For every action, there is an equal and opposite reaction.
Isaac Newton probably wasn’t referring to the red marks on the inside of my thighs or the bloody cut on my left heel when he coined that phrase.
About a month ago, I took up the at first horribly exhausting, but later oddly invigorating hobby that is outdoor running. The horribly exhausting bit lasted a full two weeks.
I’d encountered this uphill battle - literally, I was running up hills - a handful of times since my late teens. Like many, I'd long had the vague ambition that I wanted to be one of those effortlessly fit individuals who seem to run a marathon every other weekend.
And so it happened again. You know the feeling; somebody or something - often an irritatingly boastful Twitter user - inspires you to get up off your arse and give this running thing a go.
All of a sudden, you’re excited at the prospect of becoming that healthier version of yourself you just know has been hiding somewhere deep inside, raging to get out.
You head into Penney’s and spend a good €20 on a high-tech polypropylene t-shirt and shorts. You may even go the whole hog and drop another €70 on a fancy pair of runners.
You feel absolutely fantastic about yourself as you trot off home, bag in hand. You think you’re one-in-a-million as you pull on your new gear, stand in front of the mirror and convince yourself that you too can be the running sort.
You realise this is a piece of cake as your load up your iPod will all the latest motivational dance music. Then you start running and get a stitch in your side and realise you’ve made a huge mistake.
But this time, it was different. I wish I could say I had one of those light bulb moments in which I suddenly realised that I, too, could become physically fit. In reality, I did something early on to make sure that no matter how painful my new hobby became, there was no way in hell I would allow myself to give up and walk back - slowly, while gasping for breath and clutching my side - to the couch. No longer would I allow my running gear to gather dust under my bed next to those 20kg weights I once lugged all the way home from Argos on the bus.
I ignored my better judgment, pulled out my debit card and booked myself in for a 10k charity run, knowing all too well that my frugal nature meant I’d never forgive myself for quitting after dropping €27.95 on the damn thing.
After a painful, cramp-ridden fortnight, something strange happened. No longer did I dread every single run and put it off for as long as possible. I actually started enjoying the act of throwing on my running clothes and jogging around my north Dublin suburb.
It wasn't easy. About three weeks in, when I’d upped the ante to about 10k per run, I started experiencing an awful burning sensation between my legs, otherwise known as chafing. And earlier this week, my new running shoes tore my heel open and I’ve had to wear a plaster on it ever since.
While I’m yet to become one of those effortless marathon runners - who I’m beginning to think don’t exist - I’m now more aware of my personal health and the food and drink I put in my body. Don’t get me wrong - I still indulge in a chocolate bar every day. And I continue to enjoy a greasy cheeseburger on the weekends.
But despite the burns between my legs, and that awful bloody mess on my new runners, this whole running lark doesn’t seem so horribly exhausting anymore.
Put that in your 17th century pipe and smoke it, Mr Newton.
Brian completed the Great Pink Run for Breast Cancer Ireland on Saturday.