Introduction: The regulation of balance during upright standing involves continuous muscular activity, associated with body sway. In single stance standing, the base of support is narrower compared to double stance, resulting in an increased body sway and emphasizing the role of individual muscles in regulating the sway motion. In this study, we investigated the effect of Tibialis Anterior (TA) fatigue on body sway during standing on one leg on ten able-bodied subjects.

Methods: Foot ground reaction forces, goniometry of the ankle joint, and EMG of the TA were all measured simultaneously in two tests. Each test lasted 30 sec. During which the subjects were required to stand as still as possible with their dominant leg on a forceplate and the contralateral knee flexed upward at 90 deg approximately, and their hands resting on their waists. The tests were separated by a 4 min isotonic fatiguing effort of the TA, indicated by a significant decrease of the mean power frequency (MPF).

Results: The EGM root mean square (RMS) started off at 45% MVC but, towards the end of the effort, significantly increased to 52% MVC, the latter determined in non-fatigue condition. Compared to the non-fatigue state, the following significant (p< 0.05) sway changes took place in the fatigue state: force RMS increased from 2.61 to 3.90 N and from 3.77 to 5.01 N in the mediolateral (ML) and vertical directions, respectively. The center of pressure (CoP)

RMS in the ML direction increased from 0.57 to 0.68 cm. The EMG RMS in the TA increased from 4.15 to 5.58 and the MPF decreased from 107.6 to 96.7 Hz in the fatigued state.

Discussion: An interesting finding was revealed by comparing the variations of the ankle angle to those of the center of pressure in the anterior-posterior direction taking place during standing. During the non-fatigue test the CoP moved gradually posteriorly, while the goniometer indicated an ankle change towards dorsiflexion. These obviously two opposing trends necessitate compensatory angular adjustments at the knee and/or hip joints. However, during the fatigue test, the posterior excursion of the CoP was accompanied by a consistent change in the ankle, towards plantar flexion. This seems to suggest that in the fatigue state the redundancy of the musculoskeletal system is reduced, increasing the degree of correction between ankle angle and CoP.