The weight on a barbell will never lie to you. Maybe you grew up in a world where even the dumbest kid in class got a Super Good Tryer award, but when you walk a heavy squat out of the rack, it's going to honestly tell you how strong you are.

We've all had those good days. The bar sits ominously on the floor weighted close to a personal record. You walk up, take a deep breath, and deadlift it so fast it feels like you could've power cleaned it.

Likewise, we know what the bad days feel like. You walk out a squat that should be barely over a warm up and it feels crushing. Everything hurts. All you want to do is take a nap.

The difference between your good gym days and your bad ones are determined largely by the status of your nervous system. Just as you stress a muscle in training in order to force it to adapt and become stronger, your nervous system is being stressed, recovering and attaining a new level of strength.

The cycle between stress and recovery matters to your muscles and it's a crucial factor in the health of your nervous system as well. There's a fine line between the right amount of stress, sufficient recovery, and going too far.

How do we know where we are in that continuum? What can we do to ensure that we have more good days in the gym than bad?

Enter Heart Rate Variability

There are many different ways to monitor nervous system status, from detailed sleep analysis to sticking tiny needles into nerve fibers to measure sympathetic traffic. They all work to varying degrees. The preferred method, due to ease of use, accessibility, and accuracy is monitoring heart rate variability, or HRV.

HRV allows us to take the guesswork out of day-to-day manipulation of training intensity and recovery. It helps to individualize control of the processes necessary for mental and physical performance as well as resistance to sickness and injury.

What is HRV?

To understand HRV, you need to know the basic structure of the two branches of the nervous system.

Autonomic nervous system. Controls the body's functions necessary for survival such as breathing, digestion, heart rate, blood pressure, and organ control. Voluntary nervous system. This is consciously controlled and allows you to perform daily functions like lifting weights, running, or picking up a coffee mug.

Within the autonomic nervous system there are two sub-systems that coexist in a push-pull relationship.

The first is the sympathetic nervous system, which creates the "fight or flight" response. This increases physiological performance when a stressor is introduced. The second subsystem is the parasympathetic nervous system, which counters the body's response to the sympathetic system and helps create an environment conducive to rest and recovery.

Don't think of them so much as a gas and brake pedal because they don't fight each other. Rather, consider them in terms of a continuum, working in unison to varying degrees.

Your heart doesn't beat in a perfectly steady, metronomic fashion. Rather, the frequency of your heart rate varies with respiration. Each time you exhale, within milliseconds the brain sends an inhibitory parasympathetic signal to the heart that slows it. As soon as you inhale, that signal drops away and sympathetic tone increases, causing a slight increase in heart rate.

This back and forth balance provides a window into the status of the two components of your autonomic nervous system. If the parasympathetic "rest and digest" system is strongly activated, you'll see a high level of variability in heart rate. If the sympathetic system is dominating, the parasympathetic system will be blunted and variability will be low.

This allows insight into how your body is responding to allostatic load, which is the cumulative demand placed on your neuroendocrine system to maintain homeostatic balance under dynamic conditions.

Selectivity: Strong Stress Response and Rapid Recovery

Beyond the immediately visible body composition measures seen from training, it's important to understand the deeper psychobiological side of performance.

Often the difference between an Olympic podium and "also ran" status, or a successful graduate of Special Forces Selection and washing out comes down to neurological components of performance.

Ideally, an individual has a powerful, sympathetic response to an acute stressor (like a competition), along with an equally powerful parasympathetic response when it comes time to rest and recover, whether that's on days off from training, de-load periods, or even breaks between rounds in an MMA fight or tennis match.

Compared to more average competitors, Olympic caliber athletes and Special Operations personnel have simultaneously stronger sympathetic responses during competition and higher parasympathetic input during rest. They swing further to either side of the continuum.

They tend to have lower baseline stress hormones with greater diurnal variation of cortisol, meaning that they have significantly higher levels of cortisol in the morning than in the evening, which allows their body to fluctuate between higher arousal during the day and deeper recovery at night.

Elite Performance versus "Here's your participation ribbon"

A study done by Potterat et. al. compared Navy SEALs to "non elite" men from a conventional military group. The SEALs had higher heart rate dipping, which means that during sleep, when the parasympathetic system takes over to induce recovery, their heart rates dropped by an average of 29%, whereas the non-elites dropped by only 21%.

The SEALs also had lower baseline levels of the stress hormone cortisol during the day while in a free living (non-stressed) environment, despite an equal level of cortisol release during a stressful survival course.

