Pontos de corte para diagnóstico de sarcopenia em idosos a partir da força muscular de membros superiores e inferiores com ajustes alométricos

Abstract

This thesis defends the normalization of strength and muscle mass by body size as an efficient strategy to minimize the bias of body dimensions on the functionality of the older adults. The thesis consisted of three specific objectives answered in four original studies. The first objective was to propose allometric exponents to normalize the strength of upper and lower limbs by body size and generate cutoff points for muscle weakness in the older adults, which was answered in two original studies. In Study I (Identification of muscle weakness in older adults from normalized upper and lower limbs strength: A cross-sectional study), which involved 94 older people (95% CI: 68.2 to 73.9 years) of both sexes, were muscle strength measurements were obtained (hand grip, dynamic and isokinetic knee extension), body size (anthropometry, dimensions, and indexes) and mobility. Strategies for normalization (strength/body size) and allometry (strength/body sizeb; b being the allometric exponent) associated with the mobility of the older adults were applied. Were generated 49 valid models to identify muscle weakness. Normalization increased the precision of the cut-off points in women but not in men. However, adjustments in men also made muscle strength independent of body size, reducing the bias for extreme cases. This implied a lower risk of attributing a falsepositive/ negative diagnosis to muscle weakness. And in Study II (Allometrically adjusted grip strength to identify low strength from 13,235 older adults of low- and middle-income countries), which involved older adults of both sexes from six developing countries, the normalization of muscle strength by body dimensions was replicated in different populations. The methods were like Study I, but only involved measurements of handgrip strength, height, and body mass. The non-linear relationship between strength and body mass was confirmed, except for height. Allometric adjustments made muscle strength independent of body size, and their variability highlighted the need for country-specific cutoff points. The values of allometric exponents generated for each country were very close to, confirming the universal effectiveness of this strategy. Our second objective was to comparatively apply the allometric exponents proposed in Study I and others reported in the literature, to test their effectiveness in normalizing strength and identifying muscle weakness. Thus, in Study III (Foreign allometric exponents suitably normalize isokinetic knee extension strength to identify muscle weakness and functional limitation in Portuguese older adults: A cross-sectional study) 132 Portuguese older adults of both genders performed tests of mobility, isokinetic strength of breast extension. knee and body dimension measurements to normalize strength (strength/body sizeb). Cutoff points for muscle weakness (ROC curve) were defined based on the normalized isokinetic knee extension strength, or not, identified by the lowest mobility quartile. The absolute strength cut-off points showed insufficient accuracy (AUC<0.70). Allometric exponents, although foreign (three Brazilians and one North American), improved the diagnostic accuracy of muscle weakness. We conclude that normalizing isokinetic knee extension muscle strength, even with the use of foreign allometric exponents, is better than no adjustment. Our third objective was to seek a simplified strategy to identify low muscle mass during aging, based on functional limitation for the risk of sarcopenia. Thus, for Study IV (Normalizing Calf Circumference to identify low Skeletal Muscle Mass in Older Women: A Cross-sectional Study) cut-off points were proposed for the calf perimeter, normalized by body size, to identify low skeletal muscle mass in older women. Calf circumference values for young women (n=78) were used as a reference for low muscle mass cutoff points (-2 standard deviation). The results showed that the calf perimeter normalized by BMI identified low muscle mass with greater accuracy than the absolute values. Normalization removed the usual bias of the inverted U-to-mobility ratio (6MWT), usually seen in absolute values. The accuracy obtained supported the use of PP·BMI-1 to identify low muscle mass in older women. In conclusion, the proposed allometric strategy avoids errors in the classification of low muscle strength in the older adults, resulting from the bias caused by body size. Possibly, this considerably reduces the chances of diagnosing false positive and negative cases of sarcopenia in body dimensions of older people with extreme values.