THE DYNAMIC NATURE OF HUMAN STANDING
Joseph Mizrahi (Dept. of Biomedical Eng., Technion - Israel Institute of Technology, Haifa, Israel
Bipedal standing is an unstable position that necessitates a continuous regulation process, involving periodic contractions of muscles in the lower limb and trunk. The direct result of this continuously acting stabilizing system is the existence of body sway in the sagittal, coronal and transversal planes.
Measurement of postural balance has been used in the past to assess stability in bipedal standing and walking. Postural sway tests while standing still are relatively easy to carry out, demanding minimal cooperation and effort from the tested subjects or patients. These tests also provide valuable information on the mechanisms of unperturbed balance in normal and pathological conditions. This particularly applies to patients with deficiencies in the locomotor system, such as cerebrovascular accident (CVA) patients, craniocerebral injured (CCI) patients, and amputees wearing their prostheses, as soon as they become able to stand without support.
This paper discusses the parameters which describe standing balance of able-bodied subjects and of subjects with neuromuscular and skeletal deficiencies. A special emphasis is given to the measurement of the foot-ground reaction forces in standing still. These are easily measurable quantities and they provide a natural input to biomechanical models on human standing. At the same time, they can form the basis for the clinical characterization of standing posture in subjects with balance pathologies.
CHANGES OVER EIGHT YEARS OF MUSCLE STRENGTH, MUSCLE MORPHOLOGY AND MOTOR UNIT SIZE IN SUBJECTS WITH POST-POLIO SEQUELAE
Gunnar Grimby (Department of Rehabilitation Medicine, Goteborg University, Sweden) and Erik Stbalberg (Department of Clinical Neurophysiology, Uppsala University, Sweden)
The purpose of the study was to follow longitudinally the adaptive processes for loss of motor units in subjects with post-polio sequelae and get further insight in the pathophysiological mechanisms behind a progressive loss of muscle strength.
Twenty-four subjects who had had polio 24-51 years prior to the first examination were studied on three occasions, each 4 years apart. Isometric and isokinetic muscle strength and endurance were measured for knee-extension. Muscle biopsies were taken from vastus lateralis for histochemical, enzymatic analysis, and also Macro-EMG.
On the average, mean muscle strength decreased 8-15% during the 8 years, with larger decrease in legs where new weakness was acknowledged. Muscle endurance decreased. Evidence of reduced oxidative potential was revealed. Muscle fiber size was markedly increased in most subjects but with changes in both directions. Macro-EMG, reflecting the number of muscle fibers in a motor unit, was increased in all subjects (4-42 times control). Legs with macro-EMG amplitudes above 20 times control showed decrease at the next examination.
An upper limit of the compensatory processes to maintain muscle strength may be defined and the results may be used to identify subjects at risk for progressive loss of muscle strength.