The goal of strength training is to make sure you can apply more force – that’s it. Of course, there is the matter of how you apply it. A big back squat is no guarantee for a strong lunge, even though you use the same muscles. Still, the primary goal is simple.
Now let’s take a look at a soccer player. When the soccer players sprints, he pushes into the floor to propel the body forward – the soccer player applies a lot of force to the ground. He’s too late; the ball gets kicked in a different direction, and he needs to run the other way. He pushes his feet into the floor and applies even more force so he can quickly decelerate, turn around and explode in a different direction. Aside from technique and coordination – that’s a lot of force application! This example shows us how strength training can make you faster in the context of game sports.
Let’s take a look at a marathon runner. Whenever you run, there’s a super slight twist at the middle of the back. This is easy to see by the hands alternating forward and backwards with every step. The thing is that this twisting – you need to propel yourself forward, and every bit of power that goes left or right means there is a ‘power leak’. Energy is going in a direction that is not helpful to what you’re training to do – running a marathon. To understand this better, compare a relative stiff body to a tensed up rope and a wobbly body to a hanging rope with a lot of slack. Which rope will make pulling a car towards you easier? So how do you keep the rope, i.e. your body, tight? By applying force! When the body wants to twist left or right, you use your core muscles to ensure it doesn’t. The stronger you get, the easier this gets. Long story short – yes, getting strong does help with running.
The big fear is that you would gain too much muscle from strength training, and every added kilo when running countless miles is one too many. The solution is simple – train in a way that improves your performance (through strength increases) more than it decreases your performance (through weight gain). The net result will be an improvement, considering that muscle building is a way slower process than strength improvements.
Still, strength training can make you slow, especially if you do your exercises slowly and neglect your speed/power work in favor of strength exercises. You need to be strong but in a way that works for your sport. Look at Olympic weightlifters; they’re both incredibly fast and strong – they have to balance their strength and speed work for maximum carryover to the snatch, clean and jerk. In much the same way, a soccer player or marathon runner needs to be smart about how they train.
All that’s left is to do your strength training properly so that it transfers to your primary sport! So no, bench presses may not be the best way to improve your running performance and doing all your strength work super slow and controlled may not help your transfer onto the field in rugby. It’s not that strength training is bad for those sports; it’s wrongly applied strength training that is the issue!