Lower extremity fatigue increases complexity of postural control during a single-legged stance.

BACKGROUND: Non-linear approaches to assessment of postural control can provide insight that compliment linear approaches. Control entropy (CE) is a recently developed statistical tool from non-linear dynamical systems used to assess the complexity of non-stationary signals. We have previously used CE of high resolution accelerometry in running to show decreased complexity with exhaustive exercise. The purpose of this study was to determine if complexity of postural control decreases following fatiguing exercise using CE. METHODS: Ten subjects (5 M/5 F; 25 ± 3 yr; 169.4 ± 11.7 cm; 79.0 ± 16.9 kg) consented to participation approved by Western Oregon University IRB and completed two trials separated by 2-7 days. Trials consisted of two single-legged balance tests separated by two Wingate anaerobic tests (WAnT; PreFat/PostFat), or rest period (PreRest/PostRest). Balance tests consisted of a series of five single-legged stances, separated by 30 s rest, performed while standing on the dominant leg for 15-s with the participant crossing the arms over the chest and flexing the non-dominant knee to 90 degrees. High resolution accelerometers (HRA) were fixed superficial to L3/L4 at the approximate center of mass (COM). Triaxial signals from the HRA were streamed in real time at 625 Hz. COM accelerations were recorded in g's for vertical (VT), medial/lateral (ML), and anterior/posterior (AP) axes. A newly developed statistic (R-test) was applied to group response shapes generated by Karhunen Loeve (KL) transform modes resulting from Control Entropy (CE) analysis. RESULTS: R-tests showed a significant mean vector difference (p < .05) within conditions, between axes in all cases, except PostFat, indicating the shape of the complexity response was different in these cases. R-test between conditions, within axis, differences were only present in PostFat for AP vs. PreFat (p < .05). T-tests showed a significantly higher overall CE PostFat in VT and ML compared to PreFat and PostRest (p < .0001). PostFat CE was also higher than PostRest in AP (p < .0001). CONCLUSIONS: These data indicate that fatiguing exercise eliminates the differential complexity response between axes, but increases complexity in all axes compared to the non-fatigued condition. This has implications with regard to the effects of fatigue on strategies of the control system to maintain postural control.

Reliability of mechanomyography and triaxial accelerometry in the assessment of balance.

Reliability of high-resolution accelerometery (HRA) and mechanomyography (MMG) was evaluated for the assessment of single-leg balance. Subjects (5M/5F, 25+/-3 yr; 169.4+/-11.7 cm; 79.0+/-16.9 kg) participated in fifteen (three randomized bouts of five repetitions) 15-s dominant leg stances. A single HRA was fixed superficial to L3/L4 segment to capture motions relative to the center-of-mass, and three-uniaxial accelerometers were fixed on the surface of the dominant leg correspondent to the vastus medialis (VM), vastus lateralis (VL), and soleus (SOL) muscles to record MMG. Triaxial signals from the HRA (s.r.=625 Hz) were streamed to a base station, simultaneously with MMG (s.r.=1000 Hz). Signals were sampled, recorded and later analyzed. HRAs were recorded in g's for vertical (VT), medial/lateral (ML), anterior/posterior (AP) directions, and resultant (RES) scalar. Intraclass correlation coefficients (ICC) were computed for each and Pearson's r was calculated for the relationships between MMG and HRA (alpha < or =0.05). Except for RES (ICC=0.36), all measures demonstrated moderately strong reliability (ICC=0.75, 0.73, 0.63, 0.87, 0.89, and 0.86 for VM, VL, SOL, VT, ML, and AP, respectively). HRA and MMG provide reliable information pertaining to balance, and may have application in evaluating postural control and stability.