Final answer:
The lever systems formed by muscles pulling on bones in the human body are primarily third-class levers, where the fulcrum is the joint, the effort is from the muscle, and the load is the body part moved.
Step-by-step explanation:
In the human body, the lever systems that are created by muscles pulling on bones are primarily considered as third-class levers. In these systems, the fulcrum is the joint around which rotation happens, the effort is the force applied by the muscle, and the load is the part of the body being moved or the resistance against the muscle contraction. A familiar example of this type of lever in action is when the biceps muscle contracts to lift the forearm; the elbow acts as the fulcrum, the biceps applies force at a point close to the elbow, and the load is the weight of the forearm and anything that the hand might be holding.
The majority of musculoskeletal lever systems operate with a mechanical advantage that is less than one. This implies that the muscle must exert a larger force than the load being moved. However, this configuration allows for greater range of motion and speed of movement, which are crucial in many biological functions, particularly for fine motor skills. Although muscles can only contract, they work in pairs, known as antagonistic pairs, such as the biceps and triceps, to control the movement of bones around joints.