Effect of forward and backward sloped support surfaces on postural equilibrium and ankle muscles activity

Although sloped surfaces are common in daily living, most studies of body balance are carried out on flat surfaces, and few data are available for sloping angles below 14°.

The purpose of this study was to explore the effect of forward and backward sloping surfaces at 7° and 15° on postural equilibrium and the activity of flexor/extensor ankle muscles.

Fifteen healthy subjects (8 males and 7 females) (27.67 ± 3.9 years) underwent a posturographic examination associated with a surface electromyogram (EMG) of tibialis anterior (TA), soleus (Sol) and gastrocnemius medialis (GasM) under five conditions of support inclination: 0° (H0), backward inclination at 7° and 15° (DF7 and DF15), forward inclination at 7° and 15° (PF7 and PF15).

Results showed that the center of pressure (CP) was shifted according to the surface slope, with a forward move in PF7 (p <0.001) and PF15 (p <0.001) and a backward move in DF7 (p <0.01) and in DF15 (p <0.001). The mean displacement of the CP along the anterior-posterior axis (Xm) was increased in DF15 (p <0.01) relative to the H0 condition but reduced in PF7 (p <0.01). The normalized EMG revealed higher values when the muscles were in a shortened position (PF7 for Sol, p <0.05; PF15 for GasM, p <0.01; DF15 for TA, p<0.01) and lower values of GasM and Sol when lengthened (DF15, p <0.05).

Our findings indicate that standing on a backward sloped surface impairs body balance, while low-angle forward sloped surfaces might improve postural stability. Muscular activity variations of the ankle flexors/extensors, which are stretched or shortened, also seem to be related to the length-tension relationship of skeletal muscles.