Ankle Angle but Not Knee Angle Influences Force Fluctuations During Plantar Flexion

Area of Science:

• Biomechanics
• Neuroscience
• Human Movement Science

Background:

• Muscle length and joint angles are critical factors influencing muscle force production and neural control.
• Understanding these relationships is essential for optimizing motor performance and rehabilitation strategies.

Purpose of the Study:

• To investigate how variations in ankle and knee joint angles affect force steadiness and motor unit (MU) discharge characteristics in the soleus muscle during steady isometric plantar flexions.
• To determine the role of the soleus muscle in controlling submaximal forces generated by the plantar flexor group.

Main Methods:

• 14 young adults performed submaximal isometric contractions (5-40% maximal force) at specific ankle (75° and 105°) and knee (120° and 180°) angles.
• Force steadiness was assessed using the coefficient of variation of force.
• Soleus motor unit discharge characteristics (rate, variability) and neural activation variability were recorded.

Main Results:

• Force steadiness improved with increasing contraction intensity up to 20% maximal force.
• Force steadiness was significantly better at the longer ankle angle (75°) compared to the shorter angle (105°), irrespective of knee angle.
• Soleus motor unit discharge rate, discharge variability, and neural activation variability were lower at the 75° ankle angle than at 105°.
• Knee joint angle variations did not significantly affect any measured outcome.
• Medial gastrocnemius MUs were not recordable at a knee angle of 120° due to minimal activation.

Conclusions:

• The ankle joint angle, specifically muscle length, plays a crucial role in modulating force steadiness and motor unit control during plantar flexor isometric contractions.
• The soleus muscle is a primary determinant of submaximal force control within the plantar flexor muscle group.
• Knee joint position has a negligible effect on these parameters within the tested range.