Background: We aimed to study the association between flexor muscle strength in the knee joint and cross-sectional area (CSA) on magnetic resonance imaging (MRI). We also evaluated aging-related changes in flexor muscle strength and CSA. We hypothesized that muscle strength is associated with muscle CSA on MRI and that such relationships would differ among the flexor muscles. Furthermore, we hypothesized that muscle strength and CSA would decline, and the reduced rate of muscle CSA would differ among the flexor muscles with aging.
Methods: A total of 252 patients (mean age, 33.6 years; range, 11–66 years) who visited the outpatient clinic with knee pain between September 2006 and August 2009 were retrospectively analyzed. The CSA of each knee joint flexor muscle (biceps femoris, sartorius, gracilis, semitendinosus, and semimembranosus) was measured on MRI axial images at the supra-patellar level. We evaluated flexor muscle strength (peak torque, N.m) using a Cybex dynamometer at 60 and 180°/s and evaluated its correlation with CSA.
Results: Mean CSA was 605.4 mm2 for the semimembranosus, 444.7 mm2 for the biceps femoris, 282 mm2 for the sartorius, 55.4 mm2 for the semitendinosus, and 34.1 mm2 for the gracilis. Mean peak torque was 67.4 N.m and 52.7 N.m at 60°/s and 180°/s, respectively. CSA was positively correlated with flexion strengths of 60°/s (r = 0.363, p = 0.000) and 180°/s (r = 0.354, p = 0.000). Muscle strength was associated with CSA for all muscles, except the gracilis (r = 0.056, p = 0.375). Flexion strength decreased significantly with age after the third decade of life. Total CSA decreased with age (r = −0.247, p = 0.000). CSA of the biceps femoris, sartorius, semimembranosus, and semitendinosus decreased significantly, while CSA of the gracilis tended to decrease non-significantly with aging.
Conclusions: Muscle strength was associated with total muscle CSA on MRI and the CSA of every muscle but the gracilis. Flexion strength decreased significantly with aging after the third decade of life. Total CSA decreased significantly with aging. CSA of the biceps femoris, sartorius, semimembranosus, and semitendinosus decreased significantly, whereas that of the gracilis decreased slightly with aging.
Level of Evidence: Level III, retrospective comparative study