As the effort level rises it is instinctive to lean into the handlebar. The handlebars should not be placed so low that they force a rider to adopt this position. Consider that a rider positioned with straight arms will suffer from shoulder and neck fatigue much earlier than a rider who rides with flexed elbows. Note that the postion of the torso can be accomodated with many hand positions but some of these choices are far more comfortable than others. The handlebar can be rotated in the stem to change the hand position.
Many stems are adjustable so that the handlebars can be raised and lowered.
Traditional stems are known as Quill Stems. They slip into the fork steer tube and are fastened with an internal binding wedge. These stems offer a range of vertical adjustment and have safety markings that indicate the maximum safe height. A stem should never be ridden if it is placed beyond the safety marking recomendations. Another popular style of stem is designed for use with threadless headsets.