Joe Weider popularized The Weider Principles, a compilation of guidelines to help bodybuilders achieve their maximum potential. While these principles are often dismissed as bro-science, it turns out that many have solid research to back up their use. The "Muscle Priority Principle" is one of them.
The Muscle Priority Principle
This principle maintains that you should work your weak points first in a workout. Since energy levels and mental focus are highest at this time, prioritizing training allows the use of greater intensity and effort on the muscles that need it most.
Although this principle runs counter to the commonly held notion that large muscle groups should be trained before smaller muscle groups, recent research seems to support the hypothesis. Studies consistently show that strength gains in exercises performed early in a workout are significantly greater than those performed at the end of the routine (Simao et al., 2010; Dias et al., 2010).
In a recent review of the topic, researchers concluded that, given the magnitude of strength decrements over the course of a workout, it's beneficial to structure exercise order based on the muscles in greatest need of improvement, regardless of whether they involve large or small muscle group movements (Simao et al. 2012).
If your biceps lag behind your triceps, don't hesitate to start off your workout with some curls. If your legs look like upside down bowling pins, by all means perform calf raises at the onset.
Don't blindly stick to the "heaviest compound movement first" mantra if you have a glaring imbalance. If you possess a noticeable strength or muscle discrepancy, prioritize that weak link by hitting it first in your workout.