That's a crucial point. The elite performers had stronger parasympathetic tone during non-stressful living conditions. The ability to relax is an asset.

In another study comparing Special Forces (SF) soldiers to conventional troops during an intense survival school, the SF soldiers were found to have higher levels of sympathetic-driven norepinephrine (a.k.a. noradrenaline) during the stressful scenarios. When the training was over, the norepinephrine levels of the SF soldiers returned to their pre-training baseline, whereas the non-elite soldiers had significantly depressed levels. Their sympathetic system was exhausted.

In the intensely stressful Combat Diver Qualification Course, the highest performers had the lowest rates of HRV immediately before and during the course, illustrating that when it was time to take on the challenge, the top performers responded with the strongest sympathetic responses.

Conversely, the men with highest heart rate variability at this point demonstrated the highest levels of "burnout" and lowest performance. As the researchers hypothesized, they had the lowest level of "engage threat" response.

All of this illustrates the importance of a healthy, powerful, stress response and the ability to rapidly recover from acute stressors while minimizing unnecessary chronic stress. The way you manage your training and recovery will determine your ability to develop that.

HRV and Athletes

Even if you don't plan on navigating underwater for three miles, being taken captive by enemy forces, or shooting Bin Laden in the face, these concepts have important implications for any athlete.

Similar observations of stress responses during elite level performance have been made when examining Olympic athletes, especially in the research of Dr. Eric Potterat and his colleagues at Optibrain, an institute dedicated to understanding the neurobiological underpinnings of extreme performance.

Additional illuminating research has also been conducted on HRV in athletes ranging from powerlifters to distance runners and swimmers.

In competitive powerlifters, scientists found a correlation between HRV, strength, and levels of the hormone DHEA (a hormone crucial for neural repair). Following a two-hour strength workout that included lifts at 95% of 1-rep max, lifters were monitored for a 72-hour recovery period.

Strength levels, HRV and DHEA decreased after the workout. These levels recovered to baseline and in some cases increased to new levels by the end of a 72 hour period. (See the charts below.)

HF in the above graph is a measure of parasympathetic tone

This illustrates the neural recovery process taking place following a heavy training session. The initial stress of the workout increased sympathetic output and depleted neuroprotective DHEA levels. As the recovery process took place and inflammation was reduced, HRV and DHEA levels recovered.

In a study done on middle distance runners, athletes were put through an exhaustive three-week training protocol followed by a one week de-load. As allostatic load from training stress accumulated, the athletes saw a progressive decrease of as much as 40% in HRV.

This change reversed itself during the recovery period, and the athletes ended with a net increase in HRV. This confirmed the hypothesis that repeating "consecutive high and low intensity training periods could result in a progressive cumulated increase in, mainly, parasympathetic activity, which has been shown to be directly correlated to higher VO2 max values."

Interestingly, resting heart rate was also measured during this study, and showed a change of only about ten percent (roughly 4 bpm) throughout the study. Considering the countless potential variables affecting resting heart rate and the relatively small degree of change, this makes resting heart rate a difficult means of assessing nervous system health.

HRV and Chronic Inflammation, Cortisol, Glucose, Immunity and Soft Tissue Pathologies

Heart rate variability, however, is a reliable means of predicting systemic inflammation, chronically elevated stress hormone levels, altered glucose management, immune health and soft tissue injuries.

"Decreased vagal function and heart rate variability (HRV) were shown to be associated with increased fasting glucose and hemoglobin A1c levels, increased overnight urinary cortisol, and increased proinflammatory cytokines and acute-phase proteins. All of these factors have been associated with increased allostatic load and poor health." Thayer and Sternberg, Neuroendocrine and Immune Crosstalk, Volume 1088, 2006.

In a massive study performed on competitive swimmers, they found reliable correlates between HRV and sickness as well as soft tissue pathology.

Mainly, the days immediately before sickness or soft tissue injury were preceded by an increase in HRV, whereas the weeks during the actual sickness or injury were marked by increased sympathetic tone and lower HRV.

According to the researchers, the most likely explanation for a brief increase in HRV before sickness is that the parasympathetic system increases activity in response to an immediate pathogen, in an attempt at preventing the spread of infection and inflammation.

After an incubation phase estimated to be about five days in length, the parasympathetic system's anti-inflammatory actions were overwhelmed and the pathogen or injury took precedence, leading to a spike in sympathetic-driven inflammation and decreased parasympathetic tone.

How do We Measure HRV?

The difference between the output of the parasympathetic and the sympathetic nervous systems is determined by measuring the variable spaces between R waves on an EKG (or the difference in time between heart beats). This measurement is an accurate representation of your autonomic nervous system balance because both systems share similar pathways and contribute to controlling heart rate.

