The isometric mid thigh pull test and force-time curve – made accessible
The Isometric Mid-Thigh Pull (IMTP) test is a popular strength test. It results in a valuable force-time curve, which reveals key performance metrics like: peak force, time to peak force and rate of force development. There are two issues though: the traditional IMTP test requires an expensive force plate, which most coaches do not have access to. Secondly, it measures vertical isometric (static) force, which may have limited transfer to the dynamic skill you’re trying to improve. Let’s tackle these issues one by one.
By Antonio Squillante, Ph.D CSCS*D RSCC*D
Content menu:
- Introduction to the Isometric Mid-Thigh Pull (IMPT)
- The Force-Time Curve: Force
- The Force-Time Curve: Time
- Alternative to force plates for force-time curve testing
– Peak Isometric Force
– Time to Peak Force
– Rate of Force Development - IMTP and force-time curve in the weight room
Introduction to the Isometric Mid-Thigh Pull (IMPT)
The Isometric Mid-Thigh Pull (IMTP) test is a popular assessment in strength and conditioning, sports science, and even physical therapy. Since its adoption in research in the early 90s numerous studies have explored its use in all aspects of sport performance, from athlete testing to return-to-sport protocols.
The IMTP is a standard mulit-joint isometric test. It adds great value as far as monitoring a change in strength over time, and because of its accuracy and precision it does represent a great alternative to starred 1RM testing. The scientific community wholeheartedly endorsed isometric testing when the use of force plates and isokinetic dynamometers were the closest method to a reliable and valid measurement of strength. However, since the early 1990s, the validity of isometric testing has been questioned. Notably, the paper by Wilson and Murphy in 1996 sparked discussions that continue to this day among researchers and scientists. Is isometric testing truly a valuable option when testing athletes, and does it provide actual measures of performance? Instead of delving into academic debates – which are notoriously difficult to resolve – let’s focus on some of the most practical limitations that make isometric testing less appealing to practitioners. Fortunately, velocity-based training (VBT) devices provide a practical alternative for measuring force in concentric and eccentric movements without expensive lab equipment.
The Force-Time Curve: Force
A valuable source of information for strength and conditioning practitioners is the so-called force-time curve. It provides insight into the neuromuscular system revealing how the central nervous system and skeletal muscles are adapting to resistance training. In a laboratory setting, using single-axis force plates it’s possible to accurately measure changes in vertical ground reaction force over time. On a simple force-time curve with force on the y-axis and time on the x-axis, peak force can be defined as the set of coordinates (x, y) corresponding to the highest point on the graph. This graph shows the change in force (F) over time (t) – it plots the function F(t) – and provides a visual representation of what is otherwise known as the rate of force development (RFD).
In a laboratory setting, strength is typically measured in terms of maximal voluntary isometric contraction (MVIC). On the field or in the weight room, it is more commonly referred to as peak isometric force. Although the difference between MVIC and peak isometric force is more than just semantic, for all intents and purposes, they can both be considered standard measurements of raw muscular strength. Peak isometric force is certainly a key metric to consider when evaluating an athlete’s overall physical development. Force is measured in Newtons (N), although it is best practice to report values normalized by body mass in N/kg. This allows for fair comparisons between athletes of different body sizes, and it also makes it possible to track progress – or lack thereof – while accounting for any potential changes in body composition.Having normative data for the isometric mid-thigh pull helps set realistic goals and prioritize training focus.
Normative data Isometric Mid-Thigh Pull
One of the most common ways to assess strength in both research and real-world settings is through maximal isometric testing. A simple isometric test offers three important metrics that can inform practitioners about key aspects of program design, including exercise selection, training intensity, and training volume. These metrics are:
- Peak Isometric Force
- Time to Peak Force
- Rate of Force Development
Experts such as Dr. Michael Stone at East Tennessee State University and Dr. Paul Comfort at the University of Salford have played a key role in advancing the understanding and application of this test. Originally developed in the context of Olympic weightlifting, the IMTP is now used globally by practitioners across strength and conditioning, sports performance, and rehabilitation. With over two decades of data collection, standardization of testing protocols, and the publication of normative data, this body of work represents a monumental effort and one that certainly deserves recognition.
The Force-Time Curve: Time
Peak isometric force is ultimately a measure of capacity and it does not necessarily reflect a direct change in performance. In simple terms, an increase in peak isometric force over time does not always correspond to improvements in key performance indicators (KPIs) in sport. Evidence shows that the correlation between peak isometric force and, say, top speed in soccer or pitching velocity in baseball is moderate at best. When it comes to KPIs, time is critical. The ability to assess and evaluate how force develops over time is often far more important than the peak value alone.
Alternative to force plates for force-time curve testing
Anyone who has attempted to collect data using force plates or motion capture systems knows it can be quite overwhelming. The sheer quantity of available data can make it difficult to sift through and identify what is truly relevant.
For example, a simple test like a countermovement jump – the most common test in the world of sport science, and arguably the most used one by practitioners – can generate well over 200 different metrics when using a force plate.
