BACKGROUND: Adaptation to strength training in very old mobility-limited individuals is not fully characterized. Therefore, the aim of this study was to perform a thorough investigation of the adaptation to a lower body strength training regime in this population, with particular emphasis on the relationship between changes in selected variables. METHODS: Twenty-two mobility-limited older men and women (85 +/- 6 years) were randomized to either a group performing 30 min of heavy-load strength training three times a week, with daily protein supplementation, for 10 weeks (ST), or a control group. End points were leg lean mass assessed by DXA, muscle thickness assessed by ultrasound, isometric and dynamic strength, rate of torque development, and functional capacity. RESULTS: Leg lean mass increased from baseline in ST (0.7 +/- 0.3 kg), along with increased thickness of vastus lateralis (4.4 +/- 3.2%), rectus femoris (6.7 +/- 5.1%), and vastus intermedius (5.8 +/- 5.9%). The hypertrophy was accompanied by improved knee extensor strength (20 to 23%) and functional performance (7 to 11%). In ST, neither the change in leg lean mass nor muscle thickness correlated with changes in muscle strength. However, a strong correlation was observed between the change in isometric strength and gait velocity (r = 0.70). CONCLUSIONS: The mismatch between gains in muscle size and strength suggests that muscle quality-related adaptations contributed to the increases in strength. The correlations observed between improvements in strength and function suggests that interventions eliciting large improvements in strength may also be superior in terms of functional gains in this population.

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