Using a Garmin 310XT with a PowerTap (or CinQo)
I'm Craig - husband, dad, IT geek, BOMOFOP triathlete/runner, Phish fanatic and proud Canadian from Edmonton, Alberta.
Despite being an IT geek, I actually don't have a blog (gasp!) so I host my random little projects here.
Here are a few more of mine:
-Predict your triathlon run:
-"Enhanced" race results from a few (and growing number of ) IM races:
-My Ironman Canada 2010 race report (and yes, I used my 310XT/Powertap, the Predict your Triathlon run spreadsheet and analyzed things after with the enhanced race results!):
-Craig Harris (email: email@example.com)
-Thanks specifically to The_Mickstar from ST, as well as several Endurance Nation, Slowtwitch, and Wattage folks for significant input.
v1.4 - Feb 2, 2011 - finally revised to include new firmware info (re: 3s/30s rolling averages)
v1.3 - July 7, 2010 - general update/cleanup, updated firmware info, etc.
v1.2 – February 1, 2010 – added several sections (marked with NEW) and revised a few (marked with, go figure, REVISED). Thanks to Jim Ley for lots of clarification in the Calibration section.
v1.1 – December 22, 2009 – significant cleanup, edits and corrections throughout.
v1.0 – November 2, 2009
There seems to be a lot of FUD about using a 310XT as the head unit for a PowerTap Hub. I created this to try and dispel (or prove...) some of it.
Since more and more people are looking to get into power, I figured they would be doing the same research I did - and trying to connect the same dots I have. This is an attempt at centralizing thoughts, discussing pros/cons/costs etc.
I've also included:
• information on the differences between the PT Elite+ and Pro+ (because that can be confusing as well)
• brief information on using the 310XT with a CinQo
If you have any additions/corrections/feedback, email me and I'll update this page.
This isn't an attempt at an in-depth review of the 310XT per se – if you're looking for that, head over to dcrainmaker's site where he does incredibly well-researched, meticulously documented reviews of key Garmin gear (the 310XT, but also the 500, 705, and 405):
1. Cut to the chase. Does it work and should I buy it?
Despite some of the issues listed below, I think the 310XT is a great option and I'm really happy with it (as are others). It's particularly attractive for the cost constrained athlete that doesn't already have a Garmin, but is a pretty good option for anyone. There are a few downsides (e.g. you can't check the calibration of your PT), but I don't think any of them are showstoppers anymore.
First of all, I think there are a few reasons for the FUD:
a) The gold standard is obviously the regular PowerTap head unit, or "Little Yellow Computer" (LYC) as it's affectionately known. It's been around forever and a lot of people have it, so there is more familiarity and comfort with it. And, of course, it's made by the guys that make the hub.
b) It's unclear what you can/can't do with the 310XT as compared to the LYC.
c) People feel burned by how Garmin treated people with the Edge 705. Those that weren't burned personally certainly read a lot about it. The brief backstory is that Garmin essentially treated early adopters as beta testers for months with repeated buggy firmware revisions until, finally, it seems to be largely stable/reliable.
d) There are differences between the 705 and 310XT in terms of what they do/don't do.
e) It's unclear what procedures you can and should follow (and there's confusion regarding the terminology) for things like calibration, auto-zero, zero averaging, auto pause, etc.
f) There is not a lot of carefully gathered data comparing 310XT and LYC.
g) There have been some problems with how various software packages deal with data from the 310XT.
h) Some people that are exploring this as an option are new to power (like me).
i) If you get into a situation where you need to troubleshoot the hub with Saris' help, you will need a LYC.
There are solutions to many, but not all, of these problems. Whether or not the tradeoffs are worth it to you is something that you need to evaluate and decide.
3. Why go with the 310XT instead of just using the LYC?
A lot of people are interested in power but scared by the cost. This solution is cheaper - especially if you're considering a Garmin for the run anyway. It's cheaper because you don't end up buying two units that essentially do the same thing (i.e. no duplicate head unit, HR strap, cradle, cables, etc.).
This approach also lets you purchase the PT Elite+ (the entry level wireless model) instead of the Pro+ (the next step up). More on the differences between the Elite+/Pro+/SL+/SLC+ below.
