Lisa M. Edelaar, Jaap H. van Dieën , Martin van der Esch, Leo D. Roorda, Joost Dekker, Willem F. Lems, Marike van der Leeden
Amsterdam Rehabilitation Research Center , Reade, 1056AB Amsterdam, The Netherlands: E-mail: l.edelaar@reade.nl
DOI: 10.2340/16501977-2252

Objective: To investigate whether relationships between upper leg muscle strength and activity limitations are non-linear in patients with knee osteoarthritis, and, if so, to determine muscle strength thresholds for limitations in daily activities.
Design: Baseline data were used for 562 patients with knee osteoarthritis in the Amsterdam-Osteoarthritis cohort. Upper leg muscle strength (Nm/kg) was measured isokinetically. Activity limitations were measured with the timed Get Up and Go test and timed Stair Climb Test, subdivided into stair-ascent and stair-descent. Linear and non-linear relationships between muscle strength and activity limitations were evaluated, and thresholds were determined.
Results: Non-linear models improved model fit compared with linear models. The improvement in percentage variance accounted for was 5. 9, 8. 2 and 5. 2 percentage points for the timed Get Up and Go, stair-ascent and stair-descent times, respectively. Muscle strength thresholds were 0. 93 Nm/kg (95% confidence interval (95% CI) 0. 82–1. 04), 0. 89 Nm/kg (95% CI 0. 77–1. 02) and 0. 97 Nm/kg (95% CI 0. 85–1. 11) for relationships with timed Get Up and Go, stair-ascent and stair-descent times, respectively.
Conclusion: In patients with knee osteoarthritis, relationships between muscle strength and activity limitations are non-linear. Patients with muscle strength below the described thresholds might benefit more from muscle strength training to reduce limitations in daily activities than would patients with muscle strength above the thresholds. Further research is needed to assess the clinical value of the thresholds determined.

Osteoarthritis (OA) of the knee joint is a leading cause of activity limitations and is often associated with muscle weakness. Muscle strength of the upper leg is needed to perform activities such as raising from a chair and stair climbing. The relationship between muscle strength and activity limitations has been assumed to be linear. However, daily activities likely require only a certain level of muscle strength and strength gains above this level would not improve physical functioning. Therefore, we investigated the relationship between muscle strength and activity limitations in 562 patients with knee OA of the Amsterdam-Osteoarthritis cohort. Results showed that there was indeed a threshold for muscle strength: patients with muscle strength below the threshold showed more activity limitations. This indicates that patients with muscle strength below the threshold will likely benefit more from muscle strength training to reduce activity limitations than patients with muscle strength above the threshold.

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