Why does lifting and not lowering weights cause unique effects?
Eccentric-only training is a special type of strength training used by athletes and in physical therapy, where only the lowering phase of an exercise is performed, and the muscle lengthens.
Eccentric-only training produces very specific adaptations that seem helpful for improving athletic performance, reducing injury risk, and assisting in the treatment of certain musculoskeletal conditions, so it gets a lot of press coverage.
In contrast, the opposite type of training (called “concentric-only” training), which involves only doing the lifting phase of an exercise, and where the muscle shortens, is very rarely discussed.
Yet, many Olympic weightlifting derivatives can arguably be described as “concentric-only” exercises, as the weight is dropped rather than lowered under control after it has been lifted. So it is not like the strength and conditioning community never uses this way of training.
And importantly, “concentric-only” strength training also produces a unique set of adaptations, and leads to greater gains in maximum lifting strength than in maximum lowering strength.
Why does this happen?
Why does “concentric-only” strength training cause greater gains in maximum lifting strength than in maximum lowering strength?
One obvious reason why “concentric-only” strength training might produce only small gains in maximum lowering strength is because it fails to trigger the key adaptations that are typically produced after strength training with eccentric contractions.
However, there are also two other reasons why “concentric-only” training might improve maximum lifting strength by more than maximum lowering strength, and these are:
- A greater increase in pennation angle
- A larger increase in rate coding
Let’s take a look at each of these in turn.
#1. Changes in pennation angle
Muscles are made up of multiple compartments, with each compartment containing muscle fascicles laid out in parallel to one another.
However, the fascicles do not run lengthways from one end of the muscle to the other, but at an angle, joining with the collagen layer that surrounds the muscle, and the different regions.
This angle is called the “pennation angle” and affects the ability of the muscle to produce force, especially at high speed.
A muscle with a large pennation angle can fit more fascicles into the same compartment, which means that it can develop a greater force. Admittedly, one downside of increasing pennation angle is that the force is no longer expressed in completely the right direction, so some is lost, but the trade-off is generally favorable.
Additionally, muscle fascicles rotate their pennation angles during a muscle contraction. This means they can produce a greater reduction in their effective length for the same change in fascicle length, and this increases their effective contraction velocity. Muscles with larger pennation angles tend to be capable of more rotation, and therefore display greater high-velocity force production.
Importantly, “concentric-only” strength training produces greater gains in pennation angle than eccentric-only training, which might allow it to produce larger increases in high-velocity strength.
And since any concentric (lifting) action is *always* faster than any eccentric (lowering) action (because lowering velocities are negative), this adaptation is of greater benefit for increasing maximum lifting strength, than for increasing maximum lowering strength.
#2. Larger gains in rate coding
Active muscle force is regulated mainly by the number of muscle fibers that are contracting at the time.
Each of the muscle fibers within a muscle are attached to individual motor units, and when a motor unit is switched on in response to a signal sent by the central nervous system (CNS), all of the muscle fibers that it controls contract together.
Motor units can be ranked in size order according to their recruitment thresholds. Lower-threshold motor units are recruited in response to fairly small signals from the CNS, while higher-threshold motor units are activated only in response to large signals.
Lower-threshold motor units control groups of muscle fibers that together produce small amounts of force, while higher-threshold motor units control groups of muscle fibers that produce larger levels of force.
If we gradually increase the size of the signal from the CNS, we recruit progressively more and more motor units, in size order from the lowest threshold ones right up to the highest threshold ones. There are more low-threshold motor units than high ones, which means that force can be more finely-tuned at low levels than at higher levels.
But the recruitment of motor units is only half of the picture, because muscle fibers relax almost immediately after they have been activated by a signal from the motor unit. This means that very regular signals are required (30 times per second) in order to achieve the consistently high force production necessary to lift a heavy weight.
The rate of the signal from the CNS to the motor unit and onwards to the muscle fiber is called “rate coding” and rate coding is faster in concentric (lifting) phases than in eccentric (lowering) phases of a movement, and much faster in high-velocity movements than in low-velocity movements. This is why peak force is typically reached faster during concentric (lifting) contractions compared to in eccentric (lowering) contractions.
Why does this happen?
Well, the actin-myosin bindings inside muscle fibers (which are the structures that actually cause the fiber to contract in response to a neural signal) release faster during higher velocity, concentric contractions, and this reduces the amount of force that can be produced. To counteract this, the CNS has to increase rate coding, which is what causes the released bindings to reattach.
Increases in rate coding are typically observed after high-velocity strength training, and contribute to high-velocity strength gains, and so it is not a big stretch to imagine that the same effect occurs after concentric-only strength training. This would then contribute to large gains in maximum lifting strength, without also increasing maximum lowering strength by as much.
What is the takeaway?
To improve sporting movements than involve fast, concentric (lifting) movements (like jumping), “concentric-only” strength training may actually be very valuable, because this type of training can produce key adaptations that enhance the ability to produce force quickly, while a muscle is shortening.