Original Editor - The Open Physio project.
An assessment of muscle strength is typically performed as part of a patient's objective assessment. This should assist the physiotherapist's clinical reasoning and enable them to reason an appropriate point to begin strengthening rehabilitation from. Muscle strength can be assessed by a number of methods: manually, functionally or mechanically. 
The Oxford Scale
The Oxford scale is commonly used by physiotherapists to manually assess muscle strength. According to the Oxford scale, muscle strength is graded 0 to 5. The grades are summarised below:
- Flicker of movement
- Through full range actively with gravity counterbalanced
- Through full range actively against gravity
- Through full range actively against some resistance
- Through full range actively against strong resistance
- Poor functional relevance;
- Non-linearity i.e. the difference between grades 3 and 4 is not necessarily the same as the difference between grades 4 and 5;
- A patient's variability over time i.e, alternating between grades due to fatigue;
- Intra-rater reliability;
- Only assesses muscles when contracting concentrically;
- The difficulty of applying the Oxford Scale to all patient's in clinical practice (so that strength is rarely assessed throughout full range as many patients assessed by physiotherapists do not possess full range due to their respective pathology.
Due to these shortcomings, physiotherapists commonly use modified versions of the Oxford scale in clinical practice. 
Performing Manual Muscle Tests
Check out strength testing by muscle group
- Porter S. Tidy's Physiotherapy. Edinburgh: Churchill Livingstone, 2013.
- Physiotutors. MRC Scale⎟Muscle Strength Grading. Available from: https://www.youtube.com/watch?v=LjlqP1uMUo0
- Cuthbert SC, Goodheart GJ. On the reliability and validity of manual muscle testing: a literature review. Chiropractic & Osteopathy 2007; 15:4