Yes, you can mountain bike -- nicely -- with a cardiac pacemaker. Modern pacers can help you bike by increasing your heart rate to match your exercise level. Once the pacer is "dialed in" you'll likely find that your limitations are based on your underlying health problems and not by the pacemaker.

Typical placement of a pacemaker for a right-handed person: just below the middle of the left clavicle, between the fat and the pectoral muscle. 

This article will include:
  - My own pacemaker story; why I got it, and how it's worked for me
    (Feel free to skip this long-winded part if you're in a hurry. My own condition may not apply to you!)
  - Why pacemakers are installed
  - How pacemakers work
  - Pacemaker program adjustments by your doctor
  - How you can change the paced heart rate while riding
  - Protecting the pacemaker area

By 2019, the symptoms began to hit during long climbs on regular "fun" rides. I thought I was just out of shape, as I'd been having other health issues at the time. But I couldn't seem to "get in shape" no matter how hard I worked out nor how much I rested. Then by spring 2020, it was happening on almost every ride. And the symptoms hit earlier  in the ride and at lower exertion. 

That's me in 2014, at a race in St George Utah called The Cactus Hugger. I was usually on the podium for my age category after every race, but soon I'd not even finish.

Hitting 11,000 feet elevation in 2018, on a ride of 26 miles and 3000 vertical. Yet by the early 2021, I couldn't even get one mile uphill on a gentle slope. I could only ride flat trails, while watching my Garmin closely to be sure my heart rate didn't get over 110. There's almost nothing you can ride while keeping your effort level that low!

So I decided to investigate the symptoms under the exact conditions that brought them on. I bought a battery-powered wrist blood pressure monitor (to see if my blood pressure was spiking during climbs) and a pulse oximeter (to see whether my oxygen was dropping) and a cardiac monitor (to see if what felt like a normal but heavy heartbeat was actually an abnormal rhythm).

This little baby cost $120 and recorded Lead 2 in 5-minute snippets. It had real-time display of EKG waveforms, and downloaded rhythm strips to computer or cell phone. There are fancier ones available now, including a unit that records 24 hours of 12-lead EKG. The catch is, you need to know what you're seeing, or have a doctor who's willing to look at the tracings for you.

But could I mountain bike with the oximeter on my finger and the blood pressure monitor on my wrist? Would the EKG electrodes of the monitor stay stuck to my chest once I began to sweat? I did a test spin on my basement trainer to see how it would all work on the trail. And bang! There it was.

I manufactured this monitor-holder from a "Flip Phone Wrist Band" for $8. The wire goes up the sleeve of my bike jersey to the electrodes on my chest.

Junctional ectopic rhythm. My heartbeat was coming from a spot near the AV node, between the atrium and ventricle. It was a slower rate than the sinus rhythm it replaced. And the atrium was beating at the same time as the ventricle, contracting against the closed valve and sending blood backwards into the veins of my chest.

Here's the junctional ectopic rhythm as it appears on the little monitor.

We spent a year trying rhythm-control medicine. And waiting to see if it would just go away -- for example if it was a viral infection of the heart that could run its course. I kept getting worse. 

Pacemaker scar, 10 days post surgery. Already feeling better and working out.

I could only ride short trails that were mostly flat, and only while watching my Garmin carefully to keep my effort level low. Once I was warmed up, if my heart rate stayed above 115 for a couple of minutes, the arrhythmia would begin. It began to come on during yard work like raking leaves. Finally, I couldn't even walk briskly around the block for 30 minutes without ending up sitting in the gutter.

The results from the pacemaker were dramatic.

I was immediately able to trail run, play basketball, and ride the mountain bike. And ride it hard. By 4 weeks post implant, I was climbing uphill at 8000 feet with heart rate hitting 145, putting in 20-mile rides. I quit watching my diagnostic monitor, because the pacemaker always responded immediately when I'd pedal faster, and suppressed the arrhythmia both uphill and down.

Right now, that pacemaker is my $60,000 new best friend.

Adjustments made to improve performance 5 months out:
 -  ADL "target rate" increased from 95 to 120 for higher heart rate during low-intensity cycling
 -  Activity Acceleration increased so heart rate increases more quickly during burst efforts
 -  Activity Deceleration prolonged for a longer recovery period after a burst effort
 -  Exercise Response slope steeper to achieve higher heart rate during higher effort
    Upper rate left at 160 -- I'd only been reaching 140 with highest efforts running or cycling

A pacemaker is usually implanted just below the middle of the clavicle, nestled in a "pouch" between the subcutaneous fat and the pectoral muscle. Two wires are threaded into the subclavian vein and advanced to the vena cava, then into the right side of the heart. Typically, the end of the wire has a small coil on the end that can be twisted to "screw into" the heart muscle. One is placed in the right atrium, and the other in the right ventricle.

