Setting the Proper Motivation

A pioneering thinker in the field of workplace motivation, David McClelland developed his theories chiefly while at Harvard in the 1950-60's with experiments such as this: Volunteers were asked to throw rings over pegs rather like the fairground game; no distance was stipulated, and most people seemed to throw from arbitrary, random distances, sometimes close, sometimes farther away.
However a small group of volunteers, whom McClelland suggested were strongly achievement-motivated, took some care to measure and test distances that would produce an ideal challenge - not too easy, and not impossible.
Interestingly a parallel exists in biology, known as the 'overload principle', which is commonly applied to fitness and exercising, ie., in order to develop fitness and/or strength the exercise must be sufficiently demanding to increase existing levels, but not so demanding as to cause damage or strain.
McClelland identified the same need for a 'balanced challenge' in the approach of achievement-motivated people. People with a strong achievement-motivation need set themselves challenging and realistic goals - they need the challenge, but they also need to be sure they'll accomplish the aim.