Glossary of Force Measurement and Muscle ...

Whether you're new to objective strength testing or looking for a quick reference, this glossary covers the essential terms used in force measurement, dynamometry, goniometry, and force plate analysis. Terms are organized alphabetically for easy reference.

A

Accuracy

The closeness of a measured value to the true (actual) value. In dynamometry, accuracy is typically expressed as a percentage (e.g., ±1%). A dynamometer with ±1% accuracy measuring a true force of 100N will display between 99N and 101N.

Asymmetry (Bilateral)

The difference in performance between left and right limbs, expressed as a percentage. Commonly calculated during jump testing on dual force plates. Asymmetry >10% during bilateral tasks may indicate injury risk or incomplete rehabilitation.

Active Range of Motion (AROM)

The range of joint movement achieved by the patient using their own muscle effort, without external assistance. Measured in degrees using a goniometer. Compare with Passive Range of Motion.

B

Break Test

A manual muscle testing technique where the clinician gradually increases force against the patient's limb until the patient can no longer maintain the position (the muscle "breaks"). The peak force at the moment of breaking is recorded. Compare with Make Test.

Bilateral Deficit

The phenomenon where the sum of forces produced by each limb individually exceeds the force produced during a bilateral (two-limbed) effort. Common during bilateral jumps and lifts.

C

Calibration

The process of verifying and adjusting a measurement device's accuracy against known reference standards (e.g., calibrated weights). Most modern strain gauge dynamometers maintain calibration well, but annual verification is recommended.

Center of Pressure (CoP)

The point on a force plate surface where the resultant ground reaction force acts. CoP displacement is the primary metric for balance assessment. Greater CoP excursion indicates poorer postural stability. See also: Center of Pressure explained.

Concentric Contraction

A muscle contraction where the muscle shortens as it generates force (e.g., the upward phase of a biceps curl). Compare with Eccentric and Isometric.

Countermovement Jump (CMJ)

A vertical jump test performed on force plates where the subject begins standing, performs a rapid downward countermovement (squat), and then jumps maximally. The CMJ is the most commonly used jump test in sports science and rehabilitation.

D

Dynamometer

A device used to measure force or torque. In clinical settings, handheld dynamometers measure isometric muscle strength. In research, isokinetic dynamometers measure force through a controlled range of motion. See also: How does a dynamometer work?

Dynamic Strength Index (DSI)

The ratio of ballistic peak force (e.g., CMJ peak force) to isometric peak force (e.g., isometric mid-thigh pull). DSI = Ballistic PF / Isometric PF. Values closer to 1.0 suggest the athlete is already expressing their maximal strength dynamically. Values below 0.60 suggest a need for power/speed training rather than maximal strength training. See: Dynamic Strength Index explained.

Dorsiflexion

The movement of bringing the top of the foot toward the shin, decreasing the angle at the ankle joint. Normal dorsiflexion ROM is approximately 20°. Limited dorsiflexion is associated with increased injury risk. See: Ankle ROM normal values.

E

Eccentric Contraction

A muscle contraction where the muscle lengthens under tension (e.g., the lowering phase of a biceps curl, or the braking phase of landing from a jump). Eccentric strength is critical for injury prevention and deceleration.

Extension

The movement of straightening a joint, increasing the angle between two body segments. Full knee extension (0°) is a critical milestone in ACL rehabilitation.

F

Flexion

The movement of bending a joint, decreasing the angle between two body segments.

Flight Time

The time a subject is airborne during a jump, measured by force plates (the period when vertical force = 0). Jump height can be calculated from flight time using the formula: height = (g × t²) / 8, where g = 9.81 m/s² and t = flight time. See: How to calculate vertical jump height.

Force

A push or pull that can cause an object to accelerate. Measured in Newtons (N). In clinical settings, force is often expressed in kilograms-force (kgf) for practical understanding. 1 kgf = 9.81 N.

Force Plate

A rigid platform embedded with force sensors that measures ground reaction forces during standing, walking, jumping, or other activities. Modern portable force plates use strain gauge or piezoelectric sensors and connect wirelessly to companion software.

Force-Time Curve

A graph plotting force (y-axis) against time (x-axis) during a movement. Analysis of the force-time curve shape reveals movement strategy, rate of force development, and neuromuscular function.

G

Goniometer

A device used to measure joint angles and range of motion. Universal goniometers are the traditional tool; digital goniometers use inertial sensors for improved accuracy and reliability. See: What is a goniometer used for?

Ground Reaction Force (GRF)

The force exerted by the ground on the body in response to the body's force on the ground (Newton's third law). Measured by force plates in three directions: vertical, anterior-posterior, and medial-lateral.

H

Hamstring-to-Quadriceps (H:Q) Ratio

The ratio of hamstring strength to quadriceps strength on the same limb. Normal isometric H:Q ratio is approximately 0.60–0.65. Ratios below 0.50 are associated with increased knee injury risk, particularly for ACL injury.

