This study shows that an isolated muscle's strength does not tell much about well you can do everyday activities. It is how that muscle works as part of a bigger movement that is important. Paul Chek has a good article here which explains these basic movement patterns.
The lesson is clear - especially thinking about maintaining function as we age, which is the context of this study - focus on training movements, useful movements. Don't do leg extensions.....squat instead. Don't do a cable cross over, do a pushup!
Effect of single and multi-joint lower extremity muscle strength on the functional capacity and ADL/IADL status in Japanese community-dwelling older adults
Abstract Forty-seven community-dwelling older adults aged >70 years participated in this Japanese cross-sectional study to determine the relationship between the isometric lower extremity muscle strength measured during knee extension (KE) in single-joint and total leg extension (TLE) in multi-joint tasks, physical performance tests, and functional status. The physical performance was determined by KE and TLE muscle strength, walking capacity, and balance performance tests, while the functional status was evaluated by interview using basic activities of daily living (ADL) and instrumental activities of daily living (IADL) tools.
The results indicated that the TLE muscle strength was significantly related to all the other performance tests, while the KE muscle strength was not correlated with the balance test. Also, the bilateral TLE muscle strength was significantly associated with IADL status compared with the KE muscle strength.
In conclusion, multi-joint muscle strength testing might be superior to single-joint muscle strength testing for the screening of the functional impairments of older adults.