Years and years of research have elapsed in the pursuit to completely understand the intricacies of human skeletal muscle contraction.It has included human, amphibian, and feline muscle biopsies, laboratory tests, practical hands-on experiments, and deductive reasoning. Thankfully, we now have a solid grip on evidence-based practical applications when it comes to designing and implementing resistance training programs. However, there still exists much controversy, ignorance, and confusion, even among educated trainers and trainees on this topic.
Before I begin to simplify this issue, I understand it can become an emotional topic due to the various schools of thought that exist regarding the best way to get stronger, increase power, maximize hypertrophy, improve endurance, or improve skill. Because there are a number of philosophies on the best way to attain these attributes, both ego and financial gain are at the root of this. Hey, it’s the world we live in, but hanging on to proven science and research will offer you some solace and common sense to move forward.
If everyone just accepted the following three indisputable facts, training program design and implementation would be much more objective, safer, and sensible:
Activity on Earth is governed by basic laws of physics – one being gravitational pull. The Henneman’s Size Principle of muscle fiber recruitment is the accepted gold-standard. Your genetic skeletal structure, muscle fiber endowment, and nervous system “hook-ups” cannot be ignored.
Gravity’s Pull and Resistance Training
The law of gravity clearly dictates you cannot move a relatively heavy resistance quickly. That is if a resistance moves quickly it must be “light” relative to your ability. Similarly, you can move a light resistance relatively quickly as compared to “heavy” resistance. The lighter resistance is, the faster your potential speed of movement, all other factors being equal. Common sense, people.
Take Olympic lifters. These people are strong. Look at their training regimens: they use training protocols to increase muscular strength, and then practice the skills of lifting heavy resistances with proper technique.
They can only move heavy resistances so fast and so high, so they need the ability to move fast to secure it. That is, the resistance does not move fast, but their technique does. Slower-moving front squatting, back squatting, and overhead pressing is done to get stronger. Faster-moving skill practice is then implemented to perfect the required body actions.
What about a conventional exercise such as a bench press or leg press? It’s pretty straight-forward: load more resistance on the bar or machine and it will move slower as compared to using a lighter resistance relative to your ability.
Think about it: you can surely throw a baseball further than a 16-pound shot used in the shot put. Likewise, all other factors being equal, a stronger person can throw both implements even further as compared to someone relatively weaker.
Henneman’s Size Principle: Slow vs. Fast Muscle Fiber
Muscle fibers are recruited sequentially based on need. That is, the lower the demand, the fewer fibers required and the greater the demand, the more fibers required. Low-demand efforts recruit the smaller, lower threshold, slower-to-fatigue motor units.
When more effort is required, the larger, higher threshold, faster-to-fatigue motor units are called upon. It makes perfect sense and explains why you can jog for a longer time as compared to sprinting, or why a lighter resistance can be moved for more repetitions as compared to heavier resistance.
The “slow” versus “fast” muscle fiber classification is a misnomer and has created mayhem among both the scholarly-educated and the average Joe Schmoe trainer and trainee. Conventional wisdom suggests the smaller, slow muscle fibers contract slowly and is not capable of “fast” muscle contraction.
Similarly, larger, faster muscle fibers are thought to be the only fibers recruited for lightening-fast muscle activity. Yes, slow fibers do contract relatively slower than fast fibers, but the difference is between 60 to 90 milliseconds. Yes, milliseconds. This difference is virtually negligible.
Understand the fast versus slow fiber classification does not only refer to the speed of contraction. It also refers to a fiber’s fatigue capacity. The larger, greater force-producing muscle fibers are faster to fatigue as compared to slow fibers, which exert slightly less force-output but are slower to fatigue.
An explosive, bodyweight-only vertical jump is a great example:
A single maximum-effort jump recruits both slow and fast fibers. Although it is high-effort, it creates minimal fatigue because of its brevity. Perform multiple jumps and fatigue will eventually ensue because of the greater demand and recruitment of higher threshold, faster fatiguing fibers. Now, jump while holding heavy dumbbells or wearing a weighted vest. What happens? The speed of movement and jump height will decrease due to gravitational pull, but you will be using more muscle fibers. Jump multiple times and fatigue will come sooner because more fibers are initially required (the faster-to-fatigue type). This higher-demand event cannot match the time frame as jumping without resistance. Finally, use a five-repetition maximum (5RM) resistance in a squat or deadlift and try to jump (which I don’t recommend, by the way). Because it is ultra-high demand, a large pool of muscle fibers will be recruited, the resistance cannot be moved fast, and fatigue will be realized quickly.
Genetics, Body Type and Your Ability to Contract Muscle
Touching just briefly on this topic, your body type, and the neuromuscular system can affect your ability to contract the muscle and perform, all other factors being equal:
Longer limbs may move slower than shorter limbs. Having exceptional tendon origins and insertions may allow you to exert greater force/speed as compared to poor origins/insertions. Greater muscle mass may exert more force than smaller mass. Possessing more high-threshold, fast muscle fibers may allow you to exert more force than possessing more slow-type fibers. If you don’t “look the part” (i.e., small muscles, gangly, over-fat) but can contract muscle/exert force with above-average ability, you probably have good neurological ability (muscle fiber-nervous system “hook-ups”).
Training Mode Implications
If you despise gravity, move to the Moon. Relatively heavy resistance requires the recruitment of many muscle fibers, including the high-threshold, greater force-generating fibers. High-threshold/greater force-generating fibers are used in explosive/speed movements outside the weight room in sports competition. Relatively heavy resistance cannot be moved fast. If you can move a resistance fast, it is light relative to your ability. Although inherently unsafe, moving relatively fast with resistance can recruit and overload many fibers provided maximum repetitions are achieved (i.e., aim for complete volitional muscle fatigue). If a fast speed of movement were important in resistance training, what amount of resistance would you use and how fast would you move it? 35%, 50%, or 80% of a 1RM? 115, 360, or 600 degrees per second? You don’t have to move fast when resistance training to develop power. Power = force x distance/time. Get stronger, (increase force) then practice your sports skills/timing (maximize distance and time), which leads to this: Move fast when skill training, unabated by resistance. Refine and hone sport-specific skills as they will be required in competition.
1. Brooks, G.A., T.D. Fahey, and K.M. Baldwin. (2005). Exercise Physiology: Human Bioenergetics and its Applications. New York, N.Y.: McGraw-Hill Companies.