The impact of obesity on skeletal muscle architecture in untrained young vs. old women

It is unknown whether loading of the lower limbs through additional storage of fat mass as evident in obesity would promote muscular adaptations similar to those seen with resistance exercise. It is also unclear whether ageing modulates any such adjustments. This study aimed to examine the relationships between adiposity, ageing and skeletal muscle size and architecture. A total of 100 untrained healthy women were categorised by age into young (Y) (mean ± SD: 26.7 ± 9.4 years) vs. old (O) (65.1 ± 7.2 years) and body mass index (BMI) classification (underweight, normal weight, overweight and obese). Participants were assessed for body fat using dual energy x‐ray absorptiometry, and for gastrocnemius medialis (GM) muscle architecture (skeletal muscle fascicle pennation angle and length) and size GM muscle volume and physiological cross‐sectional area (PCSA)] using B‐mode ultrasonography. GM fascicle pennation angle (FPA) in the obese Y females was 25% greater than underweight (P = 0.001) and 25% greater than normal weight (P = 0.001) individuals, while O females had 32 and 22% greater FPA than their underweight (P = 0.008) and normal weight (P = 0.003) counterparts. Furthermore, FPA correlated with body mass in both Y and O females (Y r = 0.303; P < 0.001; O r = 0.223; P = 0.001), yet no age‐related differences in the slope or r‐values were observed (P > 0.05). Both GM muscle volume (P = 0.003) and PCSA (P = 0.004) exhibited significant age × BMI interactions. In addition, muscle volume and PCSA correlated with BMI, body mass and fat mass. Interestingly, ageing reduced both the degree of association in these correlations (P < 0.05) and the slope of the regressions (P < 0.05). Our findings partly support our hypotheses in that obesity‐associated changes in GM PCSA and volume differed between the young and old. The younger GM muscle adapted to the loading induced by high levels of body mass, adiposity and BMI by increasing its volume and increasing its pennation angle, ultimately enabling it to produce higher maximum torque. Such an adaptation to increased loading did not occur in the older GM muscle. Nonetheless, the older GM muscle FPA increased to a similar extent to that seen in young GM muscle, an effect which partly explains the relatively enhanced absolute maximum torque observed in obese older females.