The force-length relationship shows us that the least amount of force can be produced when a muscle is at its most lengthened and most shortened positions. That doesn’t mean we shouldn’t train in those positions it just means we can be limited in our gains if we train primarily in those positions. The hardest part of a machine leg curl is when the knee is flexed. Since that position in a seated leg curl is in a more favorable length for force production it’s reasonable to say this is a better op