Today we’re going to discuss a handful of very popular muscle biomechanical properties that are taught in almost every exercise science class but never with any application to muscle growth. We are, of course, referring to the, “Force-Velocity Curve,” and the, “Length-Tension Curve,” and all of their related components. Teachers will often tout these as applicable measures to help students understand muscular force production and other various metrics but the reality is that they’re so much more applicable for muscle hypertrophy than anyone gives them credit for. So let’s dig in.
The Force-Velocity Curve
The force-velocity curve explains the relationship between how much force a muscle can produce and how fast it contracts (5). If you examine the curve, it’s easy to see that a muscle produces high amounts of force at slow contraction velocities and that force quickly drops as contraction velocity increases. Slow contraction velocities produce high amounts of force due to the increased amount of connections between the contractile proteins – at fast contraction velocities it’s impossible to create as many connections (5).
So how does this relate to muscle growth? We know that the biggest determinant of muscle growth is mechanical tension (14,16). Muscle fibers experience mechanical tension by producing force, and this force is determined by the speed with which the fiber contracts (14). Therefore, the more force a muscle fiber produces, the more mechanical tension it experiences.
Now, take a quick glance back at the force-velocity curve – a muscle produces its highest amounts of force at slow contraction velocities which means that it will also experience high levels of tension at these slow velocities. Great! That’s that, right?
Not so fast. There’s a highly-related physiological property to discuss when concerning this phenomenon, but this property at least gets plenty of credit when it comes to muscle growth. We refer to this as the, “Size Principle” and it’s the main factor to consider when promoting slow contraction velocities for gains. The size principle states that motor units are recruited in order from smallest to largest depending on the amount of force that is required for a specific task (5). We typically split motor units into two categories: 1) high tension motor units (HTMU), which often consist more heavily of larger fast twitch fibers and more overall muscle fibers; and 2) low tension motor units (LTMU), which are mostly composed of slow twitch fibers and are responsible for low intensity/velocity movements (5,12).
It’s important to offer the size principle debate here because it’s easy to see that LTMUs might be preferentially activated for slower movements. This idea throws a stick in the wheel because the muscle fibers in LTMUs do not typically grow as much in response to normal resistance training (11). Therefore, there’s definitely a caveat when considering slow contraction velocities and growth.
The caveat is this: you need to be performing these slow contraction velocities at high levels of load or fatigue. The goal is to activate the HTMUs at slow contraction velocities, and there’s two ways you can do that: 1) lift weights above 80% of your 1RM; or, 2) lift weights close to, or to, failure (15). Lifting heavy weights or lifting lighter loads to failure induces high levels of motor unit activation (15) but also creates a scenario in which movement and contraction velocities slow down (13).
The importance of load and repetition speed is underlined when we examine light load, high velocity movements and motor unit activation. Studies show that muscles are highly activated when performing high velocity movements at light loads (2) yet we see other studies that show that high velocity movements, like jumping, do not lead to muscle growth (3). When considering muscle growth, maximum activation is nothing without heavy/fatiguing loads and slow contractions!
In conclusion for Part 1, use the force-velocity curve to your advantage when designing resistance training workouts. You now know that you need to lift heavy weights at slow contraction velocities or lighter weights to failure to capitalize on the mechanical tension experienced by the muscle. Mix up both strategies in your training as well as the exercises you use to add variation to your effective rep schemes.
Force = Mass x Acceleration
The next concept we need to discuss is the idea of F=ma. Since force is the product of mass x acceleration, many people think that a muscle is producing a ton of force at high contraction velocities and therefore experiencing a ton of tension. But if we look at the Force-Velocity curve, that’s not the case. So what gives?
Coaches and trainers are confusing the idea of external force vs internal force . The barbell is the object that is benefiting from the acceleration when considering force, not your muscle. Therefore, the overall movement has a ton of force due to this acceleration, but your muscle did not produce a ton of force. Your muscle produced power.
Power is defined as work divided by time (W/t). And work is defined as force x distance (Fd). So, we can consider work as being the weight of the bar multiplied by how far the bar has to move. Therefore, since power is W/t, the less time it takes you to move the weight over that distance, the more power you can produce. The muscle is not producing a ton of force in high velocity movements; rather, it is producing power. If you take a look at the adjusted force-velocity curve with power added, you can see that peak power is actually produced at a relatively low force (9). Therefore, muscle fibers are not experiencing much overall tension at this load and/or movement velocity.
On that last point, it’s worth remembering that growth takes place in individual muscle fibers, rather than the muscle as a whole. Individual muscle fibers experience tension due contraction velocity, like we discuss in the Force-Velocity curve. Therefore, an explosive movement will induce very little tension on individual muscle fibers, even though it is, from a physics standpoint, a “forceful” exercise.
