Inverse relationship between the complexity of midfoot kinematics and muscle activation in patients with medial tibial stress syndrome.

Medial tibial stress syndrome is a common overuse injury characterized by pain located on the medial side of the lower leg during weight bearing activities such as gait. The purpose of this study was to apply linear and nonlinear methods to compare the structure of variability of midfoot kinematics and surface electromyographic (SEMG) signals between patients with medial tibial stress syndrome and healthy controls during gait. Fourteen patients diagnosed with medial tibial stress syndrome and 11 healthy controls were included from an orthopaedic clinic. SEMG from tibialis anterior and the soleus muscles as well as midfoot kinematics were recorded during 20 consecutive gait cycles. Permuted sample entropy and permutation entropy were used as a measure of complexity from SEMG signals and kinematics. SEMG signals in patients with medial tibial stress syndrome were characterized by higher structural complexity compared with healthy controls (p<0.001) while it was the opposite for the midfoot kinematics (p=0.01). Assessing the complexity of midfoot kinematics and SEMG activation pattern enabled a precise characterization of gait in patients with medial tibial stress syndrome. The reported inverse relationship in foot kinematics and SEMG complexity most likely point towards separated control processes governing gait variability.