts with every change in body posture and limb position. For example, raising both arms overhead shifts the COG upward, while bending forward shifts it anteriorly.
5. Clinical Relevance: Understanding COG helps physiotherapists assess postural deviations (e.g., scoliosis, kyphosis), design balance and proprioception training, and analyze gait abnormalities in neurological and musculoskeletal conditions.

Line of Gravity (LOG)

1. Definition: The line of gravity is an imaginary vertical line that passes through the centre of gravity of a body downward toward the centre of the earth, representing the direction in which gravity acts on the body.
2. Relationship with Base of Support (BOS): For a body to remain in a stable, balanced position, the LOG must fall within the base of support. If the LOG falls outside the BOS, the body loses balance and topples in that direction.
3. Anatomical Landmarks (Standard Plumb Line): In ideal standing posture, the LOG passes through: mastoid process -> through the shoulder joint -> through the hip joint (or just posterior to it) -> just anterior to the knee joint -> just anterior to the lateral malleolus.
4. Effect of Posture on LOG: Any postural deviation shifts the LOG. In an anteriorly tilted pelvis or forward head posture, the LOG is displaced forward, increasing the demand on postural muscles to maintain equilibrium.
5. Clinical Application: Physiotherapists use the plumb line assessment to evaluate postural alignment. Deviations of the LOG from ideal alignment guide treatment planning in conditions like low back pain, cerebral palsy, Parkinson's disease, and post-stroke rehabilitation.