The actual pacing is done by a tiny electrical shock from the pacemaker wire, which makes heart cells "depolarize," that is, fire. The wave of electricity spreads, making the heart muscle contract. If electrical activity is passing normally between atrium and ventricle through the AV node, the pacer may only shock the atrium, letting the electricity propagate and spread down to the ventricle in the normal fashion. If the AV node is not functional, the pacemaker will shock the atrium, delay while the atrium contracts, then shock the ventricle.

Modern pacemakers have internal electronics that calculate how fast to pace your heart. So it may pace at 60 beats per minute while you're sitting on the couch, rise to 80 as you walk around the house, then increase to 130 as you begin running around the block.

The pacer has accelerometers that detect motion. These report to a computer chip that decides whether the jiggle of the pacemaker was just a bump in the road, or whether the pattern of motion means you are now running or riding a bike. The sensitivity to motion, and the response to motion, can be programmed into the pacemaker by the specialist. Some cyclists report that it took a couple of programming sessions over several months before the pacemaker was "dialed in" for cycling. (While the pacemaker can report what's going on to your cell phone via Bluetooth, only your doctor can program the pacer. You can't make your phone tell the pacemaker to speed up or slow down -- both for hacker reasons and moron patient reasons.)

Settings on the pacemaker that can be changed may include:
- Lower Rate -  how low it will let your heart rate go
- Upper Rate -  maximum rate at which it will pace; higher rates allow more exertion in younger hearts
   -  the default rate setting is often low (130ish), so specifically ask about this setting if you're young and physically active
- ADL Response (activities of daily living -- lower levels of motion detected by pacemaker)
   -  how fast it will increase your heart rate as ADLs become more vigorous
   -  "target rate" set by your doctor, a higher target rate means you'll start your ride with higher heart rate
       and have higher heart rate at lower levels of chest motion while riding
- Exercise Response
  -  how fast it will increase your heart rate as more-vigorous motion is detected
  -  example: you run hard and heart rate only gets to 120. Increase steepness of response slope for higher heart rate during the same activity
- Activity Threshold 
  -  how sensitive the activity sensor will be to your body motion -- responding to subtle motions vs only vigorous activity
- Activity Acceleration
  -  how quickly the rate will increase when the sensor detects increasingly vigorous activity
- Activity Deceleration
  -  how quickly the rate decreases when activity stops
  -  slower deceleration is useful for recovery, or to keep the heart rate up while you stop riding and take a photo
- Optimization ON/OFF
  -  will the pacemaker adjust itself based on the average of your detected activity levels?
  -  may result in decreasing heart rate for the same stretch of trail at the same effort level over time

Most cyclists will benefit from a low Activity Threshold (responding to smaller motions for riding on smooth trails), a steeper Exercise Response, and a shorter period of waiting for Activity Acceleration. If symptoms of under-perfusion (lightheadedness, muscle weakness) occur immediately after high exertion, a longer period of Activity Deceleration can help. For most cyclists (but especially for road cycling), the Activity Threshold should be set as low as possible. A higher target rate for ADLs gives you a higher heart rate during lower-intensity riding (or road cycling).

Perhaps your pacemaker was doing fine at first, but now 6 months later you're fighting your way uphill at a heart rate that won't go over 120. If your pacemaker is consistently giving you a lower heart rate than it did before -- for an identical activity and effort -- it's likely that Optimization is enabled on your pacemaker. There are significant advantages to having Optimization turned on. However: When Optimization is ON, self-alteration of the pacer's programming can occur when you spend a lot of time bouncing down the rocks on your bike. Or if you do activities that shake the pacemaker, like operating some types of power equipment. The pacemaker sensors detect these bumps and the optimization turns down the response slope -- so it takes increasingly violent or more-frequent motion of the pacemaker to get the same high heart rate. Many bike riders, but especially road cyclists who also do other more "violent" sports, do better with Optimization turned OFF.

Note that for some pacemakers, there is no distinction between low activity (ADL Response) and sports (Exercise Response), and the rate response is a single relationship to the amount of activity detected by the sensors.

In summary:
You may need to go back to your cardiologist's office a time or two to "dial in" the pacemaker. It helps to have a log of how your heart rate is responding to specific reproducible activities, and when during those activities you have symptoms or feel you need "more" from your pacemaker.

Use a cycling computer to monitor your heart rate to see how the pacemaker is working for you. I'd strongly suggest a higher-quality unit with a chest strap. A watch that monitors your pulse is rarely accurate enough once heart disease and arrhythmias enter the picture. 

