A matter of Intensity
In normal rest position (1), the temporalis should be relaxed enough to allow for a "freeway space" between the teeth (commonly describing as "lips together, teeth apart"). Curiously, most diagrams representing the temporalis (Slide 1: images 1, 3, 10) show the teeth in occlusion, meaning that the temporalis is contracted. If one lateral pterygoid were to contract (2*) without the of contraction of a temporalis, the mandible would simply move sideways, without any resistance (i.e., tooth contact) to the lateral pterygoids attempts. If the temporalis' were to contract in the absence of lateral pterygoid contraction (3*), the mandible would be elevated until the teeth occlude. When one temporalis contracts in conjunction with an opposite lateral pterygoid (4), the result is a functional movement, necessary for mastication. Once the teeth contact, the movement is complete, and the contraction should cease.
Continued contraction of the temporalis following occluding of the teeth (3a) is a functionless act, that is, parafunction, and is referred to as clenching. The resultant degree of symptoms (headache, stiff/sore jaw, stiff/sore neck) is a function of the intensity, duration, and frequency of the activity.
Following a functional movement (4), the musculature is intended to relax, before continuing with another functional masticatory movement. Vigorous alternating contractions of the lateral pterygoids during the occluding of the teeth (by the temporalis) results in grinding the teeth, commonly referred to as bruxism. The more intense the contraction of the temporalis (clenching the jaws together), the more resistance there is to lateral pterygoid activity, thereby allowing more intense contractions of the lateral pterygoids and more severe the signs of bruxism. The severity of bruxism is dependent on the intensity of the temporalis contraction.
When one lateral pterygoid positions the mandible excursively while the opposite temporalis contracts to stabilize the mandible, the isometric contraction of the lateral pterygoid can place significant strain on the ipsilateral TMJ (4a) (E). This "excursive clenching" is the most damaging activity to the structures that lateral pterygoids are attached to: the TMJ complex, and the pterygoid plate of the sphenoid bone (often presenting as sinus and facial symptoms).