Basic Biomechanics: Terms and Definitions

Biomechanics is a fascinating field. Possessing sufficient knowledge in this area is paramount for properly understanding resistance training. I try my best to educate my readers so that over time they can build upon their knowledge and reach superior levels of understanding with regards to human movement.

I have listed some definitions below that I would like for my readers to try to familiarize themselves with as it will allow them to better comprehend future blogposts, articles, videos, and interviews. I created a special tab on the right hand column of the blog named “Biomechanics Terminology” so you can find this particular article whenever you need it.

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Force: force equals mass times acceleration and is a vector quantity, meaning that it’s displayed in a particular direction. Force is usually measured in Newtons.

GRFs: GRF stands for ground reaction force. When you jump, sprint, or perform an Olympic lift, you exert force into the ground. Force-plates measure these forces. During vertical jumping, most of the force produced is vertical. However, in sprinting, you have vertical forces as well as horizontal forces. When the foot strikes the ground during maximum speed sprinting, at first the force is projected forward which is called braking forces, and once the COM passes over the foot, the force is projected rearward which is called propulsive forces. In general, force, including GRF, is measured in Newtons.

Muscle Force: when muscles contract or are stretched, they create muscle force. This muscle force pulls on bones which creates joint torque. In general, force, including muscle force, is measured in Newtons.

Velocity: velocity is the rate of change of position of the athlete. It’s just like the term speed, but with a direction associated with it. It is usually measured in meters per
second, but can also be expressed in miles per hour or kilometers per hour.

Vector: vectors contain both magnitudes and directions. Force, velocity, and acceleration are all vector quantities.

Force-Velocity Curve: you can plot the force-velocity curve on a graph by plotng force on the y-axis and velocity on the x-axis. In strength & conditioning, the goal is to shift the curve upward and to the right so that the athlete can exhibit more force and power at every possible load. Heavy strength training tends to shift the curve more on the force end of the spectrum, whereas explosive training tends to shift the curve more on the velocity end of the spectrum.

Joint Angular Velocity: joints in the human body move through arcs and therefore accelerate through a range of angular motion. Joint angular velocity is the rate of change of joint movement, often measured in degrees per second or radians per second.

Acceleration: acceleration examines the rate of change of velocity with respect to time, and is typically reported in meters per second per second (meters per second
squared).

Power: power is the rate of doing work. It is calculated either by dividing work by time, or by multiplying force by velocity. Power is usually reported in watts.

Joint Power: it is possible to measure the power output of individual joints during movement by multiplying the torque by the joint angular velocity. It is usually reported in Newton-meters per second.

RFD: RFD stands for rate of force development and can be measured in multiple ways. RFD is believed to be highly important in sports that require rapid force generation. It is usually measured in Newtons per second.

RTD: RTD stands for rate of torque development and is usually measured in Newton-meters per second.

RER: RER stands for rate of EMG rise and represents the rate of increase in muscle activation. RER is typically measured in % of MVC per millisecond or millivolts per second.

Impulse: impulse is force multiplied by time (actually it’s the sum of net force, or the force that influences acceleration, multiplied by time over a phase of interest), and is sometimes calculated by taking the area under the forcetime curve. It is typically reported in newton-seconds.

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Work: work is equal to force times distance and is generally reported in joules.

Momentum: momentum is mass times velocity and is reported in kilogram meters per second.

Moment (Torque): a moment is the turning effect produced by a force. It is often synonymous with torque, which can be thought of as the rotational analog to linear force (turning force), and is calculated by multiplying the perpendicular force by the distance from the pivot (or axis of rotation). Resistance in strength training produces an external moment, whereas muscles produce an internal moment to counteract the external moment. Moments are usually measured in Newton-meters.

Torque-Angle Curve: you can plot the torque-angle curve on a graph by ploting the torque on the y-axis and the joint angle on the x-axis.

Stiffness: stiffness is the rigidity of an object and can be thought of as the extent to which it resists deformation in response to an applied force. The stiffer the object, the harder it is to deform. Stiffness is usually measured in Newtons per meter or pounds per inch.

Compliance: the opposite of stiffness is compliance. The more compliant the object, the easier it is to deform.

