Posture and balance. Center of gravity. Dynamic nature of center of gravity. John Milton BIO-39 November 7, 2017

Transcription

1 Posture and balance John Milton BIO-39 November 7, 2017 Center of gravity The center of gravity (COG) of the human body lies approximately at the level of the second sacral vertebrae (S2), anterior to the sacrum Body s center of mass is considered to be center of gravity Thus changes from person to person depending on their build Dynamic nature of center of gravity 1

2 Maintenance of balance Without moving Person s line of gravity must fall within the base of support Base of support: the area beneath the body that is encompassed when you connect, via one continuous line, all points of the body that are in contact with the ground (e.g. the space between the feet if standing) Moving (walking, running) Involve shifting the line of gravity beyond the base of support, then moving to re-establish a new base of support Controlled fall!! Wide base of support allows a wide excursion of the line of gravity without permitting it to fall outside of the base of support COP not same of COG As muscle activity changes in ankles and hips, the location of the COP in relation to COG continually changes In pendulum model of balance, the COP is considered to be under neural control 2

3 COP is under neural control If COP (R) moves anterior to COG (W), the COG accelerates backwards If COP (R) moves posterior to COG, then COG accelerates anteriorly Neutral alignment Parts of the body are balanced and symmetrical around the line of gravity Line of gravity passes just anterior to the ankle, though the knee, hip and shoulder joints and through the external meatus of the ear Assess using a plumb line Deviations from neutral alignment May be permanent or temporary Important to recognize because over time, the tissues (and even the bones) adapt to these postures, and cause the deviations to become permanent Individuals with any of these conditions should be referred to a physician or therapist 3

4 Role of exercise for management of postural deviations Can be improved with exercise if Caused by muscle or connective tissue imbalances Cannot be changed with exercise if Caused by abnormalities in bone Geometric representation of body Depends mainly on the information provided by the proprioceptive Ia afferents Vibration to the eye muscles, the neck muscles, or the ankle muscles of a standing subject with eyes closed produce body sway (direction depends on muscle vibrated) (no effect if eyes are open) Implies that proprioception from all parts of the body plays an important role in maintenance of quiet stance body posture Orientation of body with respect to vertical No static sensory receptors that directly monitor the center of gravity projection to the ground Two principle sources of sensory information are important regarding orientation of head position Labyrinthine sensors (otoliths) Visual sensors BUT 4

5 Neck muscle proprioceptors are more important for balance control The visual and labyrinthine inputs are located in the head, and they help orient head position However, since the position of the head is not fixed, their influence on body posture and more specifically on balance control depends on the evaluation of head position with respect to the trunk Is center of gravity regulated? Direct regulation of the center of gravity by displacement of body segments is common finding in human equilibrium control In cats, the primary factor regulated during balance control is vertical axis of legs Deviations in posture from normal alignment: Opposing effects of antagonistic pairs of muscles IMPORTANT SLIDE example 5

6 Stability of the pelvis: IMPORTANT SLIDE Line of transmission of both the trunk force from above and the ground reaction force from below pass anterior to the SIJs Clinical example: Duchenne muscular dystrophy Gower s sign 6

7 Analysis of stance in Duchenne Muscular Dystrophy Earliest weakness occurs in gluteus maximus (hip extensor) and hip abductors Consequently child must hyperextend spine to maintain weight line behind hips Weakening quadriceps implies that weight line must alone be behind knee Hypertrophy of calf muscles arises to help maintain this posture Posture analysis For a given type of posture Many combinations of muscle weakness and tightness can give rise to same posture Thus we identity the muscles that are typically weak and tight in each posture Kyphosis-lordosis Increase in the normal inward curve of the low back Accompanied by: Protruding abdomen and buttocks Increased flexion (outward curve) of thoracic spine Rounded shoulders Forward-tilted head 7

8 Lordosis (findings) Possible tight muscles Lower back (erectors) Hip flexors Possible weak muscles Abdominals (especially obliques) Hip extensors Kyphosis (findings) Possible tight muscles Internal oblique Shoulder adductors (pectoralis; latissimus dorsi) Intercostals Possible weak muscles Erector spinae of thoracic spine Scapular adductors (mid/lower trapezius) Flat back posture Decrease in the normal inward curve of the lower back Pelvis is in posterior tilt 8

9 Flat-back (findings) Possible tight muscles Upper abdominals Hip extensors Possible weak muscles Lower back (erectors) Hip flexors Sway-back Long outward curve of the thoracic spine with an accentuated lumbar curve and a backward shift of the upper trunk Accompanied by: Rounded shoulders Sunken chest Forward-tilted head Sway-back (findings) Possible tight muscles Upper abdominals Hip flexors Possible weak muscles Oblique abdominals Hip extensors 9

10 Scoliosis Lateral curve of the spine Usually two curves, on opposite sides of the spine, which compensate for each other Activation of muscles that maintain posture following perturbations (speed: e>d>c>b>a) Different strategies to correct following sudden postural deviation 10

11 Ankle strategy Movements that restore COG to position of stability occur mainly at ankle Appears to be used most commonly in situations in which the perturbation is small and support surface is firm Older adults often have impaired ankle strategies Weak ankle dorsi and plantar flexors? Hip strategy Controls COG by producing large and rapid motions at the hip joints with antiphase reactions of the ankles Used to restore equilibirum in response to larger, faster perturbations or when the support surface is compliant or smaller than the feet, e.g. standing on a beam Hip strategy seems to dominate in elderly adults May be related to weak ankles or loss of peripheral sensory function Stepping strategy When in-phase strategies (e.g. ankle or hip strategies) are not sufficient, a step or hop is used to bring the support base into alignment under the COG 11