Sarcopenia and muscle weakness are well known consequences of ageing. The aim of the present study was to ascertain whether the age associated muscle weakness was also due to a decrease in intrinsic muscle force. In vivo physiological cross sectional area (PCSA) and specific force of the lateral head of the gastrocnemius muscle (GL) were assessed in elderly males (EM, aged 73.8 ± 3.5 years, height 173.4 ± 4.4 cm, weight 78.4 ± 8.3 kg, n=19, mean ± SD) and in young males (YM, aged 25.3 ± 4.4 years, height 176.4 ± 7.7 cm, weight 79.1 ± 11.9 kg, n=12). Procedures were approved by the Manchester Metropolitan University Ethics Committee. GL muscle volume (VOL) and Achilles’ tendon moment arm length were evaluated using magnetic resonance imaging. Pennation angle (θ), and fibre fascicle length (Lf) were measured using B-mode ultrasonography during isometric maximum voluntary contraction of the plantarflexors (MVC). PCSA was estimated as VOL/Lf. GL fascicle force was calculated from the tendon force component (=tendon force/cos θ) after accounting for agonist and antagonist activation level (assessed by twitch-interpolation and EMG recordings), Achilles’ tendon moment arm length and the relative PCSA of the GL within the plantarflexors group. GL specific force was calculated as fascicle force/PCSA. Voluntary activation of the GL muscle was lower in the EM than in the YM (86 ± 7 % vs. 98 ± 2 %, respectively, P<0.05: one-way ANOVA). Compared to the YM, plantarflexor MVC torque and fascicle force of the EM were lower by 47 % and 40 %, respectively (P<0.01). Both VOL and PCSA were smaller in the EM by 28 % (P<0.01) and 16 % (P<0.05), respectively. Pennation angle was 12 % smaller in the EM, whereas there was no significant difference in Lf between the YM and EM. Remarkably, after accounting for differences in agonists and antagonists activation, the net specific force of the EM was 30 % lower than that of the YM (P<0.01). Thus demonstrating that the loss of muscle strength with ageing may not only be explained by a reduced voluntary drive to the muscle but mostly by a decrease in intrinsic muscle force. This phenomenon is likely due a reduction in single fibre specific tension; however the role of age-induced changes in tendon stiffness cannot be excluded.