Final answer:
To ensure your client reaches volitional fatigue at eight repetitions during the leg press, you should select a weight that is approximately 70-80% of their one-repetition maximum (1RM).
Step-by-step explanation:
When constructing a program for your client and you want them to complete eight (8) repetitions to volitional fatigue on the leg press, you should select an intensity that is roughly 70-80% of the 1RM (one-repetition maximum).
The one-repetition maximum (1RM) is the maximum amount of weight that a person can possibly lift for one repetition of a particular exercise. It is a standard measure to base training intensities upon. Since eight repetitions typically correspond to what is known as a strength training zone, 70-80% of 1RM provides a good balance between volume and intensity to promote muscular strength gains and induce volitional fatigue by the final repetition.
If the weight were too light (50-60% of 1RM), your client could likely perform more than eight repetitions, missing the target rep count for desired intensity. On the other hand, 90-100% of 1RM would likely limit your client to fewer than eight repetitions. A weight of 110-120% of 1RM would exceed what most people can lift for any repetitions and is not typically used in conventional programming.