I

Impulse

The product of force and time (Force × Time). Impulse equals the change in momentum and is calculated as the area under the force-time curve. In jump testing, propulsive impulse determines jump height.

Intraclass Correlation Coefficient (ICC)

A statistical measure of measurement reliability, ranging from 0 (no reliability) to 1 (perfect reliability). ICC values above 0.90 are considered "excellent" reliability. Clinical measurement tools should demonstrate ICC > 0.85 for clinical use.

Isometric Contraction

A muscle contraction where force is generated without a change in muscle length or joint angle. Isometric testing with a dynamometer is the most common clinical strength measurement method because it's standardizable, safe, and reproducible.

K

Kilogram-force (kgf)

A unit of force equal to the gravitational force on a 1 kg mass. 1 kgf = 9.81 Newtons. Many clinicians prefer kgf because it's intuitive ("the patient can resist 25 kg of force").

L

Limb Symmetry Index (LSI)

The ratio of involved (or weaker) limb performance to uninvolved (or stronger) limb performance, expressed as a percentage. LSI = (Involved / Uninvolved) × 100. An LSI of 100% indicates perfect symmetry. LSI ≥90% is a common criterion for return-to-sport clearance.

Load Cell

A transducer that converts force into an electrical signal. Strain gauge load cells are used in most modern dynamometers and force plates due to their accuracy, linearity, and durability.

M

Make Test

A manual muscle testing technique where the patient pushes maximally against the clinician's stationary resistance. The clinician does not attempt to overcome the patient's force. Peak force during the isometric effort is recorded. Make tests tend to be more reliable than break tests.

Maximal Voluntary Contraction (MVC)

The maximum force a person can generate during a voluntary isometric contraction. MVC is the gold standard strength measurement and serves as the reference for all relative strength comparisons.

Minimal Clinically Important Difference (MCID)

The smallest change in a measurement that is meaningful to the patient. For handheld dynamometry, MCID values vary by muscle group but are typically 10–20% of baseline values.

N

Newton (N)

The SI unit of force. One Newton is the force needed to accelerate a 1 kg mass at 1 m/s². Gravity exerts approximately 9.81 N on a 1 kg mass. In dynamometry, force measurements are often reported in both Newtons and kilograms-force.

Normative Data

Reference values derived from testing healthy populations, organized by age, gender, and other relevant demographics. Normative data allows clinicians to compare a patient's measurements against population standards. See: What is normative data?

P

Passive Range of Motion (PROM)

The range of joint movement achieved by an external force (the clinician moves the limb) without active muscle contribution from the patient. PROM is typically greater than AROM.

Peak Force

The maximum force value recorded during a measurement trial. In isometric testing, peak force represents the strongest moment during the contraction. In jump testing, peak force occurs during the propulsion phase. See: What is peak force?

R

Range of Motion (ROM)

The full arc of movement available at a joint, measured in degrees. ROM is measured using goniometers and is one of the most fundamental clinical assessments. See: Range of motion test guide.

Rate of Force Development (RFD)

The speed at which force is generated, calculated as the change in force divided by the change in time (ΔF/Δt). Measured in N/s. RFD reflects neuromuscular function and explosive ability. Deficits in RFD may persist even when peak force has recovered.

Reactive Strength Index (RSI)

A measure of the ability to quickly transition from an eccentric (landing) to a concentric (jumping) movement. RSI = Jump Height / Ground Contact Time. Tested using drop jumps on force plates. Higher RSI values indicate better reactive strength and stretch-shortening cycle function.

Reliability

The consistency and reproducibility of a measurement. Expressed as ICC (see Intraclass Correlation Coefficient). Intra-rater reliability = same tester, repeated measurements. Inter-rater reliability = different testers, same subject.

Resolution

The smallest change in force that a device can detect. A dynamometer with 0.1 kg resolution can distinguish between 24.0 kg and 24.1 kg. Higher resolution allows detection of smaller changes over time.

S

Sampling Rate

The number of data points recorded per second, measured in Hertz (Hz). Force plates require high sampling rates (≥1,000 Hz) to accurately capture fast movements like jump landings. Dynamometers typically sample at 100–500 Hz.

Strain Gauge

A sensor that measures deformation (strain) in a material under load. Strain gauges are the most common force sensing technology in clinical dynamometers and force plates due to their accuracy, linearity, and long-term stability.

T

Torque

A rotational force that causes or tends to cause rotation around an axis. Torque = Force × Distance (from the axis of rotation). Isokinetic dynamometers report torque (Nm), while handheld dynamometers report force (N). To estimate joint torque from HHD force, multiply by the lever arm length.

V

Validity

The degree to which a test measures what it claims to measure. Concurrent validity is established by comparing a new device against an accepted reference standard. A dynamometer is "valid" if its measurements closely match those of a calibrated reference device or isokinetic dynamometer.

Vertical Ground Reaction Force (vGRF)

The component of ground reaction force acting perpendicular to the force plate surface (straight up). vGRF is the primary variable in most force plate assessments including jump testing, balance, and gait analysis.