By Matt Fitzgerald | For Active.com

Each time you go for a run you carry with you a hope and an expectation that the workout will have a positive effect on your fitness. There are two fundamental variables that determine how a run affects your body: duration and intensity.

Duration can be measured directly by time or indirectly by distance. The longer a run lasts, the more pronounced its training effects. This doesn’t mean every run should be as long as you can make it. The reason is that longer runs generate more fatigue as well as stronger fitness adaptations, and your body can only handle so much fatigue. But as a general rule, as the duration of your runs increase; your fitness level will also increase.

Intensity works differently. All running intensities are beneficial for fitness, but each intensity level is beneficial in its own way. Easy jogging enhances fat-burning capacity, sprinting improves running technique, and intensities between these extremes yield other benefits. The most effective training regimen combines efforts at a range of intensity levels.

It’s one thing to create or select a training plan that balances different training intensities appropriately, but it’s another thing to execute that plan correctly. Doing so requires that you actively monitor and control your intensity during every run.

There are three practical metrics that runners can use to monitor and control their running intensity: pace, heart rate and perceived effort. Each method has advantages and disadvantages, but you will get the best results from your training if you use all three in the proper way.

Pace

Most runners are familiar with pace, which is generally measured in minutes and seconds per mile. It’s easy to measure pace on the roads and trails by wearing a stopwatch and tracking time elapsed between mile markers, or by wearing a speed and distance device that provides continuous pace monitoring. On the track, you can measure pace as the time it takes to complete each 400-meter lap.

Pace is not a direct measure of physiological intensity. For example, whereas one runner’s maximal rate of fat burning might fall at a pace of 10:15 per mile, another, more gifted runner’s maximal rate of fat burning might correspond to a much faster pace of 7:00 per mile.

To use pace monitoring effectively in your training, you need to choose pace targets for different types of workouts that are tailored to your ability and fitness level and therefore elicit the right physiological response. Tools such as Greg McMillan’s Running Calculator make this easy. Just enter a recent race time and the calculator will spit out appropriate target paces for various workouts.

The main advantage of pace monitoring is that, as a performance metric, it pushes runners to run harder for better “splits.” But this is also its main drawback. You shouldn’t push yourself in every run.

Monitoring your pace is helpful in key workouts such as intervals and tempo runs where aiming for a high level of performance is appropriate. All too often, however, runners who monitor their pace in easy runs go faster than they should because the clock puts them in a performance mindset. If you’re susceptible to this temptation, it’s best that you forgo pace monitoring in easy runs.

Heart Rate

Each runner’s heart responds differently to exercise. For one runner, the intensity level known as the ventilatory threshold (where talking becomes difficult) might correspond to a heart rate of 150 beats per minute (bpm), whereas for another runner the same threshold may occur at 160 bpm. To train effectively by heart rate you need to establish target heart rate zones that are tailored to your fitness level.

The traditional way to do this is to perform some kind of heart rate assessment test, use the results to calculate custom heart rate zones, and program these zones into a heart rate monitor. This process can be a bit of a hassle and has prevented many runners from using heart rate monitors. Recently however, new devices have been developed to automate these calculations. Look for products such as PEAR Mobile.

The simplest way to benefit from the use of a conventional heart rate monitor is as follows: Warm up and then run as far as you can in 30 minutes. Note your average heart rate during the last 10 minutes of the time trial. This is your approximate lactate threshold heart rate. Multiply this heart rate by 0.89. Keep your heart rate at or below the resulting number in all runs that are intended as easy runs.

For example, if your average heart rate in the last 10 minutes of your time trial is 160 bpm, do your easy runs at a heart rate of (160 x 0.89 =) 142 bpm or below.

Perceived Effort

Perceived effort is essentially how hard a running effort feels. The advantage of perceived effort relative to other intensity metrics is that it’s a global indicator that encompasses all of the physiological factors that determine exercise intensity.

The main disadvantage of perceived effort is that it’s subjective and therefore can be unreliable. For example, research has shown that most recreational runners believe they’re running at a low intensity (below the ventilatory threshold) when in fact they’re running at a moderate intensity.

It’s important to be mindful of perceived effort every time you run. When properly interpreted, perceived effort is the best indicator of your body’s overall status and limitations.

Any given pace or heart rate will not feel the same in every run. If you feel much worse than normal one day at a familiar pace or heart rate, this is a reliable sign that something is off physically or psychologically and you should probably slow down. On the other hand, if you feel exceptionally good in a workout, it may be beneficial to push beyond your target pace or heart rate provided it’s a workout that is supposed to be challenging.

The Right Mix

Now let’s put everything together: Use pace to monitor and control your intensity in harder workouts where performance matters, but ignore pace in easy runs. With heart rate monitoring it’s just the opposite: Use heart rate ceilings to ensure that your easy runs are truly easy. Finally, be mindful of your perceived effort in every run and use it to overrule pace or heart rate data.