So next time someone talks about how F=ma applies to strength training and muscle force, don’t be afraid to correct them; the Force-Velocity curve always applies. Just because the barbell is moving fast doesn’t mean the muscle is producing a ton of force. Power training is still an important component of getting stronger but it won’t directly have a major impact on muscle size.
The Length-Tension Curve
The next underappreciated biomechanical property of muscle, when considering hypertrophy, is the length-tension curve. The length-tension curve describes the relationship between the length of a muscle and how much tension it experiences/produces (5). If we examine this curve, it’s easy to see that, as a muscle lengthens, it experiences greater amounts of tension. The peak towards the end is caused by the addition of passive elements like connective tissue and the contractile protein, titin (5). This peak at the end of the curve is of great interest when considering muscle hypertrophy since we know tension is the main determinant of muscle growth (14,16).
So, how do you train a muscle at longer lengths to capitalize on this increase in tension? Use a full range of motion! Studies show that a full range of motion is more effective for overall size gains than a partial range of motion (1). Interestingly enough, the importance of full range of motion is further shown in studies showing that partial reps that utilize the bottom half of the ROM still result in similar levels of muscle growth to using full reps (10). This is because partial reps in the stretched half of the ROM still load the longest muscle length similarly to full ROM repetitions.
Another interesting note is that range of motion training results in specific gains. What that means is that a full range of motion squat develops the entire muscle, especially the distal portions of the quadriceps that are closer to the knee (1). In comparison, a quarter squat only develops muscle fibers closer to the hip (1). This is a unique scenario in which we see multiple types of muscle hypertrophy occurring.
The first type, which may be induced more by full ROM training, is longitudinal hypertrophy. Think about adding links to an existing chain – that’s sort of how longitudinal hypertrophy works. When a muscle is stretched past its capacity against a load, like full ROM resistance training, the muscle adapts by adding more contractile units in series to protect against that type of action in the future (17).
The second type is myofibrillar hypertrophy. Myofibrillar hypertrophy is simply the growth of the contractile proteins in response to mechanical tension (6). Since partial ROM squats still do induce mechanical tension on the muscle, you will get some growth out of them – just not nearly to the degree of full ROM squats since full ROM movements can cause growth through both longitudinal and myofibrillar hypertrophy.
Now, it’s important to note the use of partial ROM training when attempting to increase strength. Partial ROM movements can be very important when considering strength gains due to the fact that the big compound movements often have, “sticking points,” somewhere in the middle of the movement range of motion (7). This may be due to the fact that, since the majority of training involves submaximal weights, the top half of the movement is not challenged nearly as often as the bottom half of the movement.
We see this evidenced in studies showing that muscle activation greatly decreases in the second half of the bench pressing motion when using submaximal weights (4). This is simply because the initial burst of force overcomes the barbell’s inertia to the point that you no longer need to produce a significant amount of force as momentum has taken over the movement. This is why we see lifters often fail about halfway up on bench – they’re not used to having to produce force there!
Therefore, partial ROM movements can absolutely have their place in a strength training program. To improve your bench, you’ll need to do things like floor press, board press, and other triceps accessories to improve the second half of your press. Similar to squat and deadlift, you’ll have to add things like high box squats and block pulls to improve the second half of those lifts as well. Squats and deadlifts typically don’t stall quite as bad as the bench press because as you move through the ROM in the squat and deadlift you gain mechanical advantage over the bar (in the deadlift, this occurs after the bar passes the knee). That doesn’t really happen during the bench.
The most important thing to remember, when considering range of motion and gains, is that gains are always going to be specific to the range of motion that you use. A floor press will only strengthen the top half of a full bench press, just like a block pull would only increase lockout strength but not strength off the floor in a deadlift (in advanced lifters, of course). The same can be said for any muscle gains accrued through these exercises. You won’t get much pec growth out of a floor press since the pecs are not stretched to a great degree and don’t experience much tension. However, the triceps will undergo a similar range of motion through both a floor press and a normal bench press and will experience greater relative load during a floor press for more overall growth.
In conclusion for Part 3, keep in mind the idea of the length-tension curve and the specificity of strength gains when designing workouts. You want to maximize both types of hypertrophy if you’re interested in muscle gains, but if you’re a powerlifter or weightlifter, you may want to add exercises that focus on the top halves of your big lifts.
We don’t need to sum up the entire article here, but it is worth pointing out that this particular article is only covering the biomechanical components of hypertrophy. Yes, metabolic stress still likely plays a role in growth, and this role may be both additive and/or independent to/of mechanical tension. In addition, the jury is still out on what exact role muscle damage plays. However, as we state throughout this piece, mechanical stress is still the biggest determinant of growth and using some of these biomechanics tricks can help you maximize this growth stimulus.
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