The Ergogenic Effect of Amphetamine

Abstract

Amphetamine (Amp) increases exercise duration. It is thought to do so by masking fatigue, but there have been very few studies looking at the effect of amphetamine on VO_2MAX and running economy. Furthermore, it is unknown if amphetamine's effect on exercise duration occurs in a warm environment. We conducted separate experiments in male Sprague-Dawley rats testing the effect of amphetamine on maximal oxygen consumption (VO_2MAX) (n=12), running economy (n=12), and exercise duration (n=24) in a warm environment. For VO_2MAX and running economy, rats were randomized to either amphetamine at 1 mg/kg (Amp-1) or 2 mg/kg (Amp-2). Animals served as their own controls in a crossover design with the administration order counter-balanced. To study the effect of amphetamine on exercise duration, we conducted run-to-exhaustion treadmill testing on rats in a 32°C environment following administration of Amp-1, Amp-2, or Saline. Compared to control, Amp-2 increased VO_2MAX (by 861 ± 184 ml/kg/hr, p=0.005) and the time to VO_2MAX (by 2.5 ± 0.8 min, p=0.03). Amp-1 had no effect on VO_2MAX but increased the time to VO_2MAX (by 1.7 ± 0.5 min, p=0.03). Neither dose improved running economy. In the warm, only rats in the Amp-1 group (+9.4 min, p=0.02) had an increased time to exhaustion. Compared to control (41.6 ± 0.3°C), both amphetamine doses had higher temperatures at exhaustion: Amp-1 (42.0 ± 0.2°C) and Amp-2 (42.1 ± 0.2°C). Our results suggest that ergogenic effect of amphetamine occurs by masking fatigue but this effect may be offset in the warm with higher doses.

Keywords: Exertional heat stroke; Exhaustion; Running economy; VO2Max; amphetamine.

Figure 1.

Amphetamine increases aerobic capacity at room temperature. Amphetamine at a higher dose (2mg/kg) increased VO_2Max (A) and time to achieve VO_2MAX (B), while Amp at a lower dose 1 mg/kg) increased only time to VO_2Max (B). Rats received (i.p.) either saline or one of 2 doses of Amp; 5 min later they performed the incremental treadmill test at 24°C for assessment of VO_2Max. Metabolic measurements were obtained via an indirect open-circuit calorimetric system with analysis of gases collected from an enclosed motorized treadmill chamber (Oxymax). In a crossover design, at least 2 d later, the animal groups were reversed and the trial repeated. Horizontal bars represent trial means ± SEM. n = 6 per group.

Figure 2.

Amphetamine does not improve running economy at room temperature. Rats received (i.p.) either saline or 1 mg/kg (A) or 2 mg/kg (B) of Amp (n = 6 per group). Five min later, they were run at 4 different workloads on a motorized treadmill for 6 min/per load at 24°C. Running economy was assessed at each workload via analysis of expired gases for determination of steady-state O₂ consumption (Oxymax). In a crossover design, at least 2 d later, the animal groups were reversed and the trial repeated. Horizontal bars represent trial means ± SEM.

Figure 3.

Temperature curves for rats running to exhaustion in the heat. The mean temperature curves (generated between −10 and 27 min) for each group are shown. These times points were chosen as the represent the period in which no animals had yet achieved exhaustion. The corresponding squares to the right of the graph represent the grouped mean time to exhaustion and temperature at exhaustion. Error bars represent ± SEM, n = 8 for each group

Figure 4.

Amphetamine increases the temperature at exhaustion. Rats received (i.p.) either saline or Amp (1 or 2 mg/kg). Five min later they were placed on a treadmill in a warm environment (32°C) and run at 18 m/min at a 5° incline. For rats running at this constant workload in the heat, low dose amphetamin (1 mg/kg) increased time to exhaustion (A), and temperature at exhaustion (B); Amp at higher dose (2 mg/kg) increased only the temperature at exhaustion (B). Horizontal bars represent group means ± SEM, n = 8 for each group.