Hip strength in Swedish soldiers

Background: A requirement for a soldier is to be able to move long distances under short time with heavy external load, creating high requirements on stability in the lower extremities. Weakness in gluteus medius muscle (Gmed) which is one of the main muscles for hip stability have been associated with several forms of injury in the lower extremities and particularly in the tibiofemoral joint. Since strength test protocols in the Swedish armed forces does not contain instructions to control hip stability, recruits with deficient Gmed strength may acquire a position in the military and due to lack of hip stability be in greater risk of injury. Aim: The purpose of this study was to combine a two dimensional (2D) analysis of frontal plane gait kinematics with a Gmed strength test to determine if (1) there’s a correlation between subject’s Gmed strength and valgus during a 2000m self-paced run with external load (RWL) and (2) a 2000m run without external load (RNL) as well as (3) examine the difference in knee valgus between the run with and without load. Method: 10 subjects participated in a correlational study. Gmed peak strength was measured by performing a concentric side lying hip abduction in a Quantum 1080 machine. Frontal plane knee alignment was recorded in the initial and final parts of the two 2000 meters running test on a treadmill, one without external load and one with a weight vest set to 20 kg. Results: No significant correlation in terms of correlation coefficient (r) or coefficient of determination (r) was found between Gmed strength and RNL during initial recording (r = -0.04, r = 0.08, p = 0.90) or second recording (r = -0.40, r = 0.17 p = 0.25) The self-paced run with external load showed a no significant correlation in initial recording (r = -0.41, r = 0.17, p = 0.27) but a strong negative correlation on second recording (r = -0.69, r = 0.44, p = 0.02) With the exception of trend towards significance (p = 0.07) in difference of knee valgus between the second recordings of both running tests, no significant difference was found within or between the tests. Conclusion: The results showed a significant correlation between Gmed and valgus in the running test with external load and a significant change of valgus in comparison between late parts of both running test. Despite that the level of correlation in late part of running test with external load can create interest for larger studies, due to low number of subjects and no knee valgus value went beyond set boundaries, the connection to causation is in this study unclear.