What does it mean when my muscle starts shaking/trembling?

It’s a sign of fatigue. And while there’s far from perfect agreement among scientists about the exact causes of specific fatigue, it’s generally safe to say that the trembling is due to the Central Nervous System attempting to produce a voluntary contraction, but there’s a mismatch between the availability of the necessary substrate (ATP) and the demand for it.

Why do I get sore?

When  muscle tissue is stressed to a point beyond what it is used to you’re going to get microscopic tears, which are painful. The more stress you incur during the eccentric action, the higher the likelihood of micro-trauma to the muscle.


Why am I more sore the 2nd day following a workout?

The most widely accepted theory is due to inflammation and an accumulation of the metabolic by-products of exercise irritating the microscopic tears.


Is it good to be sore all the time, or is there a certain day/time at which it should plateau?

That’s a relative question depending on  several training factors, but, personally, I don’t like being sore all the time:)


If I stop getting sore does it mean it's not working anymore?

Depends on what you mean by “not working”. It really depends on what the goals are, but no it doesn’t mean that you’ve plateaued relative to strength gains or body composition gains... It does likely indicate that you’ve adjusted to the demands you’re making on your muscular system, especially the eccentric demands


How important is it to stretch before I workout? After?

It’s important to perform some type of dynamic movement preparation prior to a workout. I’d save the passive stretching for post-workout, but I don’t feel it’s important in general, unless you have a flexibility goal and that’s part of your program design.


Is it really important to take a day off? Can a “light” day substitute for a day off?

It’s important to periodize your workouts and not go “all out” all of the time. For sure, an active recovery day can certainly count as a day off. I think it’s pertinent to also get a sense that exercise comes in several forms and playing tag or soccer with your kids, throwing frisbee at the beach or in the park for a couple hours, riding your bike around town can all count as active recovery days.