How does one repetition-maximum (1RM) increase after training?
Maximum concentric strength can be defined as the peak force exerted while the muscles are shortening. Thus, it is most commonly measured while lifting an extremely heavy weight.
Conventionally, we record the heaviest barbell load that an athlete can lift as a proxy for this peak force, which is called the one-repetition maximum (1RM). Yet, the peak force must be greater than the weight of the barbell, since it is produced to break inertia as well as to combat gravity. Even so, the 1RM provides a good reference point for maximum concentric strength.
Although we tend to think of improvements in 1RM as reflecting similar improvements in strength at the muscular level, the reality is more complex. To simplify matters, we can group the ways in which improvements in 1RM occur into four basic categories.
Firstly, technique in the lift can be improved.
Improvements in exercise technique involve an increase in the total system force produced during the exercise, but the joints involved in the lift do not produce a larger net joint torque.
Essentially, as a result of the strength training program, the athlete becomes more efficient at the exercise, such that they can produce a greater system force for the same net joint moments.
#2. Intermuscular coordination
Secondly, intermuscular coordination can be increased.
Like a technique improvement, this also causes system force to increase, but in this case the joints involved in the lift *do* produce more torque. On the other hand, the prime mover (agonist) muscle force does not increase, because the increases in joint torque are caused by reductions in opposing (antagonist) muscle forces.
Improvements in intermuscular coordination occur most commonly when an exercise is done in conditions that require the athlete to stabilize the weight, such as when lifting free weights. Training causes a change in intermuscular coordination such that there is a shift from opposing (antagonist) muscle activation (which hinders the production of net joint torques) to synergist muscle activation (which does not).
#3. Voluntary activation
Thirdly, voluntary activation can be increased.
Like an increase in intermuscular coordination, this also causes system force to increase, and also causes the joints to produce more torque. However, in this case, prime mover (agonist) muscle force also increases, at least during voluntary contractions.
Increases in voluntary activation occur when the central nervous system (either the brain or the spinal cord) increases the size of the signal to the muscle, and this allows a larger number of motor units to be recruited, and the muscle is more fully activated. Yet, prime mover muscle force does not increase in *involuntary* contractions, because there is no change in the capacity of the muscle itself to produce force.
#4. Muscle-tendon unit changes
Finally, the maximum force-producing capacity of the muscle-tendon unit itself can be increased.
When this happens, system force is increased, the joints produce more torque, and prime mover (agonist) muscle force increases in both voluntary and involuntary contractions. We can test this by measuring the force-producing capacity of the muscle during electrically-stimulated contractions, which remove the effect of the central nervous system.
When a substantial amount of muscle growth is possible (as in beginners and intermediates), this increase in muscle-tendon force is largely caused by hypertrophy. Yet, muscle-tendon units might also increase their ability to produce force through other mechanisms, including:
- an increase in tendon stiffness (stiffer tendons allow muscles to shorten at slower speeds, and therefore exert more force),
- an increase in the density of myofibrillar parts of the muscle cells (the sarcoplasm does not contribute to force production), and
- increases in the number of lateral attachments between the muscle fiber and its surrounding collagen layer (this increases the amount of force that is transmitted laterally, but reduces the effective fiber length).
In general, the literature suggests that myofibrillar packing density does not contribute substantially to increases in strength, but changes in lateral force transmission are quite important. Increased tendon stiffness probably contributes to a degree, the literature is less clear.
N.B. muscle architecture and joint angle-specific strength gains
One further complication is that changes in muscle architecture (such as increased or decreased fascicle length) can alter the point in the joint range of motion at which peak torque is produced.
While this does *not* necessarily alter the peak force that a muscle is capable of exerting, it may alter the peak force measured in a 1RM effort, because dynamic exercises involving lifting weights usually have a “sticking point” where the required system force is highest, which is often produced by changes in external moment arm lengths.
