Is what you measure relevant to muscle performance?
Isokinetic dynamometry has content validity with respect to specific aspects of muscle performance. For instance, maximum isokinetic derived power (during plantar flexion) is reached at the functional velocity equal to that of toe off during walking.
In 1985 Mayhew and Rothstein published work on measurement of muscle performance with instruments. In their book emphasis was placed upon the issues of reproducibility and validity.
Most of the problem have now been solved, however, a few have not been circumvented. Advances in engineering and computer science have made systems more versatile and much more accurate (whilst at the same time holding back some systems). Eccentric activity is still a point of contention but the inclusion of gravity correction strength figures and ratios are no longer misused. Data smoothing options have ruined the 'pure' world of isokinetics (but have made it easier for day to day use). Results other than just pure strength have gone to further improve the situation but with so many results available it becomes harder to know which to look at!
The reliability of isokinetic dynamometers is extremely high. The studies which have examined the accuracy of peak torque, work and power have shown correlation coefficients between 0.93 and 0.99 (Magnusson et al. 1990, Montgomery et al. 1989 Bemben et al.1989 are just a few of the independent articles available on this topic). The reliability of testing various joints is discussed below:-