You only have one device to charge, download, update, familiarize yourself with, there's a consistent UI between sports, you don't need to learn morse code to change settings, etc.
c. Data Geek
It's "cool" to be able to see everything in one place for analysis after your run/ride. Also, the Garmin displays are numerous and programmable, with up to 3 different screens per sport, and up to 4 data segments per screen. The LYC display is limited to 1 screen with 3 data segments. Whether this translates into any tangible training benefit is debatable. ;)
4. What are the main downsides?
a. Lack of Zero Averaging
This isn't a big deal, but it's good to be aware of. See #34 for more information
b. Differences between what hardware (e.g. LYC and 310XT) and software (e.g. Garmin and WKO+) display for numbers.
See FAQ item #5 for more information on this – once you understand what is/isn't happening, it's largely manageable.
c. No ability to do a static calibration or "stomp test"
See FAQ item #9 for more information.
d. Problems with 3rd party software
This appears to be more historical than anything at this point It took a while for some vendors (including Training Peaks, makers of WKO+) to make their software work with the 310XT data – this seems to be largely resolved now.
5. If I have my LYC and 310XT side by side on the same ride, I get different numbers during and/or after the ride. Why?
The million dollar question. I think the differences you see from a power perspective can largely be explained by two main factors:
a) Zero Averaging. With the 310XT, this is OFF by default and can't be changed. With the LYC, it can be configured (and I believe it defaults to ON)
b) Stopped Time. There are differences in how the LYC and Garmin deal with pauses/stops. The Garmin is quite configurable (e.g. Auto Pause, Manual Stop/Start). WKO+ also "does stuff" based on pauses and their length, etc.
Whether you notice discrepancies or not depends on the type of riding you do – are you on a trainer versus out on the road, do you stop for a beer in the middle of your ride and, if so, how long does it take you to chug the beer, etc.
Also, here is an informative page from the WKO+ folks explaining why what you see in Garmin (and other) software may not match what you see in WKO+:
(See the section "Why does data extracted from my training device through Device Agent or WKO+ not match the numbers I get when downloading to that device's manufacturer's software?")
6. What does the 310XT
"Calibrate" function do - and what's up with the number it returns?
With older versions of firmware, the Garmin may not display the Hub ID properly. Sometimes it will display a negative number. This is a result of the Garmin code not handling the returned number properly, and only displaying 4 digits. Fortunately it's a relatively harmless bug, although it can be confusing if you're expecting some other number. This appears to have been resolved in firmware 2.90
7. What is the "Auto Zero" setting in the 310XT?
The "Auto Zero" setting on the Calibration screen is the equivalent of the LYC's "Automatically Adjust Torque Offset" setting. This has the effect of zeroing torque when you are coasting. Please note, it is NOT "Zero Averaging" (see below)
8. What is "Zero Averaging"?
The LYC has a setting called "Include Zeros" for Power, Speed and Cadence. Garmin calls this "Zero Averaging" instead. This setting deals with how Cadence and Power Averages are displayed on the 310XT (i.e. it does NOT affect how data is recorded - zeros are recorded properly so when you download your ride later you will see them). Unfortunately, the 310XT does NOT have the ability for the user to set this – it is always set to OFF. With the Edge 705, you can turn Power/Cadence Zero Averaging on or off but it's all or nothing. With the Edge 500, you can turn them on or off individually.
9. What about calibration of the PT?
Technically, you can't actually calibrate your PT. What you can do is check its accuracy using the "Stomp Test," as described here by Dr. Coggan:
Do you need to this? It's a good idea - some would argue that if you're willing to spend >$1K to measure power, you should regularly check to make sure it's not drifting out of whack.
Unfortunately, if you only have a 310XT, the only way to do this is to borrow a LYC and use that. Hopefully Garmin will offer a "Torque display mode" feature that would allow this to be done with the 310XT in the future. Alternately, there are some other computers coming out which, if they perform reliably, may be cheaper than buying a LYC.
Another option that will be available in the near future is to use Golden Cheetah with quarqd (a cool little piece of software from Quarq) and your Garmin ANT+ USB stick.
10. What is this "Smart Recording" I keep hearing about?
With the Edge 705 (and ForeRunner 305), you could enable/disable "Smart Recording." This led to a lot of problems, since Smart Recording is pretty dumb when it comes to recording power. With the 310XT, you can no longer enable/disable Smart Recording. It is always enabled for everything (e.g. altitude, speed, HR, etc.) EXCEPT power. Power is always recorded at 1s intervals (which is good). The Edge 500 is similar in this regard.
11. What software can I use with it?
WKO+: Had some 310XT problems with the old version (2.2), but the new version (3.0) appears to be a lot better dealing with 310XT data.
