It's always been a common thought that endurance athletes, if they lift at all, should lift lighter weights while using a higher repetition scheme. The philosophy behind this is the fact that endurance athletes use a disproportionate amount of slow-twitch muscle fibers vs. fast-twitch. Those slow-twitch muscle fibers are stimulated more with the lighter load and higher rep scheme rather than the fast-twitch muscles that are used for immediate force in actions like sprinting. Those fast-twitch fibers are the same ones used in the gym when lifting a heavier load for a shorter duration. Think heavy Squats for 3-6 reps.
The problem with this train of thought for endurance athletes is that it doesn't take into account that we have only a limited effect on the makeup of our own muscle fibers. The proportion of slow-twitch to fast-twitch muscle fibers we are born with is determined by genetics. For the sake of argument, let's say the majority of us are made up of a 50/50 split between fast- and slow-twitch, which is roughly accurate. High level endurance athletes can carry as much as 80 percent slow-twitch fibers. In a way, they're good at what they do because they were born to be more dominant in that type of activity. While it's believed you can alter some of your muscle fiber types through specific training (though research is ongoing, the amount of fibers available for change is often agreed upon to be in the range of 10 percent), someone born with a substantially greater ratio of slow-twitch muscle fibers are at a huge advantage in endurance sports.
If you took two people and trained them exactly the same way over the exact same amount of time, the person naturally made up of more slow-twitch fibers would inherently end up as a far better endurance athlete than the person carrying predominantly fast-twitch fibers. To expect all athletes to benefit in the same manner from using the same programming, even if it's specifically set up for one particular sport, is an unreasonable methodology. Therefore, some heavy resistance training in an endurance athlete's routine isn't going to drastically alter their muscle fibers in a way that will drag down their performance. On the contrary, it actually provides some very useful benefits.
What the heavy weight training gives endurance athletes that the lighter, circuit-type routines do not is a much stronger emphasis on joint integrity. When performed correctly, lifting heavy can almost be viewed as being protective. When you load the muscle tissue and joints with heavy weight, something called bone osteoblasts occurs. This process strengthens tendons, ligaments and collagen, making them more resilient. Lifting heavy will also increase bone density, protecting against the breaks and stress fractures which are common problems for many endurance athletes. Lifting heavy will also increase production of brain-derived neurotrophic factor (BDNF), the protein related to producing new brain cells that improve cognitive function.
For endurance athletes, there are far more benefits to lifting heavy than there are to avoiding it. Though the dangers of heavy lifting without proper form are well documented, it's no different than taking on any other type of strenuous activity in your training. If you perform it wrong, you increase your risk of injury.
In terms of movements you can include in your routine, I'm a big fan of the oldies but goodies—Squats, Bench Press and rowing variations. Compound moves use several muscle groups, so by definition, these type of compound moves will stimulate the most muscle fibers. Squats are great because you need to use your shoulders, back and core to stabilize and control the weight. This same principle is true with all compound moves. The more muscles you recruit at once, the greater the overall strength benefit. Once you start pounding the pavement, the heavy weight training will pay off in your body being stronger, more resilient and more efficient.
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