Kinesiology and Biomechanics III: Equilibrium and Stability, Posture and its Deformities with their Corrective Exercises, and the Kinesiological, Muscular and Mechanical Analysis of Fundamental Movements and of Major Sports Skills
2. Posture and its deformities, with corrective exercises
Posture is the relative arrangement of the body's parts in standing, sitting, lying and moving. Good posture balances the body segments over the base of support with the least muscular effort and strain on joints and ligaments; seen from the side, a plumb line should pass through the ear lobe, the tip of the shoulder, the greater trochanter of the hip, just in front of the centre of the knee and just in front of the ankle bone. Good posture looks well, allows efficient breathing and movement and reduces strain; poor posture results from habit (slouching, heavy school bags, long screen use), muscle weakness and imbalance, fatigue, obesity, ill-fitting furniture and footwear, disease, injury, heredity and emotional state. Postural defects are functional when they are habits that disappear when the person is lying down or consciously corrects them, and structural when the bones themselves are altered, which needs medical assessment rather than exercise alone.
| Deformity | Description | Common corrective exercises |
|---|---|---|
| Kyphosis (round back) | Exaggerated backward curve of the thoracic spine | Strengthen upper-back extensors and scapular retractors; stretch the chest; dhanurasana, chakrasana, bhujangasana |
| Lordosis (hollow back) | Exaggerated forward curve of the lumbar spine, often with a forward-tilted pelvis | Strengthen the abdominals and gluteals; stretch the hip flexors and lower back; halasana, pelvic tilts |
| Scoliosis | Sideways curvature of the spine, C- or S-shaped | Functional cases: symmetrical and prescribed asymmetrical stretching and strengthening, hanging from a bar, swimming; structural cases need medical management |
| Round shoulders | Shoulders drawn forward, often with kyphosis | Strengthen rhomboids and middle trapezius; stretch the pectorals; chest-opening exercises |
| Flat foot (pes planus) | Lowered or absent medial longitudinal arch | Walking on toes and heels, picking up marbles or a towel with the toes, heel raises, skipping, rolling a ball under the sole |
| Knock knees (genu valgum) | Knees touch while the ankles stay apart | Strengthen the hip abductors and outer thigh; horse-riding, padmasana and gomukhasana are commonly recommended in Indian texts |
| Bow legs (genu varum) | Knees stay apart when the ankles touch | Walking on the inner edges of the feet and strengthening the thigh muscles; garudasana is commonly recommended in Indian texts |
3. Analysing fundamental movements
A kinesiological analysis identifies the phases of a movement and, for each phase, the joints involved, the movements at each joint, the plane and axis, the muscles acting as agonists and the type of contraction; a mechanical analysis adds the forces, levers, the path of the centre of gravity and the mechanical principles that explain success or failure. Most skills can be divided into a preparatory phase, which positions the body and stores elastic energy, a force-producing or execution phase, and a recovery or follow-through phase. Two principles recur. Summation of forces: maximal speed at the end of a chain is produced when body segments act in sequence from the largest and slowest (legs and hips) to the smallest and fastest (arm, hand, implement), each starting as the previous one reaches its peak speed, as in throwing, kicking and striking. The stretch–shortening cycle: a rapid eccentric stretch immediately before a concentric action, as in the countermovement of a jump, increases force.
Walking is a cycle of stance and swing phases with a period of double support when both feet are on the ground; running has no double support but a flight phase, and as speed rises the stance phase shortens. Running speed = stride length × stride frequency: a sprinter with a stride of 2.4 m at 4.5 strides per second runs at 10.8 m/s. In the stance phase of running, the hip, knee and ankle extensors (gluteus maximus, quadriceps and calf muscles) drive the body forward, working first eccentrically to absorb the landing and then concentrically to push off; in the swing phase the hip flexors bring the thigh through and the hamstrings flex the knee and then decelerate the lower leg before contact. In jumping the countermovement, flexing the hips, knees and ankles quickly before extending them together (triple extension) with an arm swing, raises the height reached. In an overarm throw the sequence runs from the legs, through hip and trunk rotation, to the shoulder, elbow extension by the triceps, and wrist flexion; in an instep kick the approach, the placement of the support foot beside the ball, the backswing of the kicking leg and forceful knee extension by the quadriceps follow the same summation.
4. Mechanical analysis of major sports skills
Sprint start: in the set position the hips are slightly above the shoulders and the CG is well forward over the hands, deliberately unstable; at the gun the athlete drives against both blocks (Newton's third law) with a long impulse from the rear and front legs, the arms driving vigorously in opposition, and the body rises gradually through the acceleration phase. Long jump: a fast, accurate approach; a take-off in which the last strides lower the CG slightly and the take-off leg plants briefly, converting some horizontal speed into vertical speed at an angle of about 18–25°; a flight phase in which the path of the CG is fixed and techniques such as the hang and the hitch-kick only control rotation; and a landing that extends the legs forward while bringing the CG over the feet. High jump: the Fosbury flop, introduced by Dick Fosbury at the 1968 Mexico City Olympics, uses a curved approach whose lean produces rotation, a take-off from the outside foot, and a back-first arched layout over the bar, which allows the body to clear the bar while its centre of gravity passes close to, or even below, the bar.
