Paradoxical muscle movement in human standing.

Area of Science:

• Biomechanics
• Neuroscience
• Human Physiology

Background:

• Human standing relies on ankle muscles (soleus, gastrocnemius) to counteract gravity-induced toppling.
• Previous theories proposed calf muscles lengthen with forward sway and shorten with backward sway, stabilized by muscle stiffness or reflexes.
• These theories assumed stiff passive tissues (Achilles' tendon, foot), which recent data contradicts.

Purpose of the Study:

• To investigate the in vivo behavior of soleus and gastrocnemius muscles during human standing.
• To test the hypothesis of paradoxical muscle movements (shortening with forward sway) during postural control.
• To determine the mechanism underlying human standing stabilization, differentiating between reflex and anticipatory control.

Main Methods:

• Dynamic ultrasound imaging was employed to observe muscle dynamics in real-time.
• Novel automated tracking of muscle length was developed and utilized.
• Measurements were taken in human subjects during standing and postural sway.

Main Results:

• Soleus and gastrocnemius muscles exhibited paradoxical shortening during forward body sway.
• Muscles lengthened during backward body sway or return to upright posture.
• Increased active tension was associated with muscle shortening, not stretching.
• Passive tissues (Achilles' tendon, foot) were found to be not stiff during standing.

Conclusions:

• Intrinsic ankle stiffness is insufficient for stabilizing human standing.
• Paradoxical muscle movements, driven by anticipatory neural control of muscle length, are crucial for balance.
• The findings challenge traditional models of stretch reflexes and tonic muscle activity in postural control.