Final answer:
Muscle force is reduced at longer lengths due to decreased myosin head attachment.
Step-by-step explanation:
Muscle force is reduced at longer lengths due to decreased myosin head attachment. When a muscle contracts, myosin heads attach to actin to form cross-bridges and generate muscle tension. However, cross-bridges can only form where thin and thick filaments already overlap. At longer muscle lengths, there is less overlap between the filaments, resulting in fewer available sites for myosin head attachment and therefore reduced muscle force.