Shot put: whether by the glide (the O'Brien technique, a backward glide across the circle) or the rotational technique, the aim is to maximise release speed by lengthening the path over which force is applied and summing forces from legs through hips, trunk, shoulder and arm, with a release at about 2 m and 35–38°, and a reverse (change of feet) after release to stay in the circle. Swimming front crawl: a streamlined, horizontal body position minimises drag; the hand enters in front of the shoulder, then catches, pulls and pushes backward under the body in an elongated path, producing propulsion from both drag and lift forces, while body roll about the longitudinal axis helps the recovery and breathing and a flutter kick stabilises the body. Basketball jump shot: the shot is released near the top of the jump with the shooting arm extending and the wrist flexing, and a higher arc gives the ball a larger effective target at the ring. Cricket fast bowling: the run-up builds momentum, the delivery stride braces the front leg to transfer it up through a rigid front leg, trunk and a straight bowling arm, and mixed actions that twist the spine are linked to low-back stress fractures in young bowlers.
| Skill | Key mechanical principle | Typical fault |
|---|---|---|
| Sprint start | Unstable set position; action–reaction against the blocks | Standing up too early, losing drive |
| Long jump | Projectile of the CG; take-off about 18–25° | Slowing down to hit the board |
| Fosbury flop | Arched layout lets the CG pass near or below the bar | Taking off too close to the bar |
| Shot put | Summation of forces; release speed, height and angle | Arm-only throw without leg and hip drive |
| Front crawl | Streamlining to reduce drag; drag and lift propulsion | Head up, hips and legs sinking |
Key takeaways
- Stability increases with a wider base, a lower centre of gravity, a line of gravity near the base's centre and greater mass; a sprinter in the set position deliberately gives it up.
- Good posture aligns ear, shoulder, hip, knee and ankle; kyphosis is a thoracic, lordosis a lumbar and scoliosis a sideways curve; corrective exercise suits functional defects and structural ones need medical care.
- A movement analysis names phases, joints, movements, planes, muscles and contraction types, then adds forces, levers and mechanical principles.
- Summation of forces from large to small segments and the stretch–shortening cycle explain throwing, kicking, striking and jumping; running speed = stride length × stride frequency.
- The Fosbury flop (1968) lets the CG pass near or below the bar; long jumpers take off at about 18–25°; the shot is released at about 35–38°.
Practice questions (10)
Attempt each one before opening the answer. Every explanation names the tempting wrong option as well as the right one, because that is where marks are lost.
A sprinter's stride length is 2.4 m and stride frequency is 4.5 strides per second. What is the running speed in metres per second? Type the number.
Numerical answer — type the value.
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Answer: 10.8
Running speed = stride length × stride frequency = 2.4 × 4.5 = 10.8 m/s. Speed can be raised by lengthening the stride, quickening it, or both, but in practice one tends to fall as the other rises.An exaggerated forward curve of the lumbar spine, giving a "hollow back", is called
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Answer: C — lordosis
Lordosis is the exaggerated lumbar curve. Kyphosis is the thoracic "round back", scoliosis a sideways curve and genu valgum knock knees.Which change would make a wrestler more stable against being pushed from the front?
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Answer: D — Lowering the centre of gravity and widening the base in the front-to-back direction
Stability rises with a lower centre of gravity and a base widened in the direction of the expected force, with the line of gravity kept near the centre. Standing tall, raising the arms or leaning towards the edge all reduce stability.The Fosbury flop allows a high jumper to clear a bar with less rise of the centre of gravity because
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Answer: A — the arched layout lets body parts pass over the bar in sequence while the centre of gravity passes close to or even below it
Arching over the bar places the centre of gravity outside the body, so the body can go over while the CG stays near or below the bar. The CG's path cannot change after take-off, and the approach alters neither mass nor gravity.Which corrective measures are appropriate for functional kyphosis (round back)? Select all that apply.
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Answer: A — Stretching the chest (pectoral) muscles; D — Strengthening the upper-back extensors and scapular retractors
Kyphosis is corrected by strengthening the muscles that pull the spine and shoulders back and stretching the tight chest muscles, with backbends such as dhanurasana and bhujangasana. Forward bending and one-sided loads worsen posture.Assertion (A): In an overarm throw, the fastest release is achieved when the legs, hips, trunk, shoulder, elbow and wrist act in sequence. Reason (R): In the summation of forces each segment starts to move as the previous, larger segment reaches its peak speed. Choose the correct option.
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Answer: A — Both A and R are true, and R is the correct explanation of A
Both are true and R explains A: sequencing from large, slow segments to small, fast ones transfers and builds speed along the chain, so the hand and ball leave at the highest speed. Moving all segments at once wastes that transfer.In a good standing posture viewed from the side, the plumb line should pass through all of the following EXCEPT
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Answer: C — the tip of the nose
The side-view plumb line passes through the ear lobe, the tip of the shoulder, the greater trochanter, just in front of the knee's centre and just in front of the ankle bone. The nose lies well in front of this line.During the stance phase of running, the quadriceps first lengthen under tension as the knee bends on landing and then shorten to extend the knee. These two actions are, in order,
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Answer: B — eccentric, then concentric
Lengthening under tension to absorb landing is eccentric, and shortening to push off is concentric; together they form a stretch–shortening cycle. Reversing the order describes the wrong phase of the stride.Which of the following describe the long jump correctly? Select all that apply.
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Answer: B — Once airborne, the path of the centre of gravity cannot be changed by the hitch-kick; D — The take-off angle is typically about 18–25°, well below 45°
Long jumpers take off at about 18–25° because they cannot convert high horizontal speed into a steep take-off, and flight techniques only control rotation. The CG's parabola is fixed at take-off, so neither the hitch-kick nor pushing on air adds distance.A gymnast's weight of 500 N acts 0.3 m horizontally from the hands, which act as the axis. What is the torque about the hands in newton-metres? Type the number.
Numerical answer — type the value.
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Answer: 150
Torque = force × perpendicular distance from the axis = 500 × 0.3 = 150 N·m. If the weight acted directly over the hands the distance would be zero and there would be no turning effect, which is why a balanced handstand keeps the CG over the base.