Thus far you have gotten great “text book,” answer’s. Not that these answers are incorrect by any stretch of the imagination, but movement had to deal with gravity and the body has to react to gravity a little different than most textbooks will illustrate. So, on with a discussion in functional anatomy and remembering that concentric contains are only part of what muscles do. To answer your question, I will quivkly discuss walking because everyone does it.
The first stage of of gait is weight acceptance. During weight acceptance the lead foor is hitting the ground with a slightly bent knee as that leg is traveling forward. Yes, the joint itself is flexing like the others have said, but because of ground reaction forces we need to slow down this movement, to prepare for the swing phase as we transfer weight to the other leg. Slowing the knee down is primary responsibility of the rectus femoris and the soleus muscle. These two muscles are really only able to their job effectively because the psoas at the hip and the tibialis posterior and anterior are doing their job by acting eccentrically versus gravity.
With that being said there is all kinds of chasing reactions that occur due to how or muscles absorb energy from the ground with movement. Depending on the population that you are training this stresses the importance of thinking about how the body works in the real world. The real world isn’t full of leg extension and hamstring curl machines, its full of b natural movements. If you are trying to prepare folks to manage their movements a little better in life, its time to start seeing how muscles work against gravity.
Hope this helps,