TrainingPeaks: Online version of WKO+.
RaceDay: The main alternative to WKO+ - top notch support - often from Dr. Phil Skiba himself.
SportTracks: Free, with more interesting plugins (e.g. Power, Cadence, Quadrant Analysis) all the time.
Training Center: Garmin's desktop software.
Garmin Connect: Garmin's online solution.
Power Agent: Saris' desktop solution.
12. Tell me more about WKO+ - I heard there were problems with it?
With WKO+ version 2.2, there were problems with how WKO+ dealt with 310XT data. With WKO+ version 3.0 (including Device Agent 3.0), it now appears to support it. Here is the updated page for the 310XT:
13. What version of firmware should I use?
You can get the latest version here (there is also a revision history):
As people with the Edge 705 will tell you, the latest version isn't always the "best" version - sometimes a new version will create more problems than it solves.
As of February 2, 2011, 3.20 is the latest version and appears to be stable. 3.00 was a major functionality release that *finally* included 3s/30s rolling power averages. 3.10 and 3.20 were minor bugfixes.
14. What about firmware upgrade problems?
Some people have reported bricking (i.e. killing) their unit when they try to upgrade firmware. This is always a risk when flashing devices, and it may be magnified because the entire process is done wirelessly. However, many (including myself) have done it with absolutely no problems.
The most common problem is the infamous "loader loading" problem. It appears to get stuck during the update process. The solution is usually just to do a soft reset and then do the update again. I'll document the process the next time I update the firmware on my 310XT.
15. What about the Saris Joule?
The Joule is the next-generation version of the LYC. It's a pretty cool little machine, and has greatly improved display options compared to the LYC.
The main advantage it has is that it provides real time (and historical) TSS/NP/IF. Some people think this is important to have, others aren't so sure.Its major downside is that it's pricey.
16. What about the Edge 500 and 705
The Edge 705 is a very nice bike computer. But, it's kinda big, and not great for running. (I've seen more than one thread about how to use it for that, though). Here's a review:
The Edge 500 is a smaller, cheaper, simpler version of the 705 with mapping functionality removed and a few features added. Here are a couple reviews:
17. Any other things about the 310XT I should know?
Some of this information is covered in better detail elsewhere on the web, but off the top of my head:
a. The 310XT has a glass screen. If you drop it from any height onto something hard, it could crack. It happens...so be careful.
b. The GPS reception is not reliable under water. The best workaround is to put it under your swim cap and use your goggle straps to hold it in place (careful!). Firmware 2.90 adds an open water swim mode, but testing by others seems to indicate the cap method is still most accurate.
c. The HR reception also is not functional under water. The frequency that ANT+ uses (2.4GHz) does not propagate through water (darn physics!) The 301 used a different frequency and actually worked quite well (but NOT the 305 - which also uses ANT+ for HR).
d. You can't look at the History while you're in the middle of the workout – you can only review it after you've finished the workout. You used to be able to do this in the 301/305. But, inexplicably, Garmin removed this functionality for the 310XT.
e. There is an obscure problem where if your 310XT doesn't get a satellite lock, it can't tell time properly. This is a hardware problem that only shows up on certain watches – the only way to fix it is to get a new unit from Garmin.
f. The HR strap is new and improved. More fabric, less plastic, more comfortable. The 310XT will also work with other ANT+ HR straps (ie. the 305 strap, the Saris strap, etc).
18. What about the Quick Release kit?
You really should buy this – it's pretty slick. There are two versions:
-The old version includes an ugly utilitarian black plastic strap instead of the snazzy (and more comfortable/functional) orange one that comes with the watch. With that being said, the old QR watch strap isn't all bad. It's very low profile (for wearing without the watch under a wetsuit and on your bike). And, the system requires a 1/4-turn to mount/dismount so it's solid. People had problems with the watch popping off of the 305 QR mounts, and the 305 QR strap could not be worn under a wetsuit without serious risk of getting snagged and ripping the suit.
-The new version has the same wider and nicer-looking orange/grey watch strap and the bike mount is subtly improved as well.
19. What about speed/cadence?
Garmin makes a speed and cadence sensor (GSC 10 – there are other newer models as well but they're all pretty similar). You attach the sensor to your bike, and a magnet to a rear spoke and your crank arm. You enter the wheel circumference - either manually from a measurement, or it can auto-calculate this using GPS if outdoors. It uses this to calculate speed and also measures cadence. Since the magnet is on the rear wheel, it makes it possible to use on an indoor trainer, too, which is a very nice feature.
