Age-related changes in quadriceps stiffness and strength: a longitudinal observation in community-dwelling older adults

In aging muscles, altered elastin and collagen type III and extracellular matrix may be responsible for the age-related increase in muscle stiffness, which may compromise muscle force production. Indeed, cross-sectional studies have compared quadriceps stiffness between older and younger adults, yet their findings are contradictory. We aim to longitudinally observe quadriceps stiffness/strength changes, and to examine how those changes are associated in elders. This was a prospective longitudinal observation. Community-dwelling adults were recruited if they were aged 60–80 years and capable of walking 10 m independently. We excluded participants with knee injury/surgery, knee/hip joint replacement, cognitive impairment, or neurological conditions. Quadriceps stiffness and strength were assessed at baseline and the follow-up, using shear-wave ultrasound elastography and a Cybex dynamometer, respectively. Paired t-tests were used to examine changes in quadriceps stiffness and strength from baseline to the follow-up. Linear regression was performed to investigate the association between age-related changes in quadriceps stiffness and strength. Repeated measures ANOVA was used to assess differences in stiffness changes across rectus femoris (RF), vastus lateralis (VL), and vastus medialis (VM). A total of 172 eligible participants (age: 65.7 ± 4.0 years; 64.0% females) completed the baseline and 18-month follow-up assessments. Quadriceps stiffness increased by 0.76 (95% CI: 0.56, 0.97) kPa, while quadriceps strength decreased by -0.15 (-0.18, -0.12) Nm/kg. Estimated annual changes in quadriceps stiffness and strength were 9.14% (7.02, 11.26) and − 7.95% (-9.78, -6.12), respectively. Greater increase in quadriceps stiffness was significantly associated with larger strength reduction in females (R²adjusted = 0.106; P = 0.021). RF stiffness underwent the most obvious increase, followed by VL and VM (1.41, 0.62, 0.26 kPa, P < 0.001). However, only increased quadriceps stiffness (0.76 > 0.71 kPa), particularly the RF stiffness (1.41 > 1.24 kPa), exceeded minimum detectable changes over this 18-month observation. Quadriceps stiffness is increased with aging, and it is associated with strength decrements in female older adults. Age-related increases in quadriceps stiffness are most prominent in RF, followed by VL and VM. Alongside muscle strengthening, interventions for reducing age-related increases in quadriceps stiffness may be important in geriatric rehabilitation.