First, be aware that it's motion in your upper body that activates the pacemaker's heart rate algorithms. So anything that DECREASES the rate or intensity of upper body motion will result in a LOWER heart rate as you exert. For example, wearing a backpack. The added mass means more inertia -- increased resistance to being moved by a bump in the trail or by motion of your legs -- so the chest and shoulder will move less when hit with the same bump in the trail. And while wearing a heavy backpack you'll keep your upper body more still, on purpose. Overall, wearing a backpack makes your pacemaker less effective.

Any addition of mass to your body or your bike means you'll move down the trail slower when expending the same effort. And that means the bumps hit at a lower frequency, and with less intensity. So the pacemaker response will be slower, when compared to riding free of that mass, just when you need the pacer to fire more quickly because you're working hard. So seriously consider what you carry with you. Perhaps your riding partner can be the guy with the heavy tool kit that you all use. And if you must take it with you, get the heavy stuff as far away from the pacemaker as possible -- down in the jersey pockets or on the bike frame instead of in a backpack.

Another issue is the slope of the trail. A steeper trail means you climb slower. So bumps in the trail come slower. You're working harder than ever, but the motion sensors receive less stimulation. So the pacemaker heart rate slows just when you need it to speed up.

Discuss your biking experiences with your cardiologist at your pacemaker checkup. For example, I requested that the pacer can be programmed to taper the rhythm over a longer period of time. I'm hoping to keep my heart rate high, and thereby avoid an arrhythmia, when I stop riding to set up a trailside camera or deploy the drone. And I hope to keep the heart rate high for a longer time during recovery from a trail run or hard climb, so I don't have to pace around for 20 minutes bouncing on my heels (to artificially keep the heart rate high during recovery and prevent the junctional ectopic rhythm).

If your cardiologist can't "tune" the pacemaker so it responds appropriately to mountain biking, it's up to you. Increase the stiffness of your suspension (set your shocks so they're "harder" and a bit "bouncier"). Change how you ride and what you take with you. Try riding a lighter stiffer bike: perhaps an XC racer rather than an all-mountain rig. Consider moving your gear and fluids out of the camelbak.

So here's your basic checklist to improve the pacemaker's detection:
1- Get rid of the backpack and go to essentials-only riding
2- Increase the stiffness of your suspension
3- If you have gel gloves, go to standard gloves
4- Consider a harder (less plush) handgrip
5- Consider riding a stiffer bike (hardtail or XC full-suspension)

And if you're climbing a trail that's dusty and smooth and you're not seeing the heart rate that you expect, here are some things you can try. Many of them you'd normally want to avoid. But these tricks, I've found, make the pacemaker sense more motion and thereby increase the heart rate.
1- Go to a lower gear and spin a higher cadence
2- Get forward and put more weight on your hands
3- Lock out your rear suspension
4- Lock out your front suspension
5- Stand periodically to pedal at a lower gear than comfortable (higher cadence)
6- Have a riding partner carry part of your weight

For some activities -- for example when riding on a super-smooth paved bike trail with a plush full-suspension bike -- I need do special tricks to help the pacemaker "know" that I'm exercising. It's just too smooth. So I deliberately aim for the rocks and roots. I rock side to side on straight smooth stretches of trail. I pedal with "bad technique" to bounce around on the bike seat. I bounce the handlebars a few times while holding my arms stiff. And last resort, I'll take my right hand off the handlebar and tap the pacemaker about 20 times to drive the heart rate up. (Note: if your pacemaker has Rate Profile Optimization, putting this sort of kinetic input into the pacemaker's motion sensor on a regular basis can make it re-adjust its response profile, resulting in a lower heart rate when cycling in the future. As I said, this should be a last-resort option!)

One quick note here:  If your heart is being paced from the ventricular lead, rather than the atrial lead, your heart rate monitor may not report the heart rate accurately! This is especially likely to happen at higher heart rates (example 120-150 bpm). If you're riding and the heart rate from your Garmin is all over the place and doesn't correspond to the pulse rate you feel with your fingers, that may be what's happening. (The wide QRS followed by a tall T wave, coming closely after the previous beat, makes it hard for the unit's beat-detection algorithm to correctly count your heart rate.)

You can go to an easier gear to increase your pedal cadence. Deliberately let yourself bounce a little. When you see the response you want, you can back off to a smaller cog for a while.

You can rock your shoulders as you pedal. This works best at higher cadences.

You can slam the handlebars repeatedly with your arms as you pedal or while coasting.

One solution of course is to always ride your road bike on routes that feature non-stop bumpy broken-up shoulders. The bumps make the pacer think you're running.

Dedicated roadies have learned how to give the pacemaker some artificial motion by riding with "bad technique" as discussed above. This tells the pacer you're exercising, so it increases the heart rate. A heart rate display, such as a Garmin, helps you monitor this as you ride.

