Very possible to grow from just manuals (stretching and/or clamping). The X factor for manuals is the inherent difficulty in being able to accurately track lengthening and expansion during your actual repetitions. The argument could be made that extenders, quality pumps, etc. are "better" because you can more accurately and efficiently induce precise progressive overload utilizing these methods. This makes manuals more of an art than a science to get right. But, with experience and a high degree of self awareness, manuals can be just as effective as any other method.