When we speak about the body as a functional unit, we often speak of a "Kinetic chain". This is the concept that our body moves together. Nothing happens in isolation. When one thing moves, another thing has to move too.
Shout out to my main man Sir Isaac Newton and his 3rd law of motion! (I'm not a nerd, you're a nerd) For every action there is an equal and opposite reaction. The body is no exception. When the body moves, there are "links" in the chain that inevitably move in reaction to that movement.
You can picture the links as follows: Fingers -> Wrists -> Forearms -> Elbows -> Upper arms -> Shoulders -> Shoulder blades -> Spinal column making up the Upper Kinetic Chain.
The Lower Kinetic Chain can therefore be pictured as: Toes -> Feet -> Ankles -> Lower legs -> Knees -> Upper legs -> Hips -> Pelvis -> Spine.
When we think about adding resistance to a movement in order to bring about some kind of desirable change, we should also take into consideration how the rest of the natural "chain" is moving.
We place types of movements in regards to the kinetic chain into two categories: Open and Closed.
Open Kinetic Chain (OKC)
OKC movements involve the distal portion of the extremity (hand or foot in this case) is free to move in space. In other words, you're not pressing against a surface with your hands or feet. Bicep curls, knee extensions, hamstring curls, and leg lifts are some examples of OKC exercises.
Most importantly, some characteristics of OKC movements are they're typically characterized by a rotation at the primary joint, only one area moves at a time, and only the muscles on one side of the limb are being targeted.
Closed Kinetic Chain (CKC)
A CKC movement involves the same distal portion of the extremity (hand or foot) being fixed against a stationary surface. A bridge, squat, lunge, push-up, and pull-up are examples of CKC exercises.
So what do CKC exercises do for me, you're obviously asking yourself? Well listen to these phrases of what CKC exercises are characterized by: linear stress patterns, movement occurring at multiple joints and multi-joint axes, more than one area moving at one time, promotes joint stabilization. Now doesn't that sound better, more efficient, and more functional? Sure does to me!
So who cares?
First off, I do. If we want to build a functional unit that works together, and not a body that is a compilation of body parts, we need to pay more attention to this concept. Write that down!
Do I think one is better than the other? Yup!
Do I think there are benefits to both? Yup!
What if your goal is to strengthen a muscle or group of muscles in isolation? We use OKC in this case early in a rehab program to gain neuromuscular control. It can also help to build up muscle fiber size (hypertrophy) if you're going down the bodybuilding route.
But, CKC exercises are used to enhance function, promote control, and optimize stabilization (write that one down too!). These concepts are what makes the body strong, mobile, and resilient!
So, yeah, my response to the title question "Open or Closed Chain Exercise" is undoubtedly an enthusiastic "It depends" for the reasons I just mentioned. So everything has its place! One is not necessarily better than the other. There's no weak link in this chain. See what I did there???