Everyone knows you must lift heavy some of the time. As long as you push yourself and try to add weight to the bar whenever possible it’s tough to screw up maximal strength training.
Ask any lifter with more than a year of training experience how you should develop maximal strength and he’ll probably get it right. Pick a couple of compound exercises and perform a few sets of a few reps with the heaviest load you can muster a few times per week.
But you can’t train heavy all the time, nor should you. There are times when you need to train with submaximal loads to build muscle, boost endurance, or groove a motor pattern. This is the part of the equation that many people screw up.
The path to effective training with submaximal loads is paved by neuroscience research. Specifically, I’m talking about the orderly system your nervous system follows to produce higher levels of force through motor unit activation. You must focus on developing the highest levels of force with each rep to get the most bang for your training buck.
By following the information in this article you’ll build more muscle, recover faster, and minimize the risk of injury.
The Science of Motor Unit Recruitment
A motor unit consists of a motor neuron and all the muscle fibers it innervates. Motor units are recruited in a fixed order from smallest/weakest to largest/strongest. This orderly recruitment of motor units is based on the electrical properties of the motor neurons it contains. Small motor neurons get activated first, followed by progressively larger motor neurons as more force is required. This is known as Henneman’s size principle (1).
The skinniest motor neurons are connected to a small bundle (e.g., 10 muscle fibers per motor unit) of weak, type I slow-twitch muscle fibers that can fire for hours. These are known as slow (S) motor units, they produce low levels of force, and they always come into play first.
As higher levels of force are required, the fast-twitch, fatigue-resistant (FFR) motor units get activated next. The FFR motor units contain a larger bundle (e.g., 100 muscle fibers per motor unit) of stronger, type IIa muscle fibers that can fire for minutes.
When maximal levels of force are required, the fast-twitch, fast-fatigable (FF) motor units get activated last. These FF motor units contain a huge bundle (e.g., 1000 muscle fibers per motor unit) of type IIb muscle fibers that produce the most force but can only sustain their activity for seconds.
So you have three types of muscle fibers that coincide with three types of motor units:
- S motor units: contain a small bundle of slow-twitch, type I high-endurance muscle fibers.
- FFR motor units: contain a moderate bundle of fast-twitch, type IIa moderate-endurance muscle fibers.
- FF motor units: contain a large bundle of fast-twitch, type IIb low-endurance muscle fibers.
Importantly, each category of motor units has a range of sizes and force-producing capabilities. For example, you have FFR motor units with a bundle of 100 muscle fibers that get activated before a FFR motor unit with a bundle of 150 muscle fibers. Keep this in mind because it becomes essential later on.
Whether you’re pulling a max deadlift or performing a set of 20 swings, the goal is to tap into the largest, FF motor units as quickly as possible. You must produce high levels of force to activate these motor units. Since the FF motor units require the largest synaptic input to fire, they’re often referred to as "high-threshold" motor units (2).
Imagine you’re performing a touch and go sumo deadlift for 10 reps. You could perform those 10 reps with a slow, easy-going tempo or you could apply maximum acceleration right from the start and knock out 10 powerful, ballistic reps. The easy set would recruit 100% of your S motor units and, say, the first 60% of your FFR motor units. The max acceleration set, however, would recruit 100% of your S motor units, 100% of your FFR motor units, and also tap into your FF motor units. (You can’t recruit all of your FF motor units unless it’s a life-threatening situation, but that’s not important here.)
Since your largest FFR motor units and any of your FF motor units contract with the most speed and force they could just as easily be called "high acceleration" motor units. Indeed, when you apply max acceleration to a submaximal load you’re tapping into the largest, high-speed motor units that slow reps can’t touch.
Now, this is where things get really important.
What Your Speed Tells You
Let’s continue with that touch and go sumo deadlift I mentioned. And let’s say you’re an iron masochist who loves to perform sets to failure. So you grab a couple of moderately heavy kettlebells and start knocking out reps with all the effort you can muster. The first 10 reps were very fast, but by rep 16 your set was in slow-grind territory. At rep 25 your lifting velocity came to a halt.
The set was tough as hell, especially the last eight or nine reps, so you consider it a success. But was it?
Based on the orderly recruitment of motor units, this is what happened. When your set merged into slow-grind mode at rep 16 the largest motor units started dropping out. We know this because the largest motor units that have the most potential for acceleration have the lowest endurance capacity. We also know that the largest motor units get recruited last but drop out first.
Indeed, the loss of your high-acceleration motor units is the reason why your acceleration slowed down. As your acceleration dropped even further, so did the motor units that were activated. By the time you reached rep 22 or 23, the only motor units that were activated were the smaller, weaker ones that could barely keep the lift moving.
In a perfect world, there would be research that shows this drop in motor unit activity as you merge into slow-grind reps. Unfortunately, science has only come up with electromyography (EMG) to analyze motor units. But EMG measurements are crude and can’t accurately determine what’s happening as your speed slows down. Indeed, there are many additional complex factors, independent of motor unit activity, that can affect and skew EMG recordings.
Nevertheless, there are three reasons why you should keep your ballistic sets out of slow-grind territory.
