Women’s upper arm strength increases when they have children. Why? Picking up babies and toddlers require the use of their biceps. However, an imbalance occurs since few moms use the triceps with the same frequency. Muscles contract in one direction. Muscles attach to joints and work in opposing pairs to create movement. A balanced strength training routine develops the muscles in opposition to improve movement and prevent injury. While one muscle is contracting (active), the opposing muscle is relaxing (passive). The goal of strength training is to ensure that each muscle is stressed and actively stimulated.
The process is called Muscle Balance or the Principle of Opposition. Here is a good explanation from Building Strength and Stamina:
For example, when the muscle on one side of a joint (e.g., biceps) contracts and shortens, the muscle on the other side (e.g., triceps ) must simultaneously relax and lengthen for productive movement to occur. (pg. 12)
Terms used for Muscle Balance
Agonists (Prime Mover): muscles responsible for a given movement
Anagonists: muscles act in opposition to the agonists
Stabalizers: muscles that hold or fix a joint or part while the movers produce movement around it
Neutralizer: muscles which contract to prevent unwanted actions
The muscle that is active requires the force and energy to overcome resistance and become stronger. When developing or performing a strength training routine, it is important to pay attention to all parts of the body and ensure the opposing muscle receives attention. Different muscles are meant to perform at a different ratio, so as long as you’re paying attention to each of the muscles, you will develop the muscles proportionally.
You're posting is RIGHT ON... So many times we see individuals engaged in strengthening programs that target the 'problem areas', but forget the muscles on the other side. A quick, hit-list of exercises that target balance are as follows:
If You Work Here Then Also Work These
Biceps (arm curls) Triceps (triceps kickbacks)
Quadriceps (leg extension) Hamstrings (leg curls)
Abdominal (crunches) Back Extensors (back extensions)
Pectorals (push ups) Trapezius (rear rows)
Hope these get readers on the right track to muscle balancing.