A biomechanist will most likely be working in a research capacity within a university, a clinical setting or a sports science. Biomechanics is a sub-field of the broader discipline, kinesiology, (the study of how people move.) Although Biomechanics is a research discipline, practical applications do exist as well.
Advice to athletes for fine tuning their performance may well come from the biomechanist. It is quite a glamorous profession in that those with only one discipline: engineering, biological sciences, sports sciences, will not have the wherewithal to tackle the work that you, the biomechanist, can do.