Smaller Muscle Size Explains Age-Related Deficit in Knee Extensor Torque in Males But Not in Females

Muscle weakness is a well-established characteristic of older age and can be explained primarily by smaller muscle size. Specific torque (maximal torque per fat-free muscle size) is used to evaluate the relationship between muscle size and strength and is most often measured during isometric contractions. Although more pronounced weakness is observed at greater contraction velocities in older adults, differences in specific torque across a range of velocities generally are not evaluated, despite the functional relevance of dynamic contractions to mobility. Further, females typically have lower torque and muscle size compared with males, but the influence of sex on age-related differences in specific torque is not clear. Finally, intramuscular fat content is not always accounted for in studies of muscle size and specific torque. The purpose of this study was to characterize knee extensor muscle (KE) fat-free specific torque in young (20-40 yr) and older (65-80 yr) adults over velocities from 0-240°·s -1 . We hypothesized that specific torque would be lower in females compared with males and in older compared with young, and that these differences would be greatest at faster contraction velocities. To test these hypotheses, 27 young (33 ± 6 yr, mean ± SD, 52% female) and 50 older (71 ± 3 yr, 60% female) healthy participants completed magnetic resonance imaging and dynamometry testing. Serial KE fat-water images were collected along the length of the thigh at 3 Tesla. The KE muscle group was manually outlined on each slice, and the maximal, fat-free muscle cross-sectional area (mCSA, cm 2 ) was recorded. Peak KE torque (Nm) by dynamometry was determined as the greatest torque achieved during 2-3 maximal isometric contractions (0°·s -1 ), and during the isovelocity phase of sets of 3 maximal dynamic contractions at 120 and 240°·s -1 , with specific torque calculated as peak torque/mCSA (Nm·cm -2 ). To characterize habitual physical activity, participants wore an accelerometer for 7 days to determine daily minutes of moderate-to-vigorous physical activity (MVPA). The modified Short Physical Performance Battery characterized mobility. Statistics: 2-factor (age, sex) ANOVA, with velocity as a repeated measure for torque and specific torque. Daily MVPA did not differ across groups (young males [YM]: 41 ± 22 min, young females [YF]: 35 ± 21, older males [OM]: 38 ± 23, older females [OF]: 37 ± 22; p≥0.597). There were age and sex main effects but no interactions for overall mobility scores (YM: 2.92 ± 0.25, YF: 2.68 ± 0.25, OM: 2.43 ± 0.22, OF: 2.28 ± 0.27; p≤0.022). Torque decreased with increasing velocity, female sex, and older age (YM (0, 120, 240°·s -1 ): 236 ± 56 Nm, 165 ± 43, 130 ± 43; YF: 154 ± 32, 109 ± 21, 80 ± 17; OM: 167 ± 40, 123 ± 21, 94 ± 18; OF: 102 ± 17, 69 ± 12, 51 ± 10; p<0.001, all). mCSA was lower in females and older (YM: 75 ± 13 cm 2 , YF: 49 ± 9, OM: 57 ± 8, OF: 38 ± 5; p<0.001, all). Specific torque decreased with velocity (p<0.001), was lower in older females (0, 120, 240°·s -1 : 2.7 ± 0.4 Nm·cm -2 , 1.8 ± 0.3, 1.4 ± 0.2) compared with all other groups (p≤0.009), and was not different between young males (3.1 ± 0.6 Nm·cm -2 , 2.2 ± 0.4, 1.7 ± 0.4), young females (3.1 ± 0.4 Nm·cm -2 , 2.2 ± 0.3, 1.6 ± 0.3), or older males (2.9 ± 0.5 Nm·cm -2 , 2.2 ± 0.2, 1.7 ± 0.2; p≥0.883). There were no velocity x age or velocity x sex interactions for specific torque (p≥0.204). Our specific torque results indicate that, in adults with comparable physical activity habits, KE weakness in older males was explained by their smaller muscle size, but that factors other than size exacerbate KE weakness in older females. Further, the low mobility score in older females suggests that the deficit in KE specific torque has implications for mobility in this segment of the population. NIH R01 AG058607, NIH R01 AG047245, NIH R21 AR073511 This abstract was presented at the American Physiology Summit 2025 and is only available in HTML format. There is no downloadable file or PDF version. The Physiology editorial board was not involved in the peer review process.