Introduction: Impaired isometric muscle strength and muscle endurance in adults with Fontan circulation has previously been reported. However, the knowledge if corresponding impairment is present in children and adolescents with Fontan circulation is scarce.

Aim: The aim was to examine the isometric muscle strength and muscle endurance of the lower limbs in children and adolescents with Fontan circulation in comparison to age and sex matched controls.

Method: In this cross-sectional study 43 children and adolescents (6-18 years) with Fontan circulation and 43 controls were included. Isometric knee extension and plantar flexion muscle strength was assessed using dynamometry (Newton:N). Unilateral isotonic heel-lift until exhaustion was used for evaluation of lower limb muscle endurance. Analysis on group level (n=43) and for the subgroups 6-12 years (n=18) and 13-18 years (n=25) was performed.

Results: On group level the children and adolescents with Fontan circulation had impaired isometric plantar flexion strength for the left leg compared to controls (393.9±181.1N vs. 492.5±241.6N, p=0.04). In addition, they had impaired isometric knee extension strength bilaterally (right 222.8±101.1N vs. 293.0±164.9N, p=0.02, left 220.7±102.7N vs. 279.5±159.1N, p=0.05). In contrast, lower limb muscle endurance did not differ. In subgroup analysis, the impaired isometric strength was only present in the group of adolescents.

Conclusion: Adolescents with Fontan circulation had impaired isometric muscle strength compared to controls. However, no corresponding differences were found in children. Further, lower limb muscle endurance did not differ. This implies that the impaired isometric muscle strength may develop during adolescence whereas the impaired muscle endurance may occur later.