Ground Reaction Forces (GRF) Guide: Running & Jumping Analysis for Coaches & Practitioners

Every stride and every jump creates force your body has to absorb and reuse.
Ground Reaction Force (GRF) is the simplest way to describe that interaction: the ground pushes back on the body during contact. Understanding GRF helps
practitioners connect what they can see on video (movement strategy) to what the athlete experiences (external loading). The goal isn’t to “avoid force”—
performance requires force—but to manage how it is produced and absorbed, then track changes over time with consistent testing.

1) What is a Ground Reaction Force?

Ground reaction force (GRF) is the force the ground applies back to the body
when the foot (or any part of the body) contacts the surface. It exists because of Newton’s third law: the body pushes on the ground, and the ground pushes back with an equal and opposite force.

GRF acts through a point: the Center of Pressure (COP)

Because GRF is a contact force, it acts through a point on the foot called the center of pressure (COP). As you move, COP typically shifts from heel toward the forefoot and can move medial-to-lateral depending on how you load and stabilize.

The three components of GRF

Vertical GRF (vGRF): supports body weight and reflects impact/loading;
Anterior–posterior (AP) GRF: braking (negative) and propulsion (positive);
Medial–lateral (ML) GRF: side-to-side loading and frontal-plane control.

Pic 1. GRF components across running and jumping phases.

Vertical GRF reflects loading, while the AP component shifts between braking early in stance and propulsion late in stance. In jumping tasks, the same directions appear across takeoff and landing, often at higher forces over shorter time windows.

2) Why GRF matters in running and jumping

GRF links what we can observe (movement mechanics) to what the body experiences (external loading). In coaching and rehab, GRF concepts help explain why two athletes can look similar yet tolerate very different workloads, or why a small technique change can reduce stress without reducing output.

Key GRF-related concepts to know

Peak force: the maximum force during contact (often normalized to body weight);
Impulse: force applied over time (area under the force–time curve);
Loading rate: how quickly force rises after contact (strongly related to tissue stress);
Contact time: how long the foot is on the ground; interacts with stiffness and reactivity;
Symmetry: left–right comparison in repeated tasks (running strides, jump landings).

Pic 2. Metric → Meaning → Action: a practical lens for translating force concepts into coachable priorities and repeatable tracking.

3) A repeatable workflow: from capture to decision

In applied settings, the biggest value isn’t a one-time interpretation—it’s repeatable monitoring. With consistent setup, you can compare the same athlete over time (baseline vs fatigue, technique change, return-to-run progression) and make decisions based on trends rather than isolated moments.

Pic 3. A simple monitoring loop. Standardize capture, identify priority metrics, apply a targeted intervention, re-test, then track trends over time.

4) GRF in running: what to look for and why

During each running step, stance includes a braking portion and a propulsion portion. How an athlete manages these forces influences efficiency, fatigue, and injury risk.

4.1 Braking vs propulsion (AP GRF)

AP GRF describes forward-backward forces: early stance typically shows braking, and late stance shows propulsion. Excessive braking can increase energy cost and load certain tissues. Better control can improve economy.

Practical note: braking and propulsive behavior changes with speed—so when comparing sessions, keep speed and conditions consistent;
Common driver of high braking: overstriding (foot landing too far in front of the center of mass);
Coaching direction: aim for a step that lands closer to the body and transitions smoothly into push-off.

Trackable proxies to monitor:

cadence;
foot placement relative to pelvis;
trunk lean;
timing of toe-off;

4.2 Vertical loading and loading rate (vGRF)

vGRF reflects how the body is loaded during stance. Peak force can be useful, but loading rate often explains why landing or footstrike feels “harsh” or impactsensitive.

A “stiff” landing strategy can raise loading rate (force rises quickly).
A “softer” strategy may reduce loading rate but can increase contact time and demand on calves/hip extensors, depending on the athlete.
Surface, speed, shoes, fatigue, and stride adjustments all influence vGRF characteristics.

4.3 Practical technique levers that influence GRF

Small changes can shift how forces are produced and absorbed. In practice, the goal is usually better timing and distribution of force, not “less force” across the board.

Cadence: modest increases can reduce overstriding and braking for some runners;
Trunk position: slight forward lean from the ankles (not the waist) can help align forces;
Pelvis control: excessive drop/rotation can shift loading and increase ML demands;
Arm swing: influences trunk stability and timing; helps manage lateral motion.

4.4 What AiKYNETIX Running focuses on

AiKYNETIX Running is designed to make GRF-relevant insights actionable without requiring force plates. The workflow emphasizes repeatable capture and clear outputs tied to external loading and efficiency.