Peak: a peak is the greatest magnitude of a set of data, or the highest point measured.

Mean: a mean is simply on average of a set of data and is calculated by combining a set of data and dividing by the number of figures.

Relative: in biomechanics, the term relative commonly means relative to one’s bodyweight and is calculated by dividing a figure by one’s bodyweight.

Absolute: in biomechanics, the term absolute commonly means the total amount regardless of bodyweight.

EMG: EMG stands for electromyography and is a technique for recording and analyzing the electrical activity produced by skeletal muscles.

Onset Time: the onset time in EMG is the time that elapses between an occurrence and detectable muscle activation.

Isokinetic: isokinetic exercise is performed on a dynamometer which provides variable resistance to movement so that regardless of the effort exerted, the
movement takes place at a constant speed.

Isoinertial: isoinertial exercise maintains constant mass and is characteristic of typical free weight exercises that are commonly employed in strength training.

PAP: PAP stands for post-activation potentiation which is a phenomenon whereby performance is enhanced following previous muscle activation.

ROM: ROM stands for range of motion and is typically measured in degrees or radians.

Displacement: displacement is a change in position of a body. It can be translational, rotational, or a combination of both.

CMJ: CMJ stands for countermovement jump and is a common test used in research to measure jumping ability. It begins in the standing position with hands on the hip and involves a rapid countermovement until the knees reach 90 degree angle, whereby the movement is explosively reversed.

Squat Jump: the squat jump (SJ) is a common test used in research to measure jumping ability and is performed starting in the bottom position at a 90 degree knee angle with the hands on the hip, no arm swing, and no countermovement.

MVC: maximum voluntary contraction (MVC), or sometimes referred to as MVIC for maximum voluntary isometric contraction, is the measurement of the greatest possible output that the individual can create by their own volition. MVC could be used in electromyography (EMG) or with torque measurements using isokinetic dynamometers.

Concentric: concentric muscle actions occur when muscles shorten under tension.

Eccentric: eccentric muscle actions occur when muscles lengthen under tension (technically it’s not a contraction).

Isometric: isometric muscle actions occur when no movement in the joint take place. The muscles will indeed shorten while the tendons will lengthen, but the term is
relaxed.

Plyometric: plyometric exercises take advantage of the stretch-shortening cycle (SSC) whereby a muscle rapidly lengthens and then explosively reverses its action.

GCT: GCT stands for ground contact time. In general, maximum speed sprinting exhibits GCTs of approximately one-tenth of a second whereas for maximum jumping it’s approximately five-tenths of a second. Running and depth
jumping can average 0.2 seconds.

Net: in biomechanics, often forces in opposite directions are combined to create a single net force. For example, net horizontal force is the sum of braking (negative) and propulsive (positive) forces.

COM: COM stands for center of mass and is the unique point where the weighted relative position of the distributed mass sums to zero. Another way of thinking of it
is as a point in space determined by a distribution of mass, whereby a uniform force acting on that mass would act as if the distribution were located at just that point. Sometimes the term center of gravity (COG) is used in place of COM.

Active: in biomechanics, active muscle forces are generated by muscle contractions, namely the sarcomeres.

Passive: in biomechanics, passive muscle forces are generated by the elastic properties of materials such as those found in muscles (collagen, titin, etc.), ligaments, bones, tendons, and fascia.

Resultant: in biomechanics, often resultant vectors are calculated, in which case a single vector is formed by combining (or summing) two or more other vectors. For example, combining horizontal and vertical forces into a resultant force.

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23 Comments

  • Kevin Butler says:

    Awesome post Bret, I would love to see more posts like this in the future (maybe about research terms, or whatever else you can think could be helpful).

    Happy hip thrusting
    -Kevin

  • Max says:

    Force is expressed in Newton. Mass is expressed in kg. Force and mass are connected through acceleration. F = m*a. Therefore a force can never be expressed in kg. In some cases a force is proportional to the mass which leads to F ~ m if the accelerations of the compared objects are the same. The forces then can be compared as masses but the force is still not a mass 😉

  • Bob says:

    Sorry to pick nits, but kilogram is a measure of mass, not force. It just so happens that, at the earth’s surface, there is a 2.2 lb gravitational force exerted on a 1 kg mass.

