KIN 147: Biomechanical Assessment EXAM 2

Vertical Ground Reaction Force (vGRF)

The upward pushing force the ground exerts back on your body when you step on it. It is the most commonly studied force in movement analysis.

Impact Peak

The first quick spike in upward ground force that happens when your foot collides with the floor at initial contact.

Active Peak

The second, broader ground force peak created when your body actively pushes against the floor to propel forward or jump.

Vertical Loading Rate Calculation

A measure of how fast impact force builds up on landing, calculated using the slope of the middle $60\%$ of the first impact peak.

Vertical Average Loading Rate (VALR)

The average speed of impact force absorption upon hitting the ground, which is strongly linked to running-related overuse injuries.

Forefoot Strike Impact Characteristics

Landing on the ball of the foot rather than the heel, which smooths out the landing and hides the distinct initial impact spike inside the main active peak.

Forefoot Strike Injury Trade-Offs

It reduces stress and injury risk at the knee, but shifts more mechanical workload and injury risk onto the calf, Achilles tendon, and foot.

Electromyography (EMG)

A technique used to detect and record the tiny electrical signals generated when muscle fibers contract, measured in microvolts ($\mu V$).

Surface EMG (sEMG) vs. Fine-Wire EMG

Surface EMG sticks sensors onto the skin to read large surface muscles during motion; fine-wire EMG inserts tiny needles directly into deep or small muscles.

sEMG Electrode Placement Guidelines

Stick two recording electrodes about $2\,cm$ apart along the muscle belly parallel to the fibers, and place one reference ground electrode on a bony area.

Standard EMG Bandpass Filter Range

A filter setting from $20\,Hz$ to $500\,Hz$ that strips away slow movement artifacts below $20\,Hz$ and high-frequency electrical noise above $500\,Hz$.

Root Mean Square (RMS)

A mathematical smoothing method that cleans up raw, jagged electrical spikes into a readable line representing muscle activity level.

Maximal Voluntary Isometric Contraction (MVIC) Normalization

Dividing the electrical signal during an activity by the signal from a participant's maximum squeeze, allowing fair comparisons across different people and days.

Limitations of MVC Normalization

Participants may not squeeze with true maximum effort, joint testing angles might not match dynamic movement, and lengthening contractions can exceed $100\%$.

Joint Moment

The twisting or rotational torque acting around a joint, calculated as $\text{Torque} = \text{Force} \times \text{Moment Arm}$.

Inverse Dynamics

A bottom-up calculation method using foot contact forces, movement tracking, and body segment weights to figure out internal joint forces and torques.

Moment of Inertia ($I$)

A measure of how much an object or body part resists being rotated, expressed in the formula $\tau = I\alpha$.

Internal vs. External Joint Moments

External moments are turning forces exerted by outside loads like gravity and ground contact; internal moments are the balancing muscle and ligament forces resisting them.

Force Plate Tri-Axial Measurement Components

The three directions of force measured by a force plate: up-down vertical ($F_z$), forward-backward ($F_y$), and side-to-side ($F_x$).

Countermovement Vertical Jump (CMVJ) GRF Phases

The stages seen on a jump force graph: resting body weight, squatting and pushing upward, flight phase ($GRF = 0\,N$), and the landing impact peak.

Minimalist Footwear Effect on Loading Rate

Running in minimal shoes usually increases the speed of impact force compared to cushioned shoes, unless the runner switches to landing on their forefoot.

sEMG Skin Preparation Protocol

The routine of shaving hair, gently abrading skin with fine sandpaper, cleaning with alcohol, and applying conductive gel to lower electrical resistance.

Factors Influencing sEMG Signal Quality

Key factors that alter signal quality, including body fat thickness, muscle depth, skin cleanliness, electrical interference from neighboring muscles, and sensor movement.

sEMG Activation Timing Determination Methods

Identifying when a muscle switches on or off using either mathematical threshold formulas (standard deviations or percentage cutoffs) or researcher visual inspection.

Distal-to-Proximal Sequence in Inverse Dynamics

Calculating joint forces from the ground up: starting at the foot to solve the ankle, moving to the shin for the knee, and ending at the thigh for the hip.

Standard Units and Normalization for Joint Moments

Quantified in Newton-meters ($N\,m$) and commonly adjusted for body size as Newton-meters per kilogram ($N\,m/kg$).

Biomechanical Sources of Internal Joint Moments

The internal turning torque produced together by pulling muscles, stretching tendons and ligaments, and joint bones pushing against each other.

vGRF Curve Differences: Walking vs. Running

Walking produces two equal peaks with a dip in the middle, whereas running produces a sharp initial impact spike followed by one much larger active propulsion peak.