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Developing Energy Systems for the Spring-Drag-Carry

Updated: Oct 30, 2019

The Sprint-Drag-Carry (SDC), event four of the Army Combat Fitness Test (ACFT), measures more fitness in less than 2 minutes than an entire APFT. One could also make a strong argument that the components of fitness measured by the SDC more closely resemble the physical demands of combat than any other event on the Army’s new test. According to the principle of specificity, when training for the SDC, we must stress the most critical movement patterns and develop the appropriate energy system.

Here’s a quick introduction to energy system development:

We use three energy systems to generate ATP (our body’s energy currency). The Immediate energy system (also called the ATP-PCr or Phosphagen system) provides energy for short bursts of rapid activity up to 10 seconds in duration. Most of the ATP used during this time frame already resides in the muscle or is generated rapidly from phosphocreatine (also present in the muscle). ATP is generated without oxygen by the ATP-PCr system (so it is anaerobic), and the chemical processes do not generate lactate (so it is alactic). For this reason, training to increase capacity in this domain is sometimes referred to as alactic-anaerobic training. This system supplies most of the energy required to perform the MDL and SPT.

Like the ATP-PCr system, the Intermediate energy system, or fast glycolysis, also produces ATP quickly via anaerobic means. Unlike the immediate energy system, the intermediate system depends on the production of lactate to keep producing ATP at a high rate. As long as the need for ATP does not significantly outpace the system’s ability to buffer pH, fast glycolysis will continue to support high intensity activity for up to 90 seconds. Glycogen (sugar stored in the muscle) and glucose (sugar in the blood) are the primary energy substrates used to generate ATP via the intermediate system, making it an anaerobic process.

bioenergetics, energy system development, Army Combat Fitness Test, ACFT, Training For 600
Understanding Energy System Contribution to various exercise durations

The third energy system, the aerobic or oxidative system, provides the majority of our ATP needs for work that endures beyond 2 minutes. As its name indicates, the oxidative system is aerobic and relies on ATP produced slowly in the presence of oxygen in the cell’s mitochondria. True aerobic exercise is fueled by this slower process, and great endurance athletes can exercise in the aerobic domain for several hours (even at sub 6 minute per mile paces). One of the efficiencies great endurance athletes rely upon is a high anaerobic threshold (AT). This means that despite a relatively quick pace, almost all of their ATP is produced aerobically. You’ve probably felt the transition from aerobic work to anaerobic work if you’ve ever “turned it on” too early during your 2-mile run on the APFT. Save the “kick” for the last 800 meters and you might make it, but kick too soon