Pennation angle changes with time after exercise and the muscle thickness depends on the ultrasound systems as well as with the person that made the measurements.
Article Changes in muscle architecture induced by low load blood flo...
Muscle thickness, pennation angles and fascicle lengths are not a way of studying muscle architecture, they are rather parameters reflecting a muscle's architecture. If the question is whether they techniques we're currently using to obtain these parameters in vivo (mostly sagittal-plane ultrasound scans, very rarely also MRI-based diffusion tensor imaging) are valid and reliable, you might be interested in two very recent reviews (see below). From my perspective, ultrasound has several significant limitations, including a small field of view, tissue compression and user dependency, but it still is a fairly accurate and certainly the most readily available technique.
(1) Kwah et al. J Appl Physiol 114:761-769, 2013
(2) Cronin et al. Gait Posture 37: 305-312, 2013
Aside from limitations regarding 2D ultrasound imaging which has been addressed by Robert, I would look into more variables than pennation angle and muscle thickness. Fascicle length changes of a muscle under passive movement or active contraction may of interest and is readily available to measure though 2D ultrasound.
http://www.ncbi.nlm.nih.gov/pubmed/26049793
Thank you for your contribution. In fact, I totally agree with your comments.
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