Overall, you have decent knowledge of musculature, but as a few others mentioned before me, your characters tend to resemble human-shaped bags of oatmeal...I think your sketches would certainly look a little better if they had more actual shading in them instead of just marks to indicate where that would be...some of your main weak points (at least from your earlier works) are the neck/traps area (especially in the Captain America sketch on page 1) and disproportionate stomach/waist (most notable in Colossus, also on page1)...but just keep working at it.