STUDY DESIGN: This study analyzed force plate, kinematic, and metabolic energy data of 14 able-bodied subjects standing statically with upright and trunk-flexed postures. OBJECTIVE: To explore the effect of trunk-flexed postures on balance and metabolic energy expenditure during standing. SUMMARY OF BACKGROUND DATA: Abnormal trunk posture often occurs in the presence of spinal deformities, such as lumbar flatback. It is unclear whether alterations in trunk posture affect energy expenditure and the location of the body's center of mass in the transverse plane (BCOMtrans) during standing. METHODS: Kinematic, kinetic, and energy expenditure data were collected with upright trunk alignment and with 25 degrees +/- 7 degrees and 50 degrees +/- 7 degrees of trunk flexion from the vertical. The mean location of the BCOMtrans was estimated from the net center of pressure (COP), which is a weighted average of the COP beneath both feet. RESULTS: The fore-aft position of the net COP under the base of support was not significantly different between postures (P < 0.08). At each posture, the net COP was located 16% of the foot length anterior to the ankle joint centers. However, with increasing trunk flexion, there was a significant increase in oxygen consumption rate (P < 0.001 for all postures). CONCLUSION: Compensatory actions, such as ankle plantarflexion and hip flexion, allowed the mean position of the net COP to remain within a narrowly defined region irrespective of trunk posture. Changes in muscle activity associated with a trunk-flexed posture and the associated compensations likely contributed to the increased energy expenditure.