here is the link below explaining when you shoud use the Manual Muscle Testing , I hope it will be helpful! http://highered.mheducation.com/sites/0071474013/student_view0/chapter8/manuaul_muscle_testing.html

As noted above it's situational. 

Utilizing a hand held dynamometer like the Nicholas manual muscle tester or the Microfet 2 hand held dynamometer may provide you with good objective data if done in a standardized manner. There have been many studies that have looked at the reliability related to knee strength, here is an example;

American Journal of Physical Medicine & Rehabilitation:

November 2004 - Volume 83 - Issue 11 - pp 813-818

Research Article: Strength Assessment

Reliability of Hand-Held Dynamometry in Assessment of Knee Extensor Strength After Hip Fracture

Conclusions: Hand-held dynamometry is a reliable and valid tool for assessment of knee extensor strength after hip fracture.

I hope this helps.

Dear Colleagues,

Thank you for your help.

Saeed