N. Disser, P. Henricksen, S. Lamarche, D.B. Thorp

Gonzaga University, Spokane, WA

PURPOSE: The aim of this study was to compare the metabolic energy cost of repeated and sustained voluntary isometric contractions in healthy young adults. METHODS: Seventeen Gonzaga University students (9 male, 8 female), aged 19-22, participated in this study. Maximal isometric knee extension contractions were performed while the rate of oxygen consumption (VO2), heart rate (HR), rate of perceived exertion (RPE), and force were recorded. The sustained protocol consisted of 6x6 second isometric contractions with 14 seconds of rest, and the repeated protocol consisted of 12x3 second contractions with 7 seconds of rest. Each protocol was repeated twice with 5 minutes of recovery in between, and the order was randomly assigned. VO2was integrated to give the oxygen cost of the exercise, and force was converted to torque and integrated to give the angular impulse produced by the knee extensors. The two trials were averaged and paired t-tests were used to compare VO2(L/min), oxygen cost (L), HR, RPE and angular impulse between contraction protocols. RESULTS: During contraction bouts, no difference in oxygen cost was detected between the repeated (0.365 +0.203 L) and sustained (0.361 +0.179 L) protocols. However, in recovery, repeated contractions (0.275 +0.178 L) required 46.6% more oxygen than sustained contractions (0.188 +0.216 L) (p=0.005). In total, repeated contractions (0.640 +0.343 L) required 17.3% more oxygen than sustained contractions (0.548 +0.352 L) (p=0.008). There were no differences in HR (repeated: 96.9 +13.6 bpm, sustained: 99.2 +14.6 bpm) or RPE (repeated: 12.1 +2.4, sustained: 12.5 +2.4) between conditions. Additionally, there were no differences in angular impulse between the repeated (4372.1 ± 2363.4 J*s) and sustained (4450.6 ± 2417.5 J*s) protocols. CONCLUSIONS: Repeated isometric contractions require greater oxygen consumption compared to sustained contractions when the time in contraction is held constant, and this difference is observed during the recovery period. From this data, we conclude that repeated bouts of isometric contractions are more metabolically costly than sustained contractions despite the same force being applied, and these energy requirements are met by anaerobic metabolic pathways during contraction and replenished by oxidative metabolism during recovery.

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