Open and closed chain kinetic exercises

I feel that definitions of open and closed kinetic chain exercises are slightly contradictive and unclear.

One definition says that in open chain exercises, the distal part of the extremity can move, so bench press and leg press with seat fixed are open kinetic chain exercises.

Another definition from a book says

"In applying the closed kinetic chain definitions to clinical situations, an exercise in which resistance is placed through the distal aspect of the extremity and remains fixed to the extremity is considered a closed kinetic chain"

This is a very open definition and would classify the above exercises are closed.

Which is right?

I would consider leg press, squat and bench press to be closed kinetic chain exercises as there is weight applied to that distal segment. I tend to think of open as exercises where the joint is not being used in the normal way it would to support it. A squat or leg press will use gluteals, hamstrings, quads, ad/adductors, calf muscles etc during the exercise. A knee extension however uses mainly the quads in an abnormal or more 'non-functional' movement of the knee and I think of it as a more open chain exercise.

You are correct in that the definitions are confusing and not well thought out in terms of what is relevant. In general think about normal movement in exercises when trying to rehabilitate an upper or lower limb biomechanics or strengthen support around a joint e.g the knee of shoulder. Use open perhaps at times where there is a specific weakness in a specific muscle that needs strengthening ahead of performing closed chain exercise.

Assuming that the second definition is right, what exactly is meant by the distal aspect of the extremity? For example, is seated hamstring curls an open or closed chain exercise? (there is no force through the foot)

I think the text book wouldn't have been misleading .Any way,Close chain kinetic exersice are such carried out with the extremeties fixed with a non movable object,like squatting.I.e lower limb vs Gym floor.Open chain are such done with the extremeties free to move with or without resistance .eg Quard setting.
So close chain delivers static musscular contraction and increase myocardiac oxy expenditure compare to open chain if the emphacy is on the upper limb.yelufem