Final answer:
The greater the load on a muscle, the longer the muscle contraction takes and the less it shortens, due to increased resistance and the length-tension relationship governing muscle tension.
Step-by-step explanation:
The relationship between load and speed of muscle contraction is such that the greater the load on a muscle, the longer it takes for the muscle to shorten, and the less it shortens. This is because to move an object (the load), the sarcomeres within the muscle fibers must shorten, creating muscle tension. Under heavier loads, there is more resistance to overcome, leading to a slower contraction speed. As for the extent of shortening, a muscle fiber loaded heavily will not shorten as much compared to when it is under a lighter load. This is explained by the length-tension relationship, where the optimal overlap between actin and myosin (the thin and thick filaments) allows for the greatest force generation.