OBJECTIVE Overweight individuals sway more than normal weight individuals. Major weight loss improves their balance control despite a related decrease in muscle strength. Presumably, muscular strength is an important factor for balance control. This study investigated the effect that a change in body mass has on relative strength and balance control. METHODS Force (isometric knee extension) and balance control (center of pressure speed and range) were studied in three groups; normal weight (BMI <25 kg m(-2)), obese (30 kg m(-2) <BMI< 40 kg m(-2)) and excess obese (BMI >40 kg m(-2)) Caucasian male individuals. RESULTS The excess obese individuals who underwent bariatric surgery as a weight loss strategy were studied before, 3 and 12 months after losing on average, 66.9 kg (+/-95% CI 55.8, 77.9 kg; on average, 45% of their weight). The obese individuals who underwent diet modifications were studied before dieting and when resistance to weight loss occurred after losing on average 11.7 kg (+/-95% CI 9.3, 14.2 kg; on average, 12% of their weight). The control group was studied twice, 50 weeks apart. In obese and excess obese individuals, losing weight reduced absolute knee muscular strength on average, by 8.2 kg (+/-95% CI 3.9, 12.5 kg; on average, 10% of their strength) and 23.9 kg (+/-95% CI 12.1, 35.8 kg; on average, 33% of their strength). However, it also increased balance control measured with speed of the center of foot pressure, on average, by 0.10 cm s(-1) (+/-95% CI 0.05, 0.14 cm s(-1); or increased of 12%) and 0.28 cm s(-1) (+/-95% CI 0.07, 0.47 cm s(-1); increased of 27%), respectively. Relative strength increased approximately by 22% for only the excess obese group 12 months post surgery. CONCLUSIONS This suggests, in overweight individuals, weight loss is more efficient at improving balance control than increasing, or even maintaining muscle strength. In these individuals, training programs aimed at improving balance control should primarily target weight loss.