For some activities, such as swimming or isolation dumbbell workouts, you may never get a satisfactory increase in heart rate because the accelerometers simply can't detect how hard you're working. So you may find yourself out of breath and exhausted because your heart rate has stayed too low. Some sports simply work better than others for individuals with pacemakers.

If you need the heart rate to remain high while temporarily resting (for example when you stop riding to talk to friends or take a break from climbing), pace around or bounce on your heels. Or you can gently tap the skin near one corner of the pacer with your fingertips. Tap around twice per second. After around 10 to 20 seconds (depending on your pacemaker programming), the paced rate will increase as though you were running or riding bumpy trail. But remember that tapping the pacemaker can make it readjust itself and may work against you in the long run.

If you're regularly doing things that may strike the pacemaker, you can wear body armor or buy a dedicated pacemaker protection shield that inserts into a special undershirt. (These commercial protection systems are surprisingly expensive. Like $200 and up. But you can put one together yourself with a sewing machine. Sew a pocket into the undershirt over the pacer area. Make it around 6 by 6 inches. When riding, insert a flat piece of milk-jug plastic into that pocket to cover the pacer. If you need to shape the plastic, warm it carefully with a propane torch and bend it to fit.)

Note in this photo how my backpack strap hits the healing pacer scar. Over half of the incision is now under the strap. (The backpack is empty, but it hurt to take this photo.) And the strap would also tend to tilt the pacemaker and shove the it toward the sternum when the backpack strap pulls tight.

Once you resume use of the backpack, you may still need to add special protection from the backpack strap (see below) depending on your body shape, amount of fat, and weight of the filled backpack.

But see my discussion above about how a backpack decreases your pacemaker's ability to detect that you're riding. Try eliminating the backpack for a few rides and see how things go.

If you don't enjoy stuffing pockets, or ride with a pocket-less jersey, get yourself a hip pack. These are surprisingly nice to ride with. Compared to a backpack, it puts your center of gravity lower for fancy riding and it puts less strain on your shoulders and back.

One simple solution to keep the strap from digging into the pacemaker area is to make a wide cloth "donut" that surrounds the pacemaker and takes the pressure off it. Stuff it with acrylic padding.

This donut in the photo is a 5-inch Himalayan Singing Bowl Pillow that I bought for $7 on EBay. If the donut feels uncomfortably hard, you may need to unpick some stitching, remove padding, and sew it up again.

Get a snug-fitting undershirt or tunic. Figure out where the donut should go to surround the pacemaker.

Now you can sew a pocket to hold the donut. Or sew bits of velcro to both shirt and donut that allow you to place it in a specific spot. Or just safety-pin it in place.

Place your regular biking jersey over the tunic.

This works for light backpacks, but...

This holds the strap away from your chest, so it touches you only on the clavicle. There is now air between the backpack strap and the pacemaker scar. The strap can't touch the pacemaker no matter how the backpack shifts around during rough riding.

Here's my stand-off pacemaker protector! I made this 100% from stuff that was hanging out in my garage.

You can get straps and buckles at the hardware store, or you can cannibalize an old broken Camelbak. You'll need one long strap (which will be used to cover the backpack's shoulder strap) and a T-strap (which will form a loop to hold the backpack's chest strap and then clips into the long strap overtop of the shoulder strap). Any strap joints -- in particular the T joint -- can be sewn, or glued and riveted. I suggest a strap-on with buckles rather than adding a permanent part of the Camelbak. You can transfer the stand-off protector to any backpack with a chest strap.

To rivet a strap joint, glue it first. Then melt a hole through all the straps with a hot nail and place the rivet! (Propane lighter, nail, and pliers to hold the hot nail not shown.)

I like E6000 or Shoe Goo for my glue projects! You can also build up surfaces with these glues.

Styrofoam packing can be cut up and sanded to shape. Make a curve that fits your ribs. You may need to do some additional shaping once the stand-off brace is fully assembled. And keep an extra unshaped piece of foam as a backup.

Now glue the shaped styrofoam to the wood. Use a flexible glue.

Before you glue! Test your glue on an unused bit of foam and wait a few minutes. Many glues -- like the E6000 shown above -- will dissolve styrofoam. If the foam starts to sink and looks pitted, try something else.

Automotive gasket compound worked nicely as a glue. It did not eat into the styrofoam!

This is the final result, seen as if you were looking down at it on your chest for a left-sided pacemaker. Insert the backpack's chest strap through the loop at right and click the chest strap. Now click the stand-off's main strap closed over the backpack's shoulder strap.

One note: You'll probably want to make the straps a little shorter (tighter) than shown here, particularly the loop that surrounds the chest strap. Shorter straps prevent it from shifting during violent riding and help keep the brace in position when you remove then replace the backpack.