This area is a little counter-intuitive and I should expand this section in the future. Essentially, if the Garmin detects a Speed sensor, it will disable the GPS and use the sensor for Speed/Distance (the rationale being that, assuming your wheel measurement is accurate, this will be more accurate than GPS). It still *records* GPS information for later analysis, though.
20. How do the numbers work out – what is this going to set me back?
Here are some approximate MSRP prices:
• Elite+ hub only: $849
• Wheel, including build fee: Prices vary with components selected.
• Garmin 310XT (with HR – careful, some 310s come without the HR strap - of course when you're training with power, it's de rigueur to no longer look at HR ;) ): $400
• Quick Release Kit: $25
• GSC 10: Speed/Cadence sensor: $60
• Footpod: (for running cadence, and distance on a treadmill): $90
• Wheel Cover: $90
If you want to play around with different pricing combinations/permutations to get an idea, try here:
(I have no affiliation with Wheelbuilder.com - except I've bought stuff from them and think they're awesome).
21. What about the good old Garmin Forerunner 305?
This is still a bombproof watch. And, it's really cheap nowadays. It can't do everything the 310XT can do (mainly swimming & power). But, then again, it can do a few things that the 310XT can't. Many people suggest getting it from Costco, or REI. (It's a bit more at REI. But, if you ever have a problem, REI will take care of it.) The 305 is arguably a bit more rugged than the 310XT. The glass screen of the 305 is recessed so, it's a bit better shielded from impact. It is a bit larger than the 310XT, though, and the quick release isn't as good.
22. What about the Garmin 405/405CX?
This watch tends to be a love/hate thing, and is more suited for just running. Some people like that it's a regular watch that you can wear all the time. But it can't do power, some people get frustrated with the bezel, and its battery life is pretty limited for IM events. As usual, dcrainmaker has got the goods:
23. OK. I bought a 310XT. What should I do?
1. Open the packaging, and tell your spouse how "cool" you're going to be with the big, dorky, orange watch on while you're running. Between that and the compression socks you're sporting, tell him/her that you're in no danger of being hit on. ;-)
2. Go through the initial setup, following the directions in the Owner's Manual. (You may have to redo this after Step #4, below. But it will go faster the second time.)
3. Install the latest software (Training Center, ANT+ agent, Web Updater).
4. Update your firmware from the Garmin website (see the question earlier in the FAQ):
5. Go through all the menus, setting it up to your preferences (Auto Lap, Auto Pause, Auto Zero, etc.) for each sport.
6. Set up the Data screens to your preference.
7. Pair with your ANT+ accessories (ie. HR strap, PowerTap, GSC 10, etc.) This is almost entirely automatic!
8. "Calibrate" your PT, as detailed above.
24. I work at Garmin. Any suggestions?
Yeah, please try improve your PR and responsiveness a bit. Assign a Tri (and/or Power) advocate that will hang out on Slowtwitch, Wattage and other forums and listen to the smart people there, and improve your product as a result. You have a great product - you are really close to having a KILLER product.
25. What are the differences between the PowerTap Elite+ and the Pro+?
• A bit lighter
• Alloy axle
• Comes with HR strap
• Comes with a slightly different head unit that lets you customize the display a bit more, and that has more memory (>7 hours - although this depends on data save rate - e.g. 1s vs other options)
• A bit heavier
• Steel axle
• No HR strap
• Head unit display customization is more limited, and it has less memory (~7 hours - although this depends on data save rate - e.g. 1s vs other options)
The Pro+ is lighter because it has an alloy axle - the Elite+ has a steel axle instead. While steel is heavier, it's also stronger - so if you're a heavier and/or more powerful rider, having the steel axle could greatly extend bearing life due to less axle flex. Also, alloy freehubs can be prone to "galling" from cassettes. Finally, since this extra weight is concentrated at the hub (i.e. it's not rotational) the added weight does not make that much of a difference.
Beginning with the 2010 models, the Pro+ and Elite+ models come with 15mm axles - this makes them closer still to the higher-end SL/SLC models. More info in this thread:
26. What about the PowerTap SL+ and SLC+?
Here's an ST thread where Jordan Rapp (aka Rappstar) discusses why you might want to make the leap from the Elite/Pro to the SL/SLC:
Apparently there is an issue with the 310XT and CinQo crank where, depending on where the watch is located (i.e. on left or right wrist, on aerobars, on top tube, etc.) you can have data dropouts. Generally speaking, the further away, or the worse the line of sight is, the bigger the problem.
