long or weak muscles, are usuaully used to describe hyotonic muscles, or underactive muscles.
Although, a muscle could be long (ie have a normal resting length) and not be weak or underative at all. Similiarly, a muscle could be strong (ie posess lots of force potential) and still be long (have a normal resting length).
Typically if a muscle is tight or overactive, its antagonist (the muscle direclty opposite of the agonist) is weak, lengthened, overstretched (any of those terms are used in textbooks). This happens because of recipricoal inhibition. Basically, recipricoal inhibition is the phenomenon in the human body that states, when a muscle contracts (agonist), its functional antagonist relaxes so as not to interfere with joint movement. Think about a biceps curl. You curl your arm. If you were to contract your tricep (the antagonist) during that biceps curl, your arm would stop in place, you probably wouldn't get very far. Since one muscle is an elbow extensor and one muscle is an elbow flexor.
So, lets apply this to something that you can visualize......
Lets say, someone with a tight chest, and rounded shoulders. okay, they have what has been termed in the physical therapy world as "upper-crossed syndrome". Basically, their chest (pec major) is tight along with other interal rotators of the shoulder like the lattisumus doris, teres major, subscapularis and the anterior delt, as well the pec minor is usually tight and causing an anterior tilt to the scapula.
So, those muscles are tight, they are basically decreasing the neural drive to the muscles that externally rotate the shoulder (infraspinatus, teres minor) as well as those muscles that depress the scapula (or keep it from anterior tilting and keep it stationary on the spine), like the mid and lower trapezius, the rhomboids and the serratus anterior. these are the muscles that are lengthend and weak (hypotonic).
Now, putting that into something you can use, you basically see that you need to stretch what is hyperonic (the internal rotators) and strengthen that which is hypotonic (the external rotators and the scpular depressors and stabilizers).
Hope that helps to clarify a little bit.