How to Calculate Rate of Force Development

Rate of Force Development Calculator

Calculate explosive strength based on force output over time.

Usually 0 at the start of a movement.

The force measured at the end of the time interval.

Common intervals: 50ms, 100ms, 200ms.

Result

0 N/s

function calculateRFD() { var iForce = document.getElementById('initialForce').value; var fForce = document.getElementById('finalForce').value; var timeMs = document.getElementById('timeMs').value; var resultDiv = document.getElementById('rfdResult'); var errorDiv = document.getElementById('rfdError'); var valueSpan = document.getElementById('rfdValue'); var explSpan = document.getElementById('rfdExplanation'); // Reset display resultDiv.style.display = 'none'; errorDiv.style.display = 'none'; // Parse inputs var force1 = parseFloat(iForce); var force2 = parseFloat(fForce); var time = parseFloat(timeMs); // Validation if (isNaN(force1) || isNaN(force2) || isNaN(time)) { errorDiv.innerText = "Please enter valid numbers for all fields."; errorDiv.style.display = 'block'; return; } if (time <= 0) { errorDiv.innerText = "Time interval must be greater than zero."; errorDiv.style.display = 'block'; return; } // Calculation: RFD = Delta Force / Delta Time (in seconds) var deltaForce = force2 – force1; var timeSeconds = time / 1000; // Convert ms to seconds var rfd = deltaForce / timeSeconds; // Update UI valueSpan.innerText = rfd.toLocaleString(undefined, {maximumFractionDigits: 0}); var interpretation = "This means force was developed at a rate of " + rfd.toLocaleString(undefined, {maximumFractionDigits: 0}) + " Newtons per second."; if (rfd < 0) { interpretation += " A negative value indicates force decreased during this interval (relaxation phase)."; } explSpan.innerText = interpretation; resultDiv.style.display = 'block'; }

How to Calculate Rate of Force Development (RFD)

Rate of Force Development (RFD) is a critical parameter in sports biomechanics and strength training. It measures how fast an athlete can develop force, which is essentially the definition of explosive strength. Unlike maximum strength, which measures the highest amount of force you can produce regardless of time, RFD measures how quickly you can reach high force levels.

This metric is crucial for sprinters, jumpers, weightlifters, and any athlete whose sport involves quick, explosive movements where the time available to apply force is limited (often less than 300ms).

The RFD Formula

Mathematically, Rate of Force Development is the slope of the force-time curve. It is calculated as the change in force divided by the change in time.

RFD = ΔForce / ΔTime

Where:

• ΔForce = Final Force (N) – Initial Force (N)
• ΔTime = Time Interval (seconds)

Example Calculation

Let's consider an athlete performing an Isometric Mid-Thigh Pull (IMTP). Using a force plate, we want to calculate their RFD during the first 200 milliseconds (0.2 seconds) of the pull.

• Initial Force (at 0ms): 0 Newtons (excluding body weight for this example)
• Final Force (at 200ms): 3,000 Newtons
• Time Interval: 200ms = 0.2 seconds

Calculation:

RFD = (3000 – 0) / 0.2
RFD = 3000 / 0.2
RFD = 15,000 N/s

Why Measure RFD at Different Intervals?

In sports science, RFD is often analyzed at specific time bands ("epochs") to understand different physiological qualities:

• Early RFD (0-50ms): Highly dependent on intrinsic muscle properties and neural firing rates (starting strength).
• Late RFD (100-200ms): More correlated with maximum strength (MVC) and cross-sectional muscle area.

How to Improve Rate of Force Development

Improving RFD requires training the neuromuscular system to fire faster and more synchronously. Common methods include:

1. Ballistic Training: Movements where the athlete jumps or throws a weight (e.g., jump squats, medicine ball throws).
2. Plyometrics: Exercises involving repeated rapid stretching and contracting of muscles (e.g., depth jumps).
3. Heavy Resistance Training: While slow, increasing maximum strength raises the "ceiling" for force production, which can indirectly improve late-stage RFD.

Units of Measurement

The standard unit for force is Newtons (N) and for time is seconds (s). Therefore, the resulting unit for RFD is Newtons per second (N/s). When inputting time into the calculator above, ensure you use milliseconds (ms), as force plates typically output data in high-frequency ms intervals. The calculator automatically converts this to seconds for the final N/s output.