These men are levy spears with some training, but they will not stand against Graeco-Roman infantry. They can give a good account of themselves as long as they hold formation.
These poorly trained, levy infantry are supplied by the great nobles (Azads) from their estates in the more settled regions of the Persian Empire. They are armed with an infantry spear and brown, leather-covered, wicker shield, a smaller version of the old spara (gerron) of imperial days, and a short sword or axe. Their primary order of battle would consist of spearmen fighting in ordered ranks. Groups of spearmen such as these are trained to form rows across and files deep and to march in step. Grouping together bolsters morale and the shield wall helps to neutralize arrows. However, the oft-repeated myth of 'roped or chained' Persian troops is an invention of literature. The Arabic term 'silsilah' is very likely a poetic device meant to imply soldiers organized into close order units. The same term is used to refer to both Sassanid Persian and Byzantine cavalry, neither of which could have conceivably been physically tied together in groups!
Historically, the Parthian Nobility displayed the same distrust of armed peasantry as many other feudal elites, The Gund-î Nîzagân were as close as they came to putting that uncomfortable idea into practice, but these foot troops were generally drawn from the poorer classes of Parthian society and were often badly equipped and barely trained. When the indifferent quality of these troops was added to the pace of Parthian warfare, it meant that the Nizag Gund would rarely be committed to heavy action. Their duties would generally include garrison and baggage guard, but they could also form a spear wall in pitched battles.
These light infantry are excellent in defense due to their good shields and their highly disciplined formation. They are able to hold the line against light and medium infantry and can pin down the enemy so that they will be easy prey for horse archers to shoot or cataphracts to unleash a crushing charge. They also make excellent garrison units.
However, they are very frail and they will not last against heavy infantry or phalanxes for so long and a devastating attack like a missile attack from the rear or a crushing cavalry charge will easily rout them.