Hi

I am doing some experiments for indentation of human skin. The indenter penetrates to the human skin with sinusoidal movement with frequency of 0.5, 0.25, 0.125 HZ. for low frequencies the hysteresis loop is clockwise and loading curve is higher than unloading curve; however for high frequency ( 0.5 Hz) the hysteresis loop is counter clockwise and loading curve is lower than unloading curve. Why?

Thanks