All of our activities of daily living depend on having sufficient muscle power to carry them out, from actions demanding high levels of power such as getting up from the ground to fine actions such as writing and typing. Loss of sensibility can be more disabling than loss of muscle power but loss of strength is important as it interferes with independent living, especially when we are older and find it difficult to rise from a chair or to climb stairs. There may be many reasons for loss of muscle power including neurological illness or stroke, joint pain, lack of use, trauma or illness and disease. Assessment of muscle strength and the progressive strengthening of muscles is a core physiotherapy skill.

The Oxford Scale is the rating system used by physiotherapists for the assessment and recording of muscle power when required. Knowledge of muscle anatomy is vital so that the joint can be positioned correctly and the tendon and muscle palpated so whether there is any muscle action can be judged. The muscle is rated on the Oxford Scale from one to five and written down as 2/5 or 4/5, at times with a plus or minus sign to show the muscle has more or less strength but not enough to go down or up the scale. The physiotherapist ensures the joint is in the optimal position to enable the muscle to function easily and for easy visualisation of the tendon and muscle.

If there is no muscle activity, either visible or with the physiotherapist feeling the tendon and muscle belly after several attempts by the patient to perform the muscle action then the muscle is graded as zero. A small muscle contraction such as a twitch, without any joint movement, is rated as one. When the muscle can do its joint action but without the force of gravity resisting the movement then this is graded as two, but the joint needs to be in the right position for correct testing. If the muscle can perform its typical action against the force of gravity then it is rated as three. An example is bending the elbow whilst standing up, where the biceps is working against gravity.

If the muscle can move the joint through the full movement both against gravity and against some resistance such as bodyweight then the Oxford Scale grading is 4/5. It is a professional judgment as to the resistance to be applied for the test, and the physiotherapist will have in mind the health, age, activity and weight of the patient. If a muscle is to be graded 5/5 it must be of normal power, but as this will vary greatly between individuals the physiotherapist must make an estimation of the expected full muscle power for that particular patient. Grade 5 for a frail sick person will be very different from grade 5 for a young, fit sports person.

Grade three out of five for the shoulder muscles might be the ability to lift the arm above the head, but if this cannot be easily done or to full range then the muscle can be graded as three minus to indicate its inability to be fully grade three. If the physiotherapist can resist the muscle firmly but it still doesn't seem to be strong enough for a five, then the rating can be four plus. Physiotherapists go through all the muscles to be tested and rate them all on a muscle testing chart as a record of the muscle strength, which can be retested over time to chart recovery.

Physiotherapists begin muscle strengthening techniques in a position where gravity is eliminated, allowing a weak muscle to be repetitively exercised. As the patient's ability increases they can perform more functional activities of daily life which strengthens the muscles in a co-ordinated way which reflects normality. Next, resistance against muscle action is increased as muscle strength improves in response to the level of intensity of resistance and not just repetition. High intensity causes muscle fibre breakdown which repairs with increased size and power until the next intensity workout repeats the process. Progression is then moved to functional exercise with bodyweight resistance as dynamic movement is more useful.