ORIGINAL ARTICLE Weight loss and muscular strength affect static balance control G Handrigan 1 , O Hue 2 , M Simoneau 1,3 , P Corbeil 1,3 , P Marceau 4 , S Marceau 4 , A Tremblay 1 and N Teasdale 1,3 1 Faculte´de me´decine, Division de kine´siologie, De´partement de me´decine sociale et pre´ventive, Universite´Laval, Que´bec, Canada; 2 De´partement dessciences de l’activite´physique, Universite´du Que´bec a`Trois-Rivie`res, Que´bec, Canada; 3 Unite´ de recherche sur le vieillissement, Centre de recherche FRSQ du CHA universitaire, de Que´bec, Que´bec, Canada and 4 Faculte´ de me´decine, De´partement de chirurgie bariatrique, Universite´Laval, Que´bec, Canada 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. Methodology: Force (isometric knee extension) and balance control (center of pressure speed and range) were studied in three groups; normal weight (BMI o25 kg m 2 ), obese (30 kg m 2 oBMIo 40 kg m 2 ) and excess obese (BMI 440 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. Conclusion: 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. International Journal of Obesity (2010) 34, 936–942; doi:10.1038/ijo.2009.300; published online 26 January 2010 Keywords: posture; balance; weight loss and force Introduction Human balance control requires a coordinated transforma- tion of the sensory signals into adequate muscular contrac- tions. Just as muscle strength limits the performance of various athletic tasks such as weight-lifting or jumping, it is also true for daily tasks. For instance, several recent studies have reported a strong association between knee extensor strength and sit-to-stand performance. 1,2 Furthermore, several studies have reported that muscle strength contributes to balance recovery. 3–5 In these cases, dynamic balance was greatly challenged (participants were either leaning forward and released or were tripped while walking) as a step was required to avoid a fall. During normal quiet stance, balance control is primarily achieved from proprioceptive feedback arising from the lower limbs and visual sources. 6 Decreased sensory information is often associated with poor balance control, although there are exceptions to this observation. 7 Generally, individuals that are less stable show a greater risk of falling. 8–10 For instance, Pajala et al, 8 showed that less stable individuals as measured with a force platform had a two- to four-fold risk for falls compared with more stable individuals. In addition, individuals with a large postural sway caused by reduced lower-limb sensory acuity or obesity likely require greater muscular strength to avoid a loss of Received 20 July 2009; revised 22 November 2009; accepted 12 December 2009; published online 26 January 2010 Correspondence: G Handrigan, Medicine Sociale et Preventive, Division de kine ´siologie, Universite ´ Laval, PEPS, 2300 Rue de la Terrasse, Que ´bec, Canada, G1K 7P4. E-mail: grant-a.handrigan.1@ulaval.ca International Journal of Obesity (2010) 34, 936–942 & 2010 Macmillan Publishers Limited All rights reserved 0307-0565/10 $32.00 www.nature.com/ijo