To improve muscle work in sitting

I'm not an expert but you may want to look into muscle exercises that build your core. Your core is more than your abdominals as core encompasses several muscle groups in the torso. I'm going to copy paste a direct quote from a body building website as I think it describes things better than I would from memory

Quote:

The core is a collection of muscles which stabilize and move the spine. Close to the spine and deep inside the abdomen is the inner core, which is comprised of the diaphragm, pelvic floor, multifidi, deep cervical flexors, and transverse abdominus. These strange-sounding muscles engage first during movement or breathing to protect the spine. The outer core muscles are also responsible for stabilizing and protecting the spine, but they also have more defined movement functions. The anterior muscles (abdominals) are the most well-known members of the outer-core assembly. The lats, spinal erectors, glute complex, quadratus lumborum, and hip flexors are also outer-core muscles.

The reasoning I give beind building core as a whole instead of one or two muscle groups is because core muscles tend to get overlooked even by regular fitness routines. While several different kinds of exercises aid 'core' many routines have a focus on the arms, upper back, pectorals and legs. Weaker core muscles are also major contributors for poor posture and back pain. You could very well be right in that your serratus anterior is over worked and if that is the case..then exercises to help build it will help.

Some general things to note for weight lifting.

Don't try for the heaviest possible thing. You should find weights that are challenging to use after the 7 or so repitition. Ideally you want to get to were you can do 10-15 repititions [reps] before upping the weight. Back in high school when I was more active weightlifter I didn't go up on a lot of weights till I could do 2 sets of 20 reps just cause I wanted to build endurance more than muscle mass.

Make sure to have milk daily when you begin to exercise. Even if you only do 1 12oz glass of milk. Add some. It will help you build quality muscle quicker and give you essential vitamins and minerals for building up muscle mass.

Don't be afraid if you gain a few pounds. Muscle weighs more than fat so after a few weeks if you are already light weight you will likely gain a few pounds. For over weight people you should loose a few pounds. Either way there will probably to a plataeu effect in weight change after about 15 pounds for a little while after which weight will change according to Calories eaten+muscle mass gain - calories burned

Don't weightlift every day. You will overwork and strain your muscles. If your doing core only then every other day at most. If you plan of incorporating core muscle building into a general weightlifting routine then fit core specific exercises in a manner similar to this

Week 1
Day 1: upper body
Day 2: lower body+ Core
Day 3: Upper Body
Day 4: Lower Body+Core
Day 5: Upper Body

Week 2 reverse your upper and lower body workouts

Day 1: Lower Body
Day 2: Upper Body+core
Day 3: Lower Body
Day 4: Upper Body+ Core
Day 5: Lower Body

This helps exercise your muscles in an even manner. If you feel good enough to do core 3x a week then do it on days 1,3,5 instead. Always pay attention to your body though and remember its normally recommended to at least check in with your doctor before beginning any major kind of fitness routine.

Also as a final note. Track your breathing. Many people breath while raising the shoulders and upper chest which is unnecessary work. Try practing drawing breath down into your diaphram.