    Mostly this matters in using F=ma, but could be relevant in recording a deadlift record on the moon. Kg would be most impressive. Assuming, of course, your federation allows a spacesuit as proper gear. 😉

    • Bret says:

      You’re not being nit picky Bob. Please scrutinize the rest, I want this to be as accurate as possible. Fixed the force definition. Thanks for your input!

      • Bob says:

        Two other smaller items caught my eye.

        1. I presume the torque-angle curve assumes zero angular velocity? (Or rather, you are plotting the torque due to the external weight, and not the torque due to the muscle.)

        2. You might be right about “moment” == “torque”. However, I always use the term “moment arm” which is different. The moment arm is the torque divided by the magnitude of the force. In a biceps curl, the torque of the weight about the elbow depends on the weight, but the moment arm does not depend on the weight. Reducing the moment arm improves mechanical advantage.

        • Bret says:

          1. Yes, the external resistance torque, not the internal muscle torque.
          2. I know that strict biomechanics terminology differentiates between a “moment” and “torque,” but I feel that these are synonymous for basic purposes. At any rate, you’re right about the moment arm, as T = Perpendicular muscle force x moment arm (internal torque that is, external torque would be the resistance x moment arm, and decreasing that moment arm indeed improves the mechanical advantage, i.e., top of a squat).

          Thanks for looking this over Bob! Much appreciated.

  • Grant says:

    I love nerdy posts like this!! Thanks Bret! The old-school anatomy drawings definitely help the overall feel of the article as well.

  • Joy says:

    Thanks for this! This kind of math I can “get”.

  • moss g says:

    bret,
    you didn’t define “strength”!

  • Rob says:

    Love this stuff Bret thanks for sharing!

  • amc says:

    Bret,

    Under Moment (Torque) you state that “Moments are usually measured in Newtons per meter or Newton-meters”. Now Moment = Force x distance so the units are Nm or Newton-meters. Newtons per meter is not the same as Newton-meters and is incorrect.

  • Bill says:

    Hey Brett, isn’t RER Respitory Exchange Rate? Gas exchange in the lungs and fuel source determination etc. Because what your talking about sounds more along the lines of QRS measurements to me.

  • Charlie says:

    Do you have any specific textbook or other resource(s) you would recommend to someone trying to get a better understanding of biomechanics?

  • Jason Lake says:

    Hey Bret,

    Great work, but impulse is the sum of net force (force that influences acceleration) multiplied by time over a phase of interest, and is normally reported in newton seconds (for some reason when used as a unit Newton becomes newton), which can be abbreviated to either Ns or N.s with a superscripted ‘-1’. If normalised relative to the mass of interest it can then become velocity, and can be reported in metres per second.

    Hope that helps,

    Jason

  • Hey Bret:

    I respect your desire to educate individuals on “biomechanics”. I have been educating personal trainers, exercise physiologists, physical therapists and athletic trainers on biomechanics for the past 20 years. Actually, biomechanics is the application of engineering mechanics to living organisms. Human biomechanics is comprised of three basic disciplines: 1) Mechanics of Human Movement or Movement Biomechanics; 2) Orthopedic Biomechanics; and 3) Fluid Biomechanics. These are very complex fields that require rigorous graduate degree engineering backgrounds.

    Sincerely
    Jerry

  • Rui says:

    Getting everyone educated in this subject its great. Any one working with clients should have some understanding of biomechanics.
    I would had Inertia to the list. That’s something that we all should have awareness when applying force in our clients joints.

    greetings

  • Surabhi says:

    Hi Bret,

    Many thanks for this article. However, I am not able to make out the difference between RFD and impulse. Could you explain a bit more if possible?

  • Michelle says:

    can you explain what producing force is?

  • Alexander Mainborg says:

    Hello,

    For the GRF, you write that when the force is projected forward it is a braking force. Since there is both a force and a reaction force, it would be less ambiguous if you specify that you are referring to the reaction force and not simply the force. Well, it’s just a suggestion from a Danish physicist – good luck with everything.

  • Roger James says:

    Good resource for review. Did find one typo. “Moment (Torque): …can be though of ” – I imagine you meant “thought of”

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