I often get asked by new athletes for the one thing that they can do to make the biggest difference in their training. Usually that's a very complex answer and has a number of caveats depending on what their goals are, so I've learned to give them the easiest thing they can do. It's something that's cheap and easy to measure, doesn't involve detailed analysis and applies to every athlete both new and advanced: cadence.
Measuring your rotations per minute on the bike or steps per minute on the run is very simple. Even the most basic bike computers and multisport watches will have sensors to do this. Usually in the form of a magnet attached to a crank arm or a pod that snaps onto your shoe laces. It's something tried and true that has been well established over years of competition. While it applies to the swim as well, it's not nearly as simple to measure so we'll leave that aside for now.
There are more than a few reasons to focus on cadence, but the first one I'll mention is efficiency. On the bike, I guide athletes to aim for a cadence that falls between 80 and 90rpms. This is regardless of the situation. Speed work, long endurance focused workouts, climbing hills... the 80-90 rule works for the over-whelming majority of people in every situation. There are obviously cases like climbing up the side of a mountain. You can keep shifting gears to get easier and easier, but at a point you run out and you're left without any choice. Your cadence might drop and that's simply the limit of your equipment.
The reason we're looking to stay in this 80-90 range is a balance of muscular and metabolic efficiency. There are people who will make an argument that lower cadences produce more power. This is absolutely true. But what that argument ignores is what happens to your body's energy systems. Triathletes are endurance athletes. Even a “sprint” will take an hour or more. This means you need to be mindful of your body's stored carbohydrates which it turns to in order to fuel your muscles. Pedaling at 70rpms might produce more power but, keeping everything else equal, that athlete will burn through their carbohydrate stores much faster than they would with a cadence for 90.
Using a VO2 test to look at how an athlete burns carbs and fat in real time, I can see the moment they shift from their small to big chain ring. Their cadence drops as they try to spin the harder gear suddenly and their fat numbers immediately drop, significantly so. While lower cadence might feel comfortable and powerful, it will eventually come back to bite the athlete in the long run.
Speaking of the run, that is the second major reason to follow your cadence. While on the bike your neuro-muscular system adapts to firing in a certain rhythm. This isn't good or bad by itself, but the body tends to adapt and get comfortable at what intervals your brain tells your muscles to fire. Trying to change this suddenly becomes very difficult. Biking for 2 hours at 70rpm and trying to get off and run at 85spm (steps per minute) if jarring. Your HR rises not because the effort is more difficult but because your body wants to stay with the rhythm you've already made it work at.
This slower cadence on the run tends to make most athletes plod along. They hit the around hard with each step. They will over stride to try and compensate, but that creates heel striking and forces engagement of more muscles groups. The athlete fatigues faster, form gets sloppy and they become more injury prone. One small thing like their cadence on the bike has a trickle down effect that can cause their whole workout/race/season to fall apart.
When we're talking about a simple magnet added to your watch or computer, it's a very small price for a huge potential benefit!