Three-dimensional hip and knee loading during the Copenhagen adductor exercise

The Copenhagen adductor exercise (CAE) is used for groin muscle strengthening. While these muscles primarily act in the frontal plane, the hip adductor muscles may also act in the sagittal and transverse planes. Therefore, the CAE likely imposes three-dimensional (3D) muscular demands at both the hip and knee. This study aimed to quantify the 3D hip and knee joint loading during variations of the CAE. Eleven men and 13 women performed the exercise with the support stand at the knee, ankle, and midway between the knee and ankle. Ground reaction forces under the stand and reflective markers trajectories were recorded to calculate hip and knee net joint moments (NJM). Hip adductor NJM was largest for the ankle support (1.54 ± 0.23 Nm·kg^-1) and smallest for the knee support (0.93 ± 0.16 Nm·kg^-1; P < 0.001). Most participants had a hip extensor NJM, that was also greater for more distal support locations (Δ_ankle-mid = -0.10 ± 0.21 Nm·kg^-1; Δ_ankle-knee =  -0.20 ± 0.27 Nm·kg^-1; P < 0.001). Support locations distal to the knee had knee adductor and flexor NJM. The hip and knee sagittal plane NJMs were strongly correlated with transverse plane thigh (r = 0.97-0.98, P < 0.017) and shank (r = 0.89-0.93, P < 0.017) orientations, respectively. Medial thigh rotation was associated with a hip flexor NJM while lateral thigh rotation was associated with a hip extensor NJM. CAE requires frontal and sagittal plane muscle efforts at the hip and knee, which are greater for more distal support locations.