Balance is a term frequently used by health professionals working in a wide variety of clinical specialities. There is no universally accepted definition of human balance, or related terms. This article identifies mechanical definitions of balance and introduces clinical definitions of balance and postural control. Postural control is defined as the act of maintaining, achieving or restoring a state of balance during any posture or activity. Postural control strategies may be either predictive or reactive, and may involve either a fixed-support or a change-in-support response. Clinical tests of balance assess different components of balance ability. Health professionals should select clinical assessments based on a sound knowledge and understanding of the classification of balance and postural control strategies.

Figure 4. Tendon mechanical propertiesA, tendon force-displacement relationship; B, stress-strain relationship; C, stiffness-force relationship; D, Young's modulus-stress relationship. Data from one subject are presented.

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A technique has been developed which allows the assessment of the variation with time of the magnitude and direction of the resultant forces transmitted at joints in the human body during normal activity. Tests have been performed on joints in the lower limb and typical patterns of variation of force with time and leg position are shown for normal test subjects walking on level surfaces. The test procedure involves the measurement of external force actions transmitted to the leg and their correlation with photographic records. This information is processed to obtain the resultant force and moment actions at the joint, taking account of the effect of gravity and accelerations. The moment actions are transmitted by the development of tension in muscular or ligamentous connections crossing the joint and the corresponding reaction at the joint. Generally, the number of muscular or ligamentous connections exceeds the number of available equilibrium equations. A guide to the muscles which are acting at any instant is obtained by the myoelectric signals picked up by electrodes secured to the skin over the muscles. Even with this information, an explicit solution is generally not possible and the information is presented as curves of upper and lower limits of joint force corresponding to the use of muscles having the greatest or least moment arm.

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