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This type of restlessness during meditation is different then mental restlessness altough they can be related. i have had a long history of muscular restlessness. It comes with not getting yourself comfortably situated, tension in muscles and not getting prepared by tensing and relaxing techniques as well as relaxing the body in preparation for meditation.

Relaxation, when it precedes meditation, is the way to the infinite -- Paramahansa Yogananda

http://www.swamij.com/relaxation.htm#beforerelaxation

Some of these tense and relaxation practices have a similarity to what I have found in the srf lessons. It is  interesting that swami J ( Swami Jnaneshvara Bharati) uses the term 'surveying' to describe the process helpful in relaxation.

It is a process of surveying inside: It has become common to refer to the surveying of the body and other internal states as a practice of relaxation. Actually, it is more accurate to call it surveying than relaxation, since the actual activity being performed is surveying. Some schools of meditation put their main emphasis on such practice.

Attention and Breathing are two key principles

Two keys: If we explore within, with our attention, particularly in conjunction with breath awareness (which is the grossest aspect of the energy flowing throughout our body), the relaxation comes of its own accord. These two principles or practices, attention and breath, are the key features in the relaxation phase of Yoga Meditation.

The importance of attention and breathing in relaxation cannot be overstated. Again, the key principles for relaxation are:

Attention to the various aspects of your being
Breath awareness
These two work together naturally in allowing the relaxation of the physical body, as well as the mind.

It is extremely useful for a practitioner of Yoga Meditation to remember these two simple principles.

In the link above swami J gives the mind something to do when it is restless through the tense and relaxation exercises.