Age-related loss of muscle mass and function represents personal and socioeconomic challenges. The purpose of this study was to determine the adaptation of skeletal musculature in very old individuals (83 + years) performing 12 weeks of heavy resistance training (3 ×/week) (HRT) compared to a non-training control group (CON). Both groups received similar protein supplementations. We studied 26 participants (86.9 ± 3.2 (SD) (83–94, range) years old) per-protocol. Quadriceps cross-sectional area (CSA) differed between groups at post-test (P < 0.05) and increased 1.5 ± 0.7 cm2 (3.4%) (P < 0.05) in HRT only. The increase in CSA is correlated inversely with the baseline level of CSA (R2 = 0.43, P < 0.02). Thigh muscle isometric strength, isokinetic peak torque and power increased significantly only in HRT by 10–15%, whereas knee extension one-repetition maximum (1 RM) improved by 91%. Physical functional tests, muscle fiber type distribution and size did not differ significantly between groups. We conclude that in protein supplemented very old individuals, heavy resistance training can increase muscle mass and strength, and that the relative improvement in mass is more pronounced when initial muscle mass is low.