In our last post we described the physiology of the Nordic curl. The focus of the blog was on how nordic curl trains the hamstring throughout the entire available muscle length and does so utilizing both eccentric, concentric, and isometric muscle contraction types. We touched on the high effort required during the performance of a Nordic and now will dive a little deeper into what that means. Exercise intensity is one of those terms that seems rather intuitive but can be misleading. The more we learn about all things strength, power, and hypertrophy training, the more that data seems to point to exercise intensity being one of the most important variables that needs to be considered to produce a sufficient stimulus for the body to adapt as long as volume is sufficient. In other words, working really hard, per set of each exercise you do, as long as you do enough overall sets and reps is the key to forcing the body to adapt to training. Of course this also assumes that your body has enough energy and resources to adapt, which is generally the case if you have been training correctly, aren’t starving, sick, or sleep deprived.
Exercise intensity can be conflated with cardiovascular intensity. This is often pictured as collapsing out of breath into pools of sweat at the end of a workout. (Crossfitters and HIIT people I am looking at you). It can also be conflated with high anaerobic intensity.
Picture the intense muscle burn that comes with a drop set of… well anything. The reality is that exercise intensity applies differently to each repetition of an exercise and each set DEPENDING on the goal of that exercise.
That brings us to an important aspect of exercise intensity and something quite relevant to why nordic curls are probably so helpful for many people, neuromuscular recruitment. Neuromuscular recruitment is the sequential process of your nervous system turning on varying amounts of muscle fibers. Each muscle fiber connected to a motor neuron (nerve) is called a motor unit. Most neurons (the nerves that signal to our muscles to contract) control many muscle fibers. Certain motor units activate really easily. These are called low threshold motor units. Others require more stimulation to activate and are called high threshold motor units. Low threshold motor units activate when moving around in our daily lives whereas thigh threshold motor units activate when we have to move something heavy, or something very quickly. This allows our body to produce movement whose force is proportional to the activity we need to do. Imagine it this way. If we didn’t have different thresholds for our motor units, every time you reached for a coffee mug intending to grasp it gently you would strike out with your arm like Rocky punching Drago (It goes without saying that we probably wouldn't survive if we moved that way).
So that’s typically that’s the order: low threshold motor units activate first, then high threshold motor units if needed. As you might imagine, following this sequence takes time. The precious milliseconds that it takes to get the high threshold motor units to fire, can be the difference between an injury and just another sprint. To get stronger we have to improve the amount and precision of neuromuscular recruitment. To get faster we have to improve the rate of neuromuscular recruitment. Here is the crux of this issue. If we train really hard, let’s say we’re a novice or intermediate nordic curler doing a nordic, with routinely getting close to the point of failing, we can safely assume that because we are fighting so hard to control the descent, we are getting the high threshold motor units to activate. With repeated training to this NEUROMUSCULAR INTENSITY, we keep dialing the high threshold motor units until eventually they get put on speed dial, and so they turn on more quickly. If we keep it up long enough they turn on BEFORE the low threshold motor units. Ultimately a lot more fancy neurological stuff happens but this is probably one of the most applicable to nordics. I think the reader can see how this might be advantageous.
Any activity that involves the need for rapid forceful hamstring contraction, requires the expression of high forces quickly. Examples of such activities include sprinting, changing direction, and jumping. Deceleration of the thigh and hip that occur during sprinting, hurdling, etc require high forces to stabilize the lower leg as the foot strikes the ground. Generally that occurs as the knee is extending and the hamstring contracts in a stretched position. Development of end-range control of bi-articular muscles like the hamstring can be lacking from athletes’ training regimens utilizing more conventional lifts, and is often under-appreciated.
We already discussed that because of human architecture, these end-range positions are where our muscles are less able to produce force and are often the positions where they are most vulnerable to injury. Although “injury prevention” is a controversial term, looking at things from neuromuscular and biomechanical perspectives, it makes sense that improving strength and control where the muscles and joints are most vulnerable could transfer to reduced injury risk while in those positions. The nordic curl is arguably one of the best tools for accomplishing this because it undoubtedly trains the hamstring muscle in a lengthened position and at a high intensity. It is hard to cheat while doing a nordic or one of its derivatives unless you are looking to lose a few teeth. As with any truthful discussion of movement there are a few caveats here. Generally speaking though, the nordic in its most basic form is a great tool to train the hamstrings effectively.
Stay tuned to hear about the evidence on injury prevention utilizing the nordic curl and some simple variations of the nordic curl that can be used to promote the development of different qualities depending on one’s goals. Until then, happy curling!