Per a thread in the google wattage group, this is apparently a known issue that CinQo is looking to try rectifying with a hardware revision in fall 2009.
28. What are the main differences between the 310XT and the 500/705?
29. Can other wireless devices interfere with my 310XT/PT?
I haven't experienced it, but I've seen a few threads floating around where people are reporting sporadic connectivity/dropout problems that seem to be related to other wireless devices (e.g. iPhones, WiFi, etc.) causing interference. Moving the devices away from the Hub/310XT apparently helps.
30. My $500 310XT can't keep time – what's up with that?
There is a known issue with certain 310XTs losing time when they don't get satellite lock. This mainly happens to people who use them indoors (e.g. treadmill or trainer) where they don't have reception.
The problem only occurs on certain units and therefore appears hardware related (i.e. at this point, there is no software fix - although they may figure out a way to fix it in the future with software). The only solution is to have Garmin repair/replace it. A thread that discusses it is here:
31. What's the absolute cheapest way to get power (and pace)?
1. Get a wired powertap ($660):
2. Get a good old fashioned ForeRunner 305 ($160):
They're both sort of "dead ends," but will be quite functional.
32. Where should I buy my watch?
One suggestion people have made is to buy your Garmin from REI (US) or MEC (Canada). You pay a bit more up front, but their warranties are flexible enough that you could return/exchange the watch if you ran into reasonable problems in the future.
33. Is the lack of 5s/30s moving averages (or "smoothing) a big problem?
Note: this is no longer an issue with firmware 3.00 and above as they added 5s/30s moving averages!
I don't think it's that big a deal if you judiciously use the Lap button once in a while. Here's a quick video (sorry about the quality, and the sound of the fan) - it shows a 705 and 310XT side by side while doing a few quick test intervals so you can decide for yourself:
34. Is the lack of "Zero Averaging" a big problem?
In my experience it's not a big deal. Here's a video (again, apologies for the quality) - it shows a 705 and 310XT side by side while I do some starting/stopping so you can decide for yourself:
Note: because of the flywheel on the Kurt Kinetic, I believe I actually have to come to a stop for it to be considered "coasting" with zero power (since the flywheel is returning stored energy).
35. Why *shouldn't* I get a 310XT?
The main reasons I can think of:
36. Any suggestions for how to configure 310XT screens for biking and running?
At a bare minimum, I would suggest "3s Power" and "Lap Power" - the remaining fields are up to you.
I have other screens setup with various lesser used fields such as Workout Time, Time of Day, Distance, Lap Distance, Speed, etc., but these are my go to screens that I find I look at the most:
Lap Time | Cadence
Lap Power | 3s Power
I also have the same screen, only with HR instead of Lap Time. I know, I know - HR is soooo last week. I still like it - I think it (and Cadence) is helping me dial in and refine RPE. For that reason, I'm trying to resist the temptation to stare at the 310, so I try gauge by feel and then sanity check once in a while.
For biking, I have auto lap disabled - I just hit the Lap button at major points of interest (start of an interval, bottom/top of a hill, etc.).
Lap Time | Cadence
Lap Pace | HR
For running, I have auto lap set to 1km and will hit lap if the intervals I'm doing are <1km.
37. Is the fact that the 310XT can only show 4 data fields at a time (as opposed to 8 for the 705/500) a big problem?
I've never found it to be a big problem. There are times when I would like to see 5 fields at once, but it's never been that much of a problem - and I LOVE data. It would be "nice" to see all of the data on an 8 fields per screen display, but I'm not really sure how often I would actually look at those "occasional" fields – and I find it's easy enough to hit the up/down arrow to peek at another screen.
I don't use autoscroll - I found it more annoying than helpful - every time I looked down it seemed to be at the wrong screen.
Someone (forget who, sorry) suggested a clever idea - setup multiple screens with the same number of fields, then duplicate the fields you really care about across the screens, and change the ones you don't care about as much. Ok, that made no sense, maybe a picture will help:
Lap Time | Cadence
Lap Power | 3s Power
HR | Speed
Lap Power | 3s Power
If you turn on autoscroll, this would have the effect of always showing you Lap Power and Power, with Lap Time/Cadence occasionally alternating with HR/Speed.
