Have a shred
The Spanish phrase 'tener un tirón' is often used to depict a situation in English where someone might 'have a shred'. In much more common terms, it might be used to indicate that someone has a minor advantage or marginally better position than others in a certain situation. It can also refer to having a small piece of chance or opportunity that can be exploited for benefit. Nevertheless, the complexity of idiomatic expressions means the exact meaning can heavily depend on context.
After exercising, many people tend to have a pull in their back.
This example refers to the common occurrence of feeling a pull or strain in the back after physical activity. 'Tener un tirón' in this context means experiencing a sharp, sudden pain typically caused by a muscle strain.
When I lifted the heavy box, I felt I had a pull in my neck.
In this case, 'tener un tirón' is being used to describe the feeling of a sudden or sharp pain in one's neck. This is often how people describe the sensation of straining a muscle, especially due to lifting something heavy.
If you feel that you have a pull in your leg, you should rest and not strain the muscle.
This is a piece of advice using the expression 'tener un tirón', referring to the sensation of muscle strain or pull in the leg. The recommended course of action in this case is to rest and avoid further straining the muscle.