Phase awareness (contact, mid-stance, toe-off) to contextualize loading;
Form metrics linked to external loading (contact time, stride timing, symmetry, alignment indicators);
Trend monitoring (baseline vs fatigue, return-to-run progression, technique changes);
Report sharing (coach–clinician–athlete alignment).

5) GRF in jumping and landing

Jumping tasks produce high forces over short time windows. The goal is not to avoid force—performance requires force—but to manage how force is produced and absorbed.

Video 1. A quick explainer on the three GRF directions (vertical, braking, propulsive) and what to look for in running and jumping when assessing performance and landing strategy.

5.1 Takeoff (propulsive phase)

Peak force and impulse contribute to jump outcome (e.g., jump height);
Coordination matters: hip–knee–ankle sequencing and trunk position influence force direction;
In repeated jumps, maintaining output with stable mechanics can indicate good reactive capacity.

5.2 Landing (absorption phase)

Landing is often the biggest concern in return-to-sport and high-volume training.
Two critical targets are:
1. Stable alignment, and
2. Controlled loading rate through appropriate joint flexion and timing.

Watch for “stiff” landings

limited hip/knee flexion
rapid force rise

Watch for collapse patterns

valgus/hip drop
trunk drift
asymmetrical loading

Track improvements

more controlled alignment
smoother deceleration
improved symmetry

5.3 What AiKYNETIX Jump focuses on

AiKYNETIX Jump supports structured assessment of jump performance and landing mechanics with repeatable tests.

Test standardization (consistent setup and instruction);
Performance + mechanics (connect output with how force is produced/absorbed);
Asymmetry checks (left vs right strategies in landing and takeoff phases where measurable);
Progress dashboards (objective evidence for readiness and development).

6) How to capture video for GRF-relevant analysis

Reliable GRF-related insights require consistent capture. Use this checklist to reduce noise between sessions.

Pic 4. Repeatable capture setup for GRF-relevant video analysis.

For running, a consistent side view helps contextualize sagittal mechanics. For jump landings, a front view helps interpret alignment and symmetry. Across both, keep the full body in frame, use good lighting, and repeat the setup each testing day.

6.1 Running capture checklist

Use a consistent view and distance (commonly side view);
Keep the full body in frame throughout stance and swing;
Use good lighting and minimize motion blur;
Record multiple strides at steady pace (avoid a single “perfect” step);
Repeat setup each testing day (same treadmill speed or same outdoor segment).

6.2 Jump capture checklist

Use a consistent view (front/back for alignment; side view for depth and timing);
Ensure the landing is fully visible (no cropping at the feet);
Standardize the task (countermovement jump, drop jump, or agreed protocol);
Capture enough trials to see a stable pattern (not just best attempt).

7) Interpreting results: practical examples

Example A: Running “impact sensitivity”

If an athlete reports shin or knee discomfort and the video shows short knee flexion at contact, a higher loading-rate strategy may be present. Interventions can include technique adjustments, graded exposure, and strength work—then retest to confirm changes in mechanics and symptom response.

Example B: Jump landing asymmetry in return-to-play

After lower-limb injury, an athlete may shift load away from the involved side. Use repeated jump trials to identify consistent asymmetry patterns, then track whether symmetry improves with rehab progress and confidence.

8) Common misconceptions (quick clarity)

Peak force isn’t the whole story. In practice, loading rate, impulse, contact time, and symmetry often explain what the athlete is actually tolerating.
Less force isn’t always better. Performance requires force. The goal is often better timing and distribution of force, not simply reducing it.
One clip doesn’t tell the story. The real value is repeatable monitoring under consistent conditions, not a single rep.

9) Quick field checklist (if you only do five things)

Standardize setup (same view, same distance, consistent conditions);
Record enough reps/strides to see a stable pattern;
Track a small set of priorities (e.g., contact time + symmetry + one alignment indicator);
Apply one targeted intervention at a time;
Re-test and compare trends, not isolated values.

10) Key takeaways

GRF is the external signature of how the body interacts with the ground during running and jumping;
In practice, loading rate, impulse, contact time, and symmetry often matter more than peak force alone;
The most valuable use case is repeatable monitoring: compare the same athlete over time in the same test conditions;
AiKYNETIX Running and Jump tools help translate video-based mechanics into GRF-relevant insights and clear reports.

Note: AiKYNETIX outputs are designed as GRF-relevant, repeatable proxies from standardized video capture to support decision-making and trend tracking; they do not replace lab-grade force-plate measurement when that level of instrumentation is required.

Ground Reaction Forces (GRF) for Running and Jumping