Background Exercise therapy is the cornerstone of knee osteoarthritis (OA) management. In particular muscle strengthening exercise, targeting the characteristic loss of muscle strength present in knee OA, is a key factor for the beneficial effects reported for exercise therapy. The optimal training intensity for resistance training in patients with knee OA, however, is not known to date. Besides resistance training, vitamin D supplementation in patients with vitamin deficiency may optimize muscle strength.

Objectives To assess (i) whether high-intensity resistance training leads to greater improvements in muscle strength compared to moderate-intensity resistance training in patients with knee OA; and (ii) whether vitamin D supplementation in combination with strength training leads to greater improvements in muscle strength compared to placebo in combination with strength training in patients with knee OA and vitamin D deficiency (25 (OH)D level > 15 nmol/L and < 50 nmol/L (in winter) or <70 nmol/L (in summer)).

Methods In a randomized controlled trial, 177 patients with a clinical diagnosis of knee OA were included. All patients were randomly allocated to a high-intensity (70-80% of the Repetition Maximum (1RM)) or a moderate-intensity (40-50% of the 1RM) resistance training program of 12 weeks. Both groups were supervised by a physical therapist twice a week and performed home exercises once a week. In addition, 50 out 177 patients had vitamin D deficiency and received supplementation of vitamin D (1200 IU vitamin D3 per day) or placebo in the 12 weeks prior to and during the resistance training program. The primary outcome measure was isokinetic (60 °/s) upper leg muscle strength (Nm/kg). In addition, the estimated 1 RM for leg press, leg curl and hip abduction were used as measures for muscle strength. Other outcome measures included severity of knee pain (NRS), self-reported and performance based activity limitations (WOMAC physical functioning (WOMAC), Get-up-and-go-test (GUG)). Measurements were performed by a blinded assessor prior to the exercise program (T0), directly after the program (T12) and at 6 months follow-up (T36). Additionally, for patients with vitamin D deficiency, measurements were also taken prior to vitamin supplementation or placebo (T-12).

Results Both the high-intensity group and moderate-intensity group improved in upper leg muscle strength over time. No significant differences between groups were found for isokinetic upper leg muscle strength (p = 0.646) (see figure 1). However, when measured by the estimated 1 RM, significant differences were found between groups in favor of the high–intensity group (p = 0.001) (see figure 1). No between-group differences were found on pain (p = 0.885), or on self-reported and performance-based activity limitations (WOMAC p = 0.968; GUG p = 0.800), although both groups improved (see figure 1). An unexpected finding was that, in the (small sample of) patients with vitamin D deficiency, the placebo group showed significant greater isokinetic upper leg muscle strength over time compared to the vitamin D group (p = 0.001).

Conclusion No differences between groups were found for isokinetic upper leg muscle strength. With the estimated 1 RM as a measure of muscle strength, high-intensity resistance training led to greater improvements in muscle strength compared to moderate-intensity resistance training in patients with knee OA. This did not result in greater improvements in pain and physical functioning in the high-intensity resistance group; both groups showed similar clinically important improvements. The added value of vitamin D supplementation on muscle strength in knee OA patients with vitamin D deficiency need further study.