Due to viscosity, drag forces and frontal resistance, water provides resistance proportional to the effort exerted against it. Experts say resistance to movement can range from approximately four to 42 times greater in water than air, making water a natural and instantly adjustable weight-training machine. Unlike most land-based exercise, water can strengthen both sides of the muscle pair simultaneously. For instance, an arm curl works both biceps and triceps, because there is resistance to the movement in both directions of the exercise. Water's resistance can be increased with speed and/or surface area and is proportional to the effort required to move against it. Adding water fittness equipment can increase resistonce to strengthen and tone the muscles.