For example, the barbell back squat displays increased external moment arm lengths of the barbell on the hip and knee joints with increasing squat depth, such that the lift is very hard at the bottom, but easier at the top. Increasing muscle fascicle lengths (to shift the joint angles at which the hip and knee extensor muscles can produce peak force) should lead to a greater 1RM and therefore greater peak forces recorded. Yet, the actual maximum force-producing capacity of the muscles themselves would not have changed. It just moved from one joint angle combination to another.
How do changes in 1RM transfer to sport? — part 1
Transfer can be calculated as the ratio of the improvement in the sporting movement or untrained exercise, divided by the improvement in the trained exercise.
When the improvement in the trained exercise is large, but the change in the sporting movement is small, the amount of transfer is also small. In contrast, when the improvement in the trained exercise is large, and the change in the sporting movement is also large, then the transfer ratio approaches 1.0, which would be a very successful result.
In general, increases in maximum concentric strength transfer weakly to improvements in sports performance. For example, very large changes in squat 1RM are required to achieve small gains in sprinting performance.
As I have explained before, this happens *partly* because the types of strength required for sprinting, changing direction, and jumping are not the same as the type of strength involved in lifting a very heavy weight. And the mechanisms through which we increase each type of strength also differ, depending on the type of strength.
But when using the 1RM to measure maximum concentric strength, we can say more than this.
How do changes in 1RM transfer to sport? — part 2
Grouping the ways in which 1RM can increase (technique, intermuscular coordination, voluntary activation, and changes in muscle-tendon unit properties) can help us understand why the research shows such poor relationships between 1RM improvements and changes in sports performance.
- Increases in 1RM because of improvements in exercise technique — cause no change in sports performance, because motor skills are very specific to the exact movement being performed.
- Increases in 1RM because of increased intermuscular coordination — cause a possible change in sports performance. Yet, changes in stability-specific strength are quite context-specific, so I wouldn’t be surprised if there was actually very little transfer effect of this adaptation.
- Increases in 1RM because of increased voluntary activation — cause improved performance in a sporting movement using the same muscle (but force production at high velocities will still have a lot of room for improvement, because the neural strategy of explosive, short-duration contractions differs from that of maximum, sustained contractions, even when the intent is maximal in both cases).
- Increases in 1RM because of changes in muscle-tendon properties — cause improved performance in a sporting movement using the same muscle (but force production at high velocities will not improve by as much as you might hope, because of increased internal moment arm lengths, tissue inertia, and a greater number of lateral attachments reducing effective fiber lengths).
Essentially, improvements in 1RM will only transfer to a sporting movement if they reflect underlying increases in voluntary activation or improvements in actual muscle-tendon properties. And even then, the transfer may not be quite as effective to high-velocity force production as you might hope.
For researchers investigating the transfer effect between changes in exercise 1RMs and performance in sporting movements (like jump height or sprint speed), this causes quite a big problem. Some athletes might record very high transfer effects, since their 1RM gains are caused by improvements in voluntary activation or muscle-tendon properties. Equally, other athletes might record very poor transfer effects, since their 1RM gains are primarily related to improved technique and intermuscular coordination. This means that the group-wide effect will not reflect the true underlying benefits of strength training.
Ultimately, when trying to achieve transferable strength gains from heavy strength training, it is not the change in 1RM that matters, it is the change in the ability to produce voluntary force.
What does this mean?
Maximum concentric strength in an exercise (the one repetition-maximum or 1RM) can increase in four ways (technique, intermuscular coordination, voluntary activation, and changes in muscle-tendon unit properties). Yet, only two of these ways (voluntary activation, and changes in muscle-tendon unit properties) can contribute to increased sports performance.
Alongside the differences in the mechanisms that determine maximum concentric strength and high-velocity strength, this framework helps explain why increasing 1RM may not always transfer as well to sporting movement as we might expect, even though the ability to produce force (under varying conditions) is the primary determinant of sporting performance.