Are you struggling with a stiff sash window? Before you worry that your window may be defective, please consider your access to the window.
Occasionally, household items and / or features such as kitchen worktops, sinks, desks etc, will prevent us from getting close to the sash window that we are trying to open; in these cases, we have no choice but to fully outstretch our arms.
Unfortunately, the muscles responsible for providing most of the lifting force required to open a sash window (the Longissmus Dorsi and Latissimus Dorsi) are located along our spines, whereas we use our arms to actually lift an object. This means that our arms act as a Lever, with our spines acting a Fulcrum, thereby increasing the lift force required as we increase the extent to which our arms are outstretched.
As you can see from the above diagram, the greater the distance between the Load (the sash window you’re trying to open) and the Fulcrum (your spine), the greater the lift force that is required. In the case of sash windows using a spiral balance system, the increased friction experienced by springs (in comparison to weights and pulleys) further exacerbates this effect.
SOURCE: University of Cambridge. Department of Physics. ‘Lifting and Handling, a Risk Assessor’s Guide’. August 2006.