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Injury prevention gets harder as you age. These methods help.

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Baby, it's cold outside, but even if you're willing to bundle up and go for a run or sprint to the gym to work out, should you?

I have a meaningful exercise routine, and I have never felt better. I am still glowing from that moment a few months ago when, at the age of 59, I crossed the finish, for the first time, at the Marine Corps Marathon.

At a certain point, when you get to a certain age — say, 60, which I'll be this month — it is not just about your willingness to exercise.

It is about your readiness for exercise.

Aging athletes know we need more time to recover from our workouts, but how can we tell if we have recovered enough? Or whether we are not pushing hard enough for our exercise to matter? When we were younger all we had to do was lace up our shoes and go.

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Are baby boomers too old to ski? Probably not.

"Whether you're a competitive athlete or a recreational one, either finding an intuitive understanding of your readiness to exercise or using some external measures can improve your overall fitness and help you avoid injury," according to sports medicine specialist and physical therapist Kevin McGuinness, who practices at Washington Orthopaedics & Sports Medicine. Exercising, particularly as you age, might also require a more scientific approach to how you are feeling and how you are doing, he said.

The good news is there is some promising research on exercise readiness, according to Carwyn Sharp, chief science officer at the National Strength and Conditioning Association in Colorado Springs. Although there are no specific guidelines yet for recreational athletes, what experts have learned so far can help us enhance our intuitive sense of readiness by throwing some objective measures into the mix.

One way is to monitor your resting heart rate, which can help you understand how well you are recovering from your previous exercise session. If you keep a log of your resting heart rate, you will get a sense of what is normal for you. If it is higher than usual, McGuinness said, that is often a sign your nervous system may be overstressed, indicating a lower level of recovery.

"There is research that shows that changes in your resting heart rate over time can be a measure of total stress level in the body," Sharp said. "And what you want to focus on is trends." For example, over time with aerobic training we should expect our resting heart rate to decrease, which is a sign of improved cardiovascular fitness, Sharp said. So if your resting heart rate is going up over time, that can be a sign your body is experiencing too much chronic stress and needs more rest.

There's huge variability in resting heart rate among people depending on any number of factors, including genetics or fitness level, Sharp said. "It's not so much, 'hey, it's really high today,' it's 'what was it yesterday?' the last few days? the last week or two? Somebody's high might be somebody else's normal." Also, for some people, a rest or recovery day might be walking the dog, for others it might be running at an easy pace or swimming laps.

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You can take your resting heart rate by lightly pressing your index and third fingers on the inside of your wrist below your thumb next to your tendon and counting the beats for 60 seconds. There are also plenty of automated options, including using an exercise tracker, which can monitor your heart rate as you sleep, or your smartphone, which may be equipped with flashlight-type technology to measure your heart rate from your fingertip.

Sharp said individuals can measure their heart rate first thing in the morning, but often they wake up startled by their alarm or stressed about getting ready for work or needing to use the bathroom.

Sharp recommends going to the bathroom as soon as you wake up but then returning to a quiet room and lying down or sitting for two to five minutes before measuring your heart rate.

"Use the same protocol each time," he added. "Then you can see how your rate is changing over time."

Testing your grip strength is another way to measure your readiness for exercise, McGuinness said. While significant grip strength research focuses on frailty and cardiovascular risk, there is a growing body of work connecting grip strength to sports recovery and performance.

Even if you're a runner and think grip strength has nothing to do with running, McGuinness says you can benefit from testing your grip strength with a dynamometer, or grip trainer, commercially available in most sporting goods stores. You can start by getting a baseline of how many pounds of force you can squeeze easily on a good day. Using that reading, stronger days may indicate you are ready for a harder workout, and weaker days may be a sign you need more recovery, he said.

Using sleep trackers to plot the amount of quality sleep you are getting, based on the time you go to bed and the time you wake up and your movement during the night, can also help you determine how well you are recovering from your training, McGuinness said.

In fact, combining multiple measures of recovery and readiness together, such as heart rate and sleep, to determine an athlete's readiness for exercise, seems to be much more valid than a single factor, Sharp said.

"If your heart rate is tracking up, that's something, but you have to look at your sleep, too," he said. The reason your heart rate is raised might be because you are not getting enough sleep. Or you might not be getting enough quality sleep. If you are getting enough good-quality sleep, he added, but your heart rate is still drifting up, it might be because your volume or intensity of training is going up too quickly and you need to back off a bit.

You also want to track how you feel, how tired you are. Sharp suggested logging your level of fatigue on a scale of 1 to 10 and noting any patterns.

"A little fatigue is good," Sharp said, "because training should be causing some low-level fatigue." Your body will only get stronger and better if you stress it beyond your normal daily levels, he said. It's when the fatigue becomes unmanageable or severe, say a 5 or a 6 for three or four days in a row, that you should take a closer look at your exercise schedule, which should include recovery days allowing your fatigue level to return to 0 or 1. If it doesn't and you continue to exercise at the same level, you risk becoming run down, according to Sharp, and susceptible to illnesses and minor injuries.

"Some coaches say, 'go easy on easy days and hard on hard days,'" Sharp added. "I say, 'go easy on easy days so you can go hard on hard days.' Because it's those hard days where you get the biggest bang for your buck."

Carolee Belkin Walker is a wellness blogger and the author of "Getting My Bounce Back: How I Got Fit, Healthier, And Happier (And You Can, Too)," out in February 2018.