Usefulness of the bioimpedance phase angle in identifying older adults with poor muscle properties: The Shizuoka study.

Leg PhA was more strongly associated with conventional physical performance measures compared with PhA of other body segments.

• Cross-sectional study of 1779 community-dwelling individuals aged 65 years or older
• Physical performance was assessed using handgrip strength, gait speed, and the five-times chair-stand test
• Segmental PhA was measured by bioelectrical impedance analysis across multiple body segments
• Leg PhA showed the strongest association among all segmental PhA measurements with physical performance

Among participants with low leg PhA, the majority (64%) had normal physical performance, indicating that low leg PhA captures muscle deterioration not detected by performance tests alone.

• Low leg PhA was defined as less than 4.5° in men and less than 4.0° in women
• 64% of participants meeting the low leg PhA threshold had normal physical performance scores
• This finding suggests that conventional physical performance assessments alone miss a substantial proportion of individuals with poor muscle characteristics

Mid-thigh skeletal muscle cross-sectional area was significantly lower in groups with low physical performance, low leg PhA, and both combined.

• Participants were divided into four groups based on low leg PhA and low physical performance status
• Low physical performance alone was associated with lower muscle cross-sectional area (β = -0.12, P < 0.001)
• Low leg PhA alone was associated with lower muscle cross-sectional area (β = -0.10, P < 0.001)
• Having both low leg PhA and low physical performance showed the greatest reduction in muscle cross-sectional area (β = -0.17, P < 0.001)

Fat infiltration into muscle (mean attenuation value on CT) was significantly decreased in groups with low leg PhA but not in the group with only low physical performance.

• Mean attenuation value on computed tomography images at the midthigh was used as the measure of fat infiltration into muscle
• Low leg PhA alone was associated with decreased mean attenuation value (β = -0.22, P < 0.001)
• Having both low leg PhA and low physical performance was also associated with decreased mean attenuation value (β = -0.19, P < 0.001)
• Low physical performance alone was not significantly associated with mean attenuation value (β = -0.03, P = 0.330)
• This indicates that leg PhA specifically captures intramuscular fat infiltration that physical performance tests do not detect

Computed tomography images at the midthigh were used to assess both skeletal muscle mass and fat infiltration into muscle as reference measures of muscle quality.

• CT images were taken at the midthigh level
• Skeletal muscle cross-sectional area served as the representative measure of muscle mass
• Mean attenuation value served as the representative measure of fat infiltration into the muscle
• These CT-derived measures provided objective validation of the muscle properties captured by leg PhA