The effects of knee injury on skeletal muscle function, Na + , K + ‐ATPase content, and isoform abundance

While training upregulates skeletal muscle Na + , K + ‐ ATP ase ( NKA ), the effects of knee injury and associated disuse on muscle NKA remain unknown. This was therefore investigated in six healthy young adults with a torn anterior cruciate ligament, ( KI ; four females, two males; age 25.0 ± 4.9 years; injury duration 15 ± 17 weeks; mean ±  SD ) and seven age‐ and BMI ‐matched asymptomatic controls ( CON ; five females, two males). Each participant underwent a vastus lateralis muscle biopsy, on both legs in KI and one leg in CON . Muscle was analyzed for muscle fiber type and cross‐sectional area ( CSA ), NKA content ([ 3 H]ouabain binding), and α 1–3 and β 1–2 isoform abundance. Participants also completed physical activity and knee function questionnaires ( KI only); and underwent quadriceps peak isometric strength, thigh CSA and postural sway assessments in both injured and noninjured legs. NKA content was 20.1% lower in the knee‐injured leg than the noninjured leg and 22.5% lower than CON . NKA α 2 abundance was 63.0% lower in the knee‐injured leg than the noninjured leg, with no differences in other NKA isoforms. Isometric strength and thigh CSA were 21.7% and 7.1% lower in the injured leg than the noninjured leg, respectively. In KI , postural sway did not differ between legs, but for two‐legged standing was 43% higher than CON . Hence, muscle NKA content and α 2 abundance were reduced in severe knee injury, which may contribute to impaired muscle function. Restoration of muscle NKA may be important in rehabilitation of muscle function after knee and other lower limb injury.