Using HRV in the Stress and Recovery Cycle

To take advantage of your HRV measurements, you need to understand the basic stages of how the body reacts to stress. This is, of course, assuming that you're providing the body with an adequate level of stress to respond to.

Following Selye's General Adaptation Syndrome (G.A.S.), the body goes through three distinct stages of stress response:

1 – Shock

Also known as the alarm stage, stage one is a healthy body's response to a new stressor. During this stage:

HRV decreases.

The body reacts with increased sympathetic tone.

Increased output of central stress hormones (CRH and ACTH).

Increased adrenal output of epinephrine, norepinephrine and cortisol.

Afferent (towards the brain) signals cause an efferent (outward) effect on the nervous system, hormonal system, and motor neurons.

2 – Overreaching

Known as the resistance stage in the G.A.S. Stage two occurs in response to an imbalance between training stress and recovery.

Sympathetic tone during stress (exercise) increases.

Parasympathetic tone during recovery increases.

HRV increases due to increased anti-inflammatory parasympathetic response.

Decreased turnover of contractile proteins (i.e., slower recovery).

Cortisol and other stress hormone levels remain elevated.

CNS responds to decreased adrenal response by increasing central stress hormone output.

Decreased Beta 2 Adrenoreceptor density reduces adrenal response to the central stress hormone ACTH (corticotropin).

3 – Chronic Stress

This is overtraining, or the G.A.S. exhaustion phase. Stage three occurs when the body continually fails to adapt to chronic stress.

CNS shuts down production of central stress hormones.

Adrenals remain resistant to the central stress hormone ACTH.

Sympathetic response is impaired.

Increased baseline cortisol.

Decreased diurnal cortisol variation.

Decreased heart rate dipping at night.

Decreased testosterone and other anabolic markers.

Psychological symptoms of "burnout".

Depressed protein synthesis (slow recovery to muscle damage).

Depressed immune function.

Increased systemic inflammation.

HRV remains elevated due to chronically increased parasympathetic response.

4 – Supercompensation or Recovery

This is the de-load phase. If you haven't slipped into stage three overtraining, you should only need about a week for this. Heavy exhaustion can take months for full recovery.

Cortisol levels decrease back to resting levels with possible improvement.

Diurnal cortisol variation increases.

CNS increases central stress hormone response to acute stressors.

Adrenals regain Beta 2 Adrenoreceptor density and sensitivity to central hormones.

Inflammation is mitigated.

HRV decreases back to baseline (Baseline likely increases if the training block focused on aerobic improvements).

Don't Break Your Body

You don't need or want to go through all three stages of the G.A.S. to attain neural supercompensation benefits. Ideally, you progress into stage two and then briefly deload before starting over. The graph above illustrates the process of stress accumulation and its effect on fitness.

How to Monitor HRV Yourself

There are a number of options for this, ranging from the gold standard Omegawave system costing thousands of dollars, to mobile smart phone apps connected to simple heart rate monitors. We're going to focus on the latter.

The most commonly used app is known as Ithlete .

Ithlete was the first easily accessible smart phone app for HRV monitoring and is simple to use. You buy a receiver that senses signals from an analog heart rate monitor and an app.

Simply plug it all in and breathe according to a fixed inhale-exhale cycle on the app for 60 seconds each morning. Ithlete will track your HRV and begin placing it into a graph based on trends.

From here, it will display your relative HRV level with daily, weekly, and monthly trends. It uses a color code system to indicate changes. Blue or green indicate no or positive change in HRV, amber is a mild decrease, and red means that you've had a significant decrease.

The idea is to use these codes to determine training intensity. If you get an amber, take it easy. If you get a red, take the day off. If you're in the blue or green, go hard.

There are some limitations to this, as this isn't an effective way to account for HRV increases caused by stage two overreaching. In other words, the program doesn't reliably distinguish between an HRV increase caused by a progression into stage two overreaching due to increased cumulative stress, or an increase in HRV caused by under-training and a recovery from stage one.

However, a new app, released in December of 2011, addresses this. It's called Bioforce .

Bioforce is similar to Ithlete, but with some advancements in predictive readings, such as the ability to distinguish between stress-caused HRV increases and recovery-caused HRV increases.

It runs off the same basic template as Ithlete, but with several differences such as a 2:30 measurement period instead of one minute, and warnings to prevent erroneous measurements.

What to Do With HRV Readings

Here's how to put all this information into good use. First, find your baseline: