How do force vectors affect the type of strength we develop?
Strength coaches often use novel exercises that have “horizontal” or anteroposterior force vectors to improve athletic performance.
Yet, exactly how these exercises differ in their effects from more traditional exercises with “vertical” or axial force vectors” is not well-understood.
Even so, we can see how exercises with axial and anteroposterior force vectors differ in at least two key ways, by (1) looking at which muscles they work hardest, and (2) identifying whether the hardest part of the movement is where the prime mover muscles are short or long.
While there are likely other factors that produce specific strength gains after training with each type of exercise (such as bar speed), these are the easist two factors to identify, if you are prepared to dig through the research and understand how (1) muscles and (2) muscle lengths change with changing joint angle.
What are force vectors, anyway?
When we are doing a strength training exercise, we have to produce force. This force is most commonly exerted to overcome gravity and inertia, but is also sometimes exerted to lengthen elastic bands or apply tension to the cables of machines.
Importantly, this force is exerted in a specific direction relative to the body.
When the force is produced parallel to the spine, we refer to the force as “axial” and when the force is produced perpendicular to the spine, we refer to it as “anteroposterior”.
When we stand on the ground to do a squat or deadlift with a barbell, this requires us to produce force to lift a weight vertically, and it involves an *axial* force vector. In contrast, when we stand upright and use a cable machine to do pull-throughs, this requires us to produce force to move a cable horizontally, and it involves an *anteroposterior* force vector.
However, moving a weight vertically does not always correspond to an axial force vector. It can correspond to an anteroposterior force vector if we are lying down to perform the exercise.
Indeed, when we lie down on the ground to do a glute bridge or hip thrust exercise with a barbell, this requires us to lift a weight vertically, but it involves an *anteroposterior* force vector. This is because the force required to overcome gravity and the inertia of the barbell is exerted perpendicular to the spine, and not parallel to it.
Why does force vector affect strength gains in lower body exercises?
When we lift a weight, the force that we exert to move it does not stay the same through the whole exercise range of motion, for two reasons.
Firstly, because we need to exert a force to overcome gravity (which applies equally at all points in the exercise range of motion) and also to overcome inertia (which only applies at the beginning of the exercise, when we accelerate the weight). Thus, many exercises are most difficult at the start, when both gravity and inertia are being overcome.
Secondly, because the force that our muscles must produce is dependent upon the turning forces at the joints, and not on the size of the weight itself. These turning forces are equal to the force (which is dependent upon the size of the weight) and the perpendicular distance between that force and the joint, which is called the “moment arm length” of the weight. Since the weight exerts force vertically downwards, this perpendicular distance is usually measured horizontally.
The way that the moment arm length of the weight changes across the whole exercise range of motion differs between axial and anteroposterior lower body exercises. This means that the exercises produce peak forces at different muscle lengths, and also load different muscles to a greater extent during multi-joint exercises.
Both of these factors lead to the two types of exercise producing different adaptations.
How does force vector cause adaptations to differ between lower body exercises?
The easiest way to see the effect of force vector on subsequent adaptations is to compare the squat and hip thrust exercises.
As we descend into the squat, the knee and hip move a large horizontal distance from the barbell. This means that the hardest part of the squat exercise is at the bottom of the movement both because of the need to accelerate the weight (and overcome inertia), and also because the moment arm length of the weight is largest at this point. In contrast, when standing upright, the knee and hip are directly underneath the barbell, so it is very easy to hold the weight at lockout.
This means that the prime movers in the squat (the quadriceps, adductor magnus, and gluteus maximus muscles) must produce their peak forces at the bottom of the squat, where they are longest. Consequently, when we use the squat in a training program, the prime mover muscles adapt to produce force most effectively at long muscle lengths.
Training at long muscle lengths *tends* to cause greater hypertrophy than at short lengths in most muscles, except those that have their best leverage at short muscle lengths, like the triceps brachii and gluteus maximus. It also increases muscle fascicle length, and thereby shifts the joint angle at which we are strongest to longer muscle lengths.
Also, since the hip has several muscles that all do the same job, the body shifts the load onto the muscles that have the most effective leverage. This leverage differs depending on joint angle. When the hip is flexed, the adductor magnus has the best leverage, so this is the hip muscle that is most effectively developed by the squat exercise.
The hip thrust
In a glute bridge or hip thrust, the moment arm length of the weight does not change as much as in the squat, because the hip and knee do not move that far from the weight throughout the exercise. And even though we still need to overcome inertia at the bottom of the movement, the exercise is still very challenging at the top, because our ability to produce a turning force at the hip is much lower when the hip is extended than when the hip is flexed. So relative to our maximum capacity, the exercise is at least as hard at the top, compared to at the bottom.
This means that the hip muscles in the hip thrust (the adductor magnus, hamstrings, and gluteus maximus muscles) must still produce high forces at the top of the exercise, where they are shortest. Consequently, when we use the hip thrust in a training program, these muscles adapt to produce force very effectively at short muscle lengths.
Training a muscle at short muscle lengths only tends to cause greater muscle growth when the muscle has its peak internal moment arm length at short muscle lengths, like the triceps brachii and gluteus maximus. Otherwise, it tends to produce strength gains at the relevant joint angle primarily through increases in neural drive.
The hip thrust also shifts the load very effectively away from the other hip muscles and onto the gluteus maximus muscle in the top position, for three reasons.
- Firstly, the hamstrings likely produce little force in this position, because they are over-shortened by the combination of hip extension and knee flexion, so the amount of actin-myosin overlap is too small for optimal force production.
- Secondly, the adductor magnus has its best leverage when the hip is flexed, and has much worse leverage in full hip extension.
- Thirdly, the gluteus maximus has its best leverage when in full hip extension.
Together, these factors combine to cause the gluteus maximus to be challenged most effectively by this exercise.
What is the takeaway?
The force vector of an exercise has two key effects. Firstly, it changes the muscle that is most effectively challenged during a multi-joint lower body movement. And secondly, it changes the joint angle at which peak forces are produced, which leads to adaptations that are typically of training at either long or short muscle lengths.
The squat has an axial force vector, and this causes the hardest part of the exercise to be at the bottom, where all prime movers are lengthened and the adductor magnus has the best leverage. Since it works the hip muscles at long muscle lengths, the squat is a good exercise for producing lower body hypertrophy in most muscles, but is especially effective for the adductor magnus.
The hip thrust has an anteroposterior force vector, and this causes the exercise to be hard at the top, where the gluteus maximus has the best leverage. Since it works the hip muscles at short muscle lengths, the hip thrust likely produces strength gains in most of the prime movers primarily through increases in neural drive and not through hypertrophy, except in the case of the gluteus maximus, because this muscle has its best leverage in full hip extension, and so gets worked far harder than the other hip extensor muscles in this exercise.