1. Power development: To produce high levels of power you must activate the high-acceleration (high-threshold) motor units. The orderly recruitment of motor units tells us that the only way to the high-acceleration motor units is through the smaller ones. Fatiguing the smaller motor units with slow-grinds can theoretically set up a roadblock to the reach the larger motor units on the next set. This is why you can’t perform two sets of 20 reps to failure with the same load, but you can perform two sets of a two-rep max without decreasing the load.
2. Fatigue management/injury prevention: Imagine your truck is stuck in the ditch and you have 10 guys at your disposal that can pull it out. If you let just three guys pull it’s more likely that one of them will get injured because they have to work considerably harder than if all 10 guys were pulling on the rope. This is how you should think of motor unit recruitment. There’s no reason to do slow-grind reps that overload fewer motor units when you could recruit all the motor units and minimize muscle strain.
3. Muscle development: High levels of muscle development coincide with high levels of acceleration. When acceleration is at its peak, all the motor units are activated so you get greater overall development of your muscles. You only need to look at the thighs of a 100-meter sprinter or the upper back of an Olympic lifter for proof.
The Solution
As mentioned at the beginning of this article, the goal of training for muscle and power is to recruit the maximum number of motor units with each rep. With maximal strength training, the heavy loads already mandate maximum motor unit recruitment. Therefore, when training with submaximal loads it’s necessary to keep acceleration at its peak.
Let’s say you like to do four sets of 25 reps with the swing to build muscle, endurance, and groove a motor pattern. By default, those last few sets inevitably merge into slow-grind reps that overload the smaller motor units and accumulate unnecessary fatigue.
So what’s the solution? First, don’t have a preset number of reps for any set. Instead, have a target number of total reps for a lift – that’s 100 swings in this case. Start with your first set and crank out as many high speed reps as possible. When you feel your speed slowing down, stop the set. Rest for 30-60 seconds and repeat for another set until your speed slows down. Continue until you reach 100 reps.
Importantly, even if you perform all your sets in this fashion your 100th rep won’t be as fast as your first rep. It doesn’t need to be. In fact, it shouldn’t be because some level of fatigue is necessary for development. However, the speed of the last rep of any specific set should be as close as possible to the first rep of that set.
There are two methods to achieve maximum motor unit recruitment with ballistic training.
1. Stop each set once your speed slows down: This should be a noticeable drop in acceleration. If you have to guess if your speed has slowed down, it hasn’t slowed down enough to stop. But don’t allow more than one rep that’s significantly slower than the first rep of that set.
2. Stop each set within 10 seconds: The largest, FF motor units can only sustain their activity for 10 seconds tops. This is why you can’t pull a deadlift with 95% of your maximum for eight reps. So another effective way to achieve maximum motor unit recruitment is to limit each set to 10 seconds.
How to Achieve 100 Fast Push-ups
The push-up is one of the best upper body muscle building exercises and the ability to knock out 100 fast push-ups is an impressive display of physical prowess. Here’s how you can reach that milestone.
1. Do 100 fast push-ups every other day regardless of how many sets it takes.
2. Your rest periods should match your current rep max. If you can only get 20-25 push-ups in one set, start with one minute of rest between each set. If you can already knock out 40 or more push-ups at once, start with 30-second rest periods.
3. Decrease the rest periods by five seconds with each workout, but only if you get more reps in the first set than you did for the last workout. Let’s say you start this push-up program on Monday. And let’s say you got 41 reps with your first set. On Wednesday, if you got, say, 43 reps for the first set drop your rest periods to 25 seconds and complete 100 total reps. If on Friday you only get 43 reps for the first set keep the rest periods at 25 seconds.
4. Follow the rest progression until you reach 10-second rest periods. Once you reach 10-second rest periods continue with the plan until you get to 100 reps for the first set. Importantly, only high-speed reps count. Stop each set once your speed slows down. The 10-second set limit I mentioned earlier doesn’t apply here.
This method can be applied to any lift with a submaximal load where a target number of continuous reps is the goal.
Final Words
A few years ago, research by Wakeling et al demonstrated that ballistic contractions might preferentially recruit the high-threshold (high acceleration) motor units (3). However, the overriding view in the neuroscience community is that motor unit recruitment is preserved in all types of contractions. In a recent, excellent review of power training, the authors stated:
"While preferential recruitment of the type II fibers remains a possibility, the current evidence for it occurring in response to exercise in humans is not convincing (4)."
The good news is that the information in this article holds up whether or not ballistic contractions preferentially recruit the high-threshold motor units.
When training with submaximal loads, speed is king. Apply maximum acceleration right from the start, stop each set when your speed slows, and forget about a specific number of reps for any set. Let your speed determine when it’s time to rest. Apply this simple principle to your training sessions and you’ll quickly develop new muscle and power.
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Chad Waterbury is a neurophysiologist and author whose unique training methods are used by a wide range of athletes, bodybuilders, figure models, and fitness enthusiasts of all ages and from all walks of life. For more of Chad’s information pick up his book Huge in a Hurry and visit www.chadwaterbury.com
1. Henneman E, Somjen G, Carpenter D. J Neurophysiol 1965; 28: 560-80.
2. Gatev P, Ivanova T, Gantchev GN. Electromyogr Clin Neurophysiol 1986 Mar; 26(2): 83-93.
3. Wakeling JM, Uehli K, Rozitis A. J R Soc Interface 2006 Aug; 22;3(9): 533-44.
4. Cormie P, McGuigan M, Newton R. Sports Med 2011; 41(1): 17-38.
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