The feet directly impact the muscular structure of the back and legs.  Generally, people who work long hours in a single position often experience swelling in the legs and other related problems.   A proper footrest can go a long way in improving comfort and increasing productivity. 

A foot rest is extremely advantageous for people who suffer from back problems or problems related to the lower limbs.  A foot rest can be of great help because, when the feet are planted firmly on the ground or supported in some other way, the blood circulation level is good and the body posture is neutral.  A thoughtfully designed foot rest provides support to the lower back, maximizes blood circulation in the legs and encourages proper posture.

Advantages of using a foot rest are -

Foot rests are relatively inexpensive tools that can modify your workstation and encourage good posture.

Feet placed on foot rests experience less stress not just in the feet but also the legs and spine. This is because of proper alignment of lower back.

Foot rests are especially handy in the absence of an adjustable seat.  As a resting place for the feet, it allows users to maintain a neutral and comfortable sitting position.

Foot rests help decrease pressure at the ischium and thigh levels.

Foot rests contribute in increasing productivity by providing higher comfort levels.  A convenient sitting position achieved through ample feet and back support translates into comfortable work environment.

Foot rests by its very design do not allow prolonged sitting.  This encourages users to take breaks or short walks. Such breaks make for increased blood circulation to the legs and minimize fatigue.

Though footrests by itself will not improve bad posture, they will definitely help control the negative effect of working in a single position by encouraging movement.

Mentioned below are some pointers that will help you in choosing the right foot rest -

A height adjustable foot rest would be an ideal choice.  One that allows up to 5-inches height is best.  A height ranging from 2-5" height fits majority of the people.

An adjustable tilt angle ranging from 0-35 degrees is recommended.  Most people find their comfort zone within this range.

A foot rest that rocks back and forth will help stretch your calf and ankle muscles.  This in turn helps in reducing muscle fatigue, increases blood circulation and improves the general comfort levels.

The foot rest should have a large surface, preferably carpeted as this will cushion the feet thus minimizing fatigue.

A simple design will be easier to clean and maintain.

The size of the footrest should be big enough to accommodate both feet simultaneously.

The surface should be a non-skid one, so the feet do not slip off the surface and is held firmly in place. 

Portability is an important factor so that it can be easily used in more than one location.

Purchase a reputed brand with warranty from a good manufacturer.