Muslim armies of the past were able to maintain a high level of effectiveness in the field due to a number of factors. One important factor was the strong sense of religious and cultural unity among the soldiers, which helped to increase morale and cohesion within the ranks. Additionally, Muslim armies were often well-trained and disciplined, and had access to advanced military technology and tactics.

Additionally, the Muslim empire was vast and diverse and the armies were composed of soldiers from various ethnic groups and regions, this diversity could be a source of strength as well as a problem, as it could lead to difficulties in communication and coordination.

Another important aspect that helped Muslim armies to maintain their effectiveness was the ability to draw on the resources of the vast Islamic empire and the support of the population. This gave them a logistical advantage over their opponents, as well as a steady stream of new recruits.

It's important to note that this is a generalization and it could vary depending on the period of time, the specific army, and the context.