38. What do the logistics look like for using the 310XT on race day?
The first decision you need to make is whether you want to use the 310XT on the swim. It doesn't work reliably for HR at all, and GPS only works if you put it under your swim cap.
The second decision you need to make is whether you want to use multisport mode or change between sports manually.
So if you're not using it for the swim, workflow is:
1. Put HR strap on under wetsuit
2. Put Quick Release wrist strap on under wetsuit
3. Turn on 310XT and put in Bike Quick Release Mount before heading to the swim (so it'll obtain sat lock while you're swimming)
4. Come into T1, transition, hop on bike, hit start and ride.
5. Just before coming into T2, remove 310XT from QR mount and put on your wrist, hit "whatever button" (depending on whether you want to configure it for multisport ahead of time, or just do it manually)
39. What's the simplest workflow for using the 310XT with WKO+/RaceDay/etc.?
1. Right click on the Garmin ANT Agent in your system tray.
2. Choose "Device Settings"
-Uncheck "Send my data to Garmin Connect"
-Uncheck "Send my data to Garmin Training Center"
3. Create a shortcut/alias to your data folder. Where it's located depends on your OS:
C:\Documents and Settings\USER\Application Data\GARMIN\Devices\SERIAL\History
Thanks to Cary Blanco for this tip.
1. When you're done your workout, toss your watch on your computer desk - it'll upload the data to the folders above (but won't start GTC).
2. Open the shortcut you created above.
3. Start WKO+
4. Open or Drag and Drop the file(s) from the directory into WKO+
40. Any suggestions for some good books/resources on training with Power?
1. Philip Skiba (the author of RaceDay software) wrote a book specifically on training for triathlon with a powermeter. It's called, funnily enough, "The Triathlete's Guide to Training With Power"
His other book, "Scientific Training For Triathletes" is more general in nature, but also very good.
You can buy both of them here:
2. Andy Coggan and Hunter Allen wrote what is generally considered the bible (more cycling focus than triathlon):
3. Endurance Nation has a Power Webinar that is $79 and is well worth the money (I'm an EN member and paid for it myself):
4. And from a running perspective, I don't you can go wrong with:
41. Hey...what does Daniels Running Formula have to do with Power?
Hey...what can I say? I like Jack Daniels' stuff. And Jack Daniel's.
42. Why does different software (e.g. WKO+ vs RaceDay/Golden Cheetah) and different books (e.g. Skiba and Coggan) use different terms for similar concepts?
The people who make WKO+ have trademarks on certain terms, so everyone else has to use different terms. While others could technically use the same algorithms (since they are public domain), they have chosen to use slightly different approaches/algorithms.
Here's a translation – it's a bit confusing at first, but not too bad once you distill it down:
RaceDay/Golden Cheetah terminology ~= WKO+ terminology
xPower ~= Normalized Power (aka NP, Pnorm)
BikeScore ~= Training Stress Score (TSS)
Relative Intensity (RI) ~= Intensity Factor (IF)
Things are calculated slightly differently, but the concepts are the same, and the numbers are quite similar.
43. Any suggestion for a good indoor trainer?
I've been really happy with the Kurt Kinetic Road Machine:
One thing that is kind of cool is that they publish a power curve for some of their trainers:
Certain speeds equate to power – if you compare your PT power numbers with your speed numbers, you'll see that they track quite closely.
44. Can you look at a 310XT workout's history while you're still in the middle of the workout?
No. You used to be able to do this with the 301/305 (and, I think, the 705).
If you have a 301/305, try this:
On the 310XT, unfortunately this isn't the behaviour - even after a Lap or Autolap, you can't view the current workout until you've "written" the entire workout by holding the lap button for 3 seconds to reset the workout.
Why is this an issue? Like I said, mainly intervals - if you're in the middle of a workout and missed the time of the last interval (because, hypothetically, you're bent over dry heaving because you're so tired), you can't go back and see what the time was. Or if, because of said dry-heaving, you lose track of how *many* intervals you've done, you can't just scroll back through the laps to see how many you've done.
45. My 310XT gives really screwy heart rate readings - spikes, etc.
You're not the only one. This is almost certainly related to static electricity, which is really bad with technical shirts. This usually manifests itself as elevated HR at the beginning of a run, that then settles down to regular readings after a while (after you've sweated a bit). To test this theory, try running in a cotton shirt and see if you have the same problem.
Some (including dcrainmaker) suggest buying the polar soft strap. You then use the garmin electronics pod with the polar soft strap and apparently have less problems.