Considering the large population of middle-aged adults, it is important to understand the age-related change in lower limb muscles and the possible mechanisms before old age (> 65 years old). The purpose of this study was to investigate age-related neural and muscular alterations of the plantar flexors in young and middle-aged women. Twenty two middle-aged (54.0 ± 5.8 yrs) and 17 young (21.8 ± 1.4 yrs) recreationally active women performed rapid maximal voluntary isometric contractions (MVIC) of the plantar flexors. Absolute and normalized rate of torque development (RTD) and electromyography (EMG) were examined. Electrical stimulation was used to examine voluntary activation and contractile properties of the muscle. Ultrasonography was used to examine medial and lateral gastrocnemius muscle thickness and pennation angle. A 6-minute walk and sit to stand task were also performed by all participants. The middle-aged women had significantly lower MVIC torque (141 ± 49 vs. 109 ± 30 Nm, P = 0.031), absolute RTD (753.0 ± 313.6 vs. 423.0 ± 156.1 Nm/s, P = 0.001), and normalized peak RTD (554.0 ± 191.0 vs. 388.0 ± 91.9% MVIC/s, P = 0.001). Normalized early RTD 0 – 50 and late RTD 100 – 200, voluntary activation, and EMG were similar between groups. Resting twitch data showed that time to peak (124.0 ± 20.4 vs. 143.0 ± 16.7 ms, P = 0.002) and half relaxation time (73.1 ± 15.2 vs. 107.0 ± 28.2 ms, P < 0.001) was significantly faster for the young women. Thickness was greater in the lateral gastrocnemius (1.6 ± 0.2 vs. 1.4 ± 0.2 cm, P = 0.006) for the young women. Pennation angle of both muscles were greater for the young women (15.8 ± 3.9 vs. 13.1 ± 2.7 degrees, P < 0.05). Performance of the 6-minute walk was similar between groups, however, the young women performed more repetitions during the sit to stand task (25.6 ± 6.7 vs. 18.3 ± 4.7 reps, P < 0.001). Compared to young women, middle-age women were shown to have lower MVIC torque, peak RTD, and functional performance. Muscle architecture and contractile properties are affected by aging. • Neural and muscular alterations of the plantar flexors were investigated in young and middle-aged women. • Preemptive efforts are urgent for middle-aged women whose neuromuscular declines in the plantar flexors are already apparent. • Our findings expanded the understanding of age-related reduction in RTD, from young to middle-age. [ABSTRACT FROM AUTHOR]