The ease of lifting something also depends on where we are in relation to the object we are lifting. If we are standing on a solid surface, and the object is at our waist level or lower, it is easier to pick up than when it is out in front of us or above us. Reaching out or above lowers our ability to lift and control what we are retrieving - often making the task difficult, riskier than it need be, or something that we choose not to accomplish.
Take picking up a one-gallon jug of water, bleach, or other liquid. This weighs approximately 8 pounds. If it is on the floor, on a low shelf, on the car seat when bringing it home from the store, or anyplace else below our waist, we can pick it up using our leg muscles and lower back to assist us. Those who might be seated (on a bench or wheelchair) due to circulatory, stamina, or mobility issues can still retrieve such an item relatively easily using their arms.
When that same item is out in front of us on a countertop or countertop equivalent (washing machine, for instance), we don't have the benefit of using our lower body to assist us in making the lift. It's all upper body, shoulders, and arms that we use.
When we have to reach at eye level or above, we must lift primarily with just our arms, using the rest of our body for balance and support. This is how injuries can happen as we pull a muscle by overextending our reach or lose control of an object we are retrieving and allow it to hit us in the face or upper body of fall and land on our foot. It could break or spill as it falls and injure or splash us or our clothing.
Sometimes, objects are difficult to ascertain how heavy they are when we are unfamiliar with them or we didn't place them where we find them. A cardboard box when it is empty is relatively easy to pick up from the floor or ground when it is empty unless it is quite large. When it is filled with papers, books, clothing, canned goods, or other household items it becomes considerably heavier. It's this additional weight that is tricky to judge. We can easily hurt ourselves by trying to lift something that is too heavy for us.
If that same box were on a shelf, we would be able to judge the weight of it more easily by the way we were able to move or slide it on the shelf.
If what we are picking up is alive - a pet or youngster, for instance - they often (though not always) can help us lift them by cooperating and coming to us as we attempt to lift them. There typically are more places to grab onto them as we pick them up also.
Of course, as we stand on a stool or ladder to reach something above us - from a higher shelf or fruit from a tall branch - there is the possibility of falling as we lose our balance or lose control of what we are retrieving.
Lifting is not always as easy as it might seem, and it can actually be somewhat dangerous depending on what we are retrieving and from what height. This risk increases as we age.