CARDIO FOR THE CALL: THE TWO SYSTEMS FUELLING FIRST RESPONDER PERFORMANCE

Strength and Conditioning is often referred to as the big ‘S’ and the little ‘c’ because conditioning is the forgotten brother of strength training. Generally speaking, tactical athletes (first responders and soldiers) are better than their sporting counterparts when it comes to addressing the cardiovascular system, but you might be surprised to know that there are two components of cardio, both equally important, and one is often overlooked.

Your body requires energy to move, and the primary source of this energy varies depending largely on the duration and intensity of the movement. The first 10-60 seconds of a high intensity movement like sprinting is fuelled primarily by the anaerobic systems (phosphagen and glycolysis), which are highly powerful energy systems that can only function for a short duration. By the two-minute mark, there will have been an observable drop in intensity as the aerobic energy system becomes the primary fuel source. The aerobic system, while not as powerful as the anaerobic systems, is capable of sustaining movement for long durations.

"While a 10-km run is a fantastic stimulus for the aerobic energy system, it does little to develop the anaerobic systems."

Contrary to what was once popular belief, these systems do not switch on and off but rather transition from one to the other. 

Just as each system specialises in energy supply at different times, the training required to develop each system must also be specialised. While a 10-km run is a fantastic stimulus for the aerobic energy system, it does little to develop the anaerobic systems.

Aerobic System

"The aerobic energy system requires oxygen to convert fuel to energy, and it takes time for your body to take in enough oxygen to fulfil its role in the energy supply process."

When most people talk about doing ‘cardio’ they’re referring to activities that will develop aerobic capacity. Running, swimming, bike riding and walking are stimuli typically associated with the aerobic system assuming the effort is longer than 2-3 minutes and at a relatively consistent intensity.

The aerobic energy system requires oxygen to convert fuel to energy, and it takes time for your body to take in enough oxygen to fulfil its role in the energy supply process. If you imagine the human body is a house, then the aerobic energy system is the solar panels. Once they absorb enough sunlight, they’re capable of supplying sustained energy for long durations, but in the absence of sunlight, another energy source is required. Just as you can add more solar panels to your house to make the capacity larger and the energy supply more efficient, you can train your aerobic capacity to make it more efficient, also.

Training your aerobic capacity with longer, steady cardio workouts like a 10-km run, a spin class, a CrossFit WOD or an open-water swim is the equivalent of adding more solar panels to your house, as it allows you to go further on that system. You can increase your body’s capacity for oxygen which makes the overall system far more effective. While there are a multitude of benefits to a powerful, well-developed aerobic capacity for first responders (more on that in future articles), a lot of the job requirements for the typical emergency service employee are fuelled by the anaerobic systems.

Anaerobic Systems (Phosphagen and Glycolysis)

"This type of training requires an exceptionally high intensity for very short periods followed by large amounts of rest."

Using the solar panel analogy, the anaerobic energy systems are the bursts of electricity your house draws from the grid in the absence of sunlight, or when the demands exceed the capacity of the panels. In the initial stages of activity, your body will draw on the anaerobic energy systems until it’s absorbed enough oxygen to fuel the aerobic system, and these initial stages in the absence of oxygen require their own specific training.  

Imagine sprinting from a vehicle to a building, taking down a resisting offender, lifting and carrying a patient from the ground, or pushing a broken-down vehicle from the road. These short, high intensity tasks are fuelled primarily by the anaerobic systems. Training with high intensity and low duration efforts with adequate recovery times can both increase the power and endurance of these systems which become crucial during high-consequence and potentially lifesaving moments.

The anaerobic systems are made up of the phosphagen (ATP-PC) and glycolysis (glycolytic/lactate) system, and they’re both trained with different stimulus.

The most noteworthy thing when training the anaerobic systems is the work to rest ratios. This type of training requires an exceptionally high intensity for very short periods followed by large amounts of rest – so much that it often feels like too much – in order to remain anaerobic and allow the energy stores to replenish prior to the next effort.  

The temptation in this type of training is to increase the intensity by extending the working effort. As you can see from Table one, extending a 95% effort from five-seconds to 10-seconds changes the focus from Phosphagen Power to Phosphagen Endurance, and increasing it again from 10-seconds to 15-seconds changes the energy system entirely. Focus instead on increasing reps. Similarly, shortening rest periods is another common mistake. The anaerobic systems require a supply of stored fuel to convert into energy, and without adequate time to replenish this fuel supply, each effort after the first one becomes aerobic, removing the desired training stimulus.

Measuring Intensity

Intensity and duration were the two key elements identified above that determine the energy system required to fuel movement. Duration is easy to measure, but intensity is far more subjective.

There is no simple way to measure intensity accurately, but %HR Max. combined with the Borg’s RPE Scale is a useful guide to inform your training, particular for high intensity intervals.  

Though other formulae exist, the simple method for calculating predicted HR Max. is 220-age. Then, by calculating your working heart rate as a percentage of your predicted maximum, you can identify your HR Zone and Energy System. Alternatively, in the absence of a smart watch or HR monitor, you can rate your effort on the Borg’s Scale from 6-20. Any rating between 17-20 will be an anaerobic effort, assuming your rating is accurate.

Summary

For first responders, conditioning isn’t just about endurance – it’s about versatility. Both aerobic and anaerobic systems play critical roles in operational performance and neglecting either can leave gaps when seconds matter most. Long, steady efforts build the foundation for sustained energy, while short, high-intensity intervals sharpen the explosive power needed for lifesaving actions. By training both systems deliberately and understanding how intensity and duration shape energy demands, you’ll be better prepared for the unpredictable challenges of the job. Cardio for the call isn’t one-size-fits-all – it’s a strategic approach to resilience, readiness, and peak performance.