Ask me anything #2

Why is it bad to "lock my knees" when standing for long periods? Why does a hot climate make this worse?

Vasovagal syncope is the major risk associated with knee locking, standing for long periods, and heat. 

 

Knees

The knee joint permits flexion (bending) and extension (straightening). At the point of full knee extension, the knee joint is shaped so that the femur bone rotates on top of the tibia bone. In this position, the structures resist flexion and hyperextension. This provides some relief to the thigh muscles. It is commonly called "locking".

 

The vascular system

While standing for prolonged periods, venous blood must flow against gravity to return to the heart. This means that venous return is reduced. At full extension, the blood vessels which pass through the knee joint are compressed. Venous return from the legs is further reduced.

In a hot climate, the superficial capillaries of the body dilate so that blood can move closer to the body surfaces. This allows heat to be transported to the surfaces so it can escape the body at a greater rate. Unfortunately, this vasodilation reduces the blood pressure.

 

Syncope (fainting)

• In the brain stem there is a column of nuclei (dense clusters of nerve cell bodies) called the nucleus of the solitary tract.  
• When stimulated, this nucleus increases parasympathetic tone (relaxation signalling) and decreases sympathetic tone (fight or flight signalling). The vagus nerve promotes a reduction in the heart rate. This causes the blood pressure to drop rapidly. 
• As the blood pressure falls, the perfusion of the brain falls, and nervous signalling in the brain falls. Consciousness stops, and muscle tone drops. 
• The body falls to the ground. Since the body is now in a horizontal position, the brain perfusion is increased again. Consciousness returns.

In vasovasgal syncope, the stimulus which triggers this event is vascular: the inadequate venous supply.

 

See here:  

• Vasovagal syncope