Your body is an amazing machine. It automatically repairs you from injury, regulates your body temperature and prioritises and absorbs from the nutrition you provide it with.
However, when making physiological modifications such as getting a more compact shape or increasing muscle size, the body needs enough force to signal change is needed. When astronauts go into to space they lose considerable amounts of muscle and bone due to the lack of gravity. This is due to a lack of stimulus through force. The same occurs when someone breaks a limb and it is immobilised for any length of time: you can lose up to 10% of strength per week (Dittmar 1993).
Within an exercise environment:
Higher load = greater stress to body = larger muscular and physiological changes
Low force exercise produces low results. The difficulty comes with performing the exercises that give you the most bang for your buck. These are the most neurologically and physiologically complex and require expert guidance or injury may occur. Within muscle you have fast twitch (type 2a) and slow twitch (type 1) muscle fibres. The fast twitch allows you to jump high, run fast or lift a heavy weight. These are usually performed for short bursts of time and are anaerobic in nature (without oxygen). The slow twitch or endurance based, allowing you to walk or run long distance. These fibres allow you to exercise constantly for a great length of time and use oxygen as the main fuel source (aerobic). You also have a pool of type 2x muscle fibres which can be converted into type 2a fibres, giving you more power or strength. These however are only activated under high loads and are wasted without enough stimulus. To activate your type 2x fibres you need a load that you can only move between 1 to 5 reps and with good form.
The body moves on the path of least resistance, meaning it will do as little as it has to. Therefore if you dont overload the muscle there is not enough stimulus for significant adaptations. During my twelve years within an exercise environment people generally tend to do what is easiest however, to get effective results you have to do what is hardest.