HSS 500 Force & RFD

Performance

Any quantitative measure of how well an organism performs an ecologically relevant task that is vital for survival

• Focus on measurable skill, task specific

• What neuromuscular factors allow us to perform a task at the highest level?

Law of Inertia

“An object remains at rest or continues to move in a straight line at constant velocity, unless acted upon by a net external force.”

• An object will not move unless force is applied

Law of Acceleration

“The acceleration of an object is directly proportional to the net external force applied to it, and inversely proportional to its mass. The direction of acceleration is the same as that of the applied force.”

• More force = more acceleration; more mass = more force applied

• F = m * a (force = mass x acceleration)

Law of Action-Reaction

“For every action, there is an equal and opposite reaction.”

• When applying force to an object, force is also “applied back” from that object

During the concentric phase, force is ____ the resistance

greater than

During the eccentric phase, force is ____ the resistance

less than

During an isometric contraction, force is ____ the resistance

equal

A _____ contraction allows you to produce the most force.

eccentric

• Ex: in a bench press greater than 1RM, although you may not be able to concentrically lift the weight, you can slow the bars descent (eccentrically)

Force

Contribution to linear motion at an endpoint; represents a key mechanical output of the neuromuscular system

• Assessing force → insight into the functional capacity of the neuromuscular system

• There is an infinite number of conditions that can be used to assess force in humans

• The movement from which force is measured determines what elements from the neuromuscular system contribute

Maximal voluntary force

Refers to the maximal linear force generated at a given point of application, typically the endpoint where the interaction with the environment occurs (e.g., feet on a force plate)

• Ex: During a mid-thigh pull, the force plate measures the net ground reaction force applied at the feet, resulting from the contribution from the muscles crossing the various joints (hip, knee, and ankle)

Rate of force development

Refers to the time-dependent increase in linear force generated at a given point of application, typically the endpoint where interaction with the environment occurs (e.g., feet on a force plate)

• RFD reflects how rapidly force is produced, particularly during the early phase of contraction

• Example: During a mid-thigh pull, RFD is quantified from the slope of the force-time curve measured by the force plate and represents how quickly net vertical ground reaction force increases

Force plates

Tool used to measure external forces generated during whole-body and multi-joint tasks (e.g., mid-thigh pull, vertical jumps, running, etc.)

• Force plates record ground reaction forces at high sampling frequencies, allowing precise characterization of force-time profiles

• This data can be used to quantify: peak force, time-to-peak force, & rate of force development (RFD)

Analog-to-digital conversion (ADC)

The process by which a continuous analog signal is transformed into a discrete digital signal, represented by a series of numerical values. During ADC…

• The analog signal is sampled at regular time intervals (sampling frequency)

• The amplitude of the signal is quantized into numerical values

• The digital signal can then be stored, processed, analyzed, & visualized

**This conversion enables the computation of key neuromuscular factors of human performance such as peak force rate and rate of force development (RFD) from the signals recorded by force plates and other measurement devices

What unit is utilized for maximum force production?

Newtons (N)

What unit is utilized for rate of force development?

Newtons per second (N/s)

Use & purpose of the isometric mid-thigh pull (IMTP) in neuromuscular assessment

Assesses maximal strength and force production via peak force and rate of force development (RFD)

• Especially useful for athlete assessment (tracking progress, asymmetries, etc.)

• Correlates with athletic movements like vertical jump & sprint speed

Typical body position during IMTP

Upright torso (shoulders back, looking up), knees slightly bent, hips slightly flexed, and feet about hip-width apart, flat on the force plate; arms are straight and holding the bar at the mid-thigh level

Importance of intent during IMTP

Intent directly influences performance and validity of results → participant must provide a maximal, rapid effort

Common sources of variability during IMTP

Bar height & placement, joint angles, verbal cues, surface/set-up, force plate software & analysis methods, personal factors (fatigue, motivation, body composition), etc

What does the VALD ForceDecks system measure?

Force, power, balance, and movement symmetry (using dual force plates)

What is the sampling frequency of the VALD ForceDecks system?

Up to 1,000Hz (1,000 times/sec.), but can be set lower for things like balance assessments (~200Hz)

What is the calibration process of the VALD ForceDecks system

Zeroing plates before each test, ensuring plates are on a flat surface, some advanced calibrations require specific weight application

What is the recommended number of trials for reliable assessment using IMTP?

3-5 trials (fatigue seen after this period)

What is the typical rest interval between IMTP attempts?

Generally 1-3 min.

How is a “good” IMTP trial recognized?

Stable baseline before pull, rapid force increase, peak force during sustained effort (not at very end), consistent effort, proper technique

• Looking for steep increase in force production followed by plateau at max. value

• Typically improvement in performance in multiple attempts (“practicing”) before decrease from fatigue

How is a “poor” IMTP trial recognized?

Use of countermovement, force applied before “go” signal, peak force at end, unstable baseline, leaning on the bar, excessive movement/foot lift

What common issues have been reported for IMTP?

High equipment costs, potential for countermovement (invalidates pure isometric data), limited transfer to horizontal/multi-plane movements, variability in protocol, need for familiarization, etc.

What bioenergetic system is utilized during the IMTP?

ATP-PCr system