In research, having more data is often beneficial, but for practitioners, access to an overwhelming amount of information can complicate decision-making. That is called paralysis by analysis and it is a common side effect of using technology in sport.
If you want to know the 3 key metrics derived from a force-time curve, tools like linear position transducers, such as the GymAware RS, provide excellent accuracy and precision.
These measurements can be collected in real-time during each training session, making relevant information available promptly to practitioners. This enables timely adjustments to training programs, ensuring continued progress in the weight room.
All that considered, here are three important metrics to consider when analyzing the force-time curve:
Peak Isometric Force
The first and most straightforward metric derived from the Isometric Mid-Thigh Pull is peak force. Peak force is simply defined as the highest force output recorded during any type of isometric test. It can be reported either as peak force (N) or peak torque (Nm), depending on the context. Force is the most common term used when measuring changes in ground reaction force (GRF) during multi-joint movements, such as an isometric squat or an isometric mid-thigh pull. Torque, on the other hand, is calculated as the product of the force being produced and the moment arm, or the perpendicular distance from the joint’s axis of rotation to the line of action of the force. This applies primarily to single-joint movements only, the likes of a leg extension or leg curl machine.
Peak isometric force tends to increase with an increase in muscle mass. There is, in fact, a strong, positive correlation between muscle cross-sectional area (CSA) and peak isometric force, as very eloquently explained – and demonstrated – by Maffiuletti et al. (2016) in a seminal paper published in the European Journal of Applied Physiology.
If you do not have access to an expensive force plate, or if you prefer to measure peak force in concentric and eccentric movements, consider using your VBT device. For instance: both GymAware RS and FLEX allow you to measure peak force during the concentric and eccentric phase of the lift, where force = mass * (acceleration + gravity).
Time To Peak Force
There is an additional piece of information embedded in this curve: time to peak force (TTPF). TTPF measures the time it takes to reach peak force from the onset of muscle contraction. It’s important to note the challenge in defining the exact onset of muscle contraction with the degree of accuracy required to produce valid and reliable data. Most force plates can only detect changes in the weight of the system, relying on an arbitrary threshold above baseline to infer the onset of muscle contraction. Otherwise, surface electromyography (EMG) would be needed to precisely assess the timing of muscle activation.
Nevertheless, among all RFD metrics derived from force plates, TTPF has consistently shown to be the most robust and reliable, with a small coefficient of variation and a high degree of test-retest reliability. For most trained athletes, TTPF falls somewhere between 300 and 500 milliseconds.There is no normative data that can be used to determine whether TTPF falls within an ideal range. The rule of thumb is, therefore, quite simple: faster is better.
While the traditional mid-thigh pull test measures time to peak force under isometric (static) conditions, GymAware RS makes it possible to capture TTPF during explosive lifts and under different loading conditions. That makes this metric far more applicable on a larger scale. GymAware RS estimates TTPF by recording the time required to reach peak force during the concentric phase of any lift.
Rate of Force Development
A third metric of great interest is rate of force development (RFD) which represents how rapidly force is produced. Oftentimes, maximum RFD is the go-to measure as it captures the steepest point on the F(t) curve or the moment at which the rate of force development reaches its peak. This typically occurs within the first 150–200 milliseconds from the onset of muscle contraction.
It is worth noting that isometric testing presents certain limitations regarding the predictive value of the metrics being collected. The very nature of the test introduces some methodological concerns, particularly in relation to its ecological validity and the strength of the correlation between metrics such as peak isometric force and peak RFD with key performance variables like impulse or concentric power during explosive movements.
Nevertheless, RFD has demonstrated strong, positive correlations with a range of field-based performance tests, including vertical jump height, acceleration, and change of direction speed. A recent meta-analysis published by Lum et al. (2024), which included 47 studies examining the relationship between isometric force-time characteristics and various explosive performance tests, from standard jump assessments to short sprints and agility drills, further corroborated these findings.
It will come as no surprise that GymAware RS can measure the rate of force development by obtaining the point at which the force changes the most (Peak RFD) / the steepest part of the force curve between 2 sample points of the concentric phase. This allows you to measure RFD in all your favourite lifts, beyond an isometric mid-thigh pull.
The Isometric Mid-Thigh Pull test remains a valuable tool for assessing force production and neuromuscular performance. While force plates provide the gold standard for generating detailed force-time curves, tools like linear position transducers offer a practical, cost-effective alternative.
IMTP and force-time curve in the weight room
In the second part of this article, we’ll head straight to the weight room to explore how to measure peak isometric force, rate of force development, and time to peak force using the GymAware RS. We’ll discuss exercises and testing protocols to ensure best practices for collecting accurate and reliable data. Finally, we’ll look at how these metrics can guide key training decisions — from exercise selection to broader aspects of program design.
Antonio Squillante
Ph.D CSCS*D RSCC*D
Antonio Squillante is an Assistant Professor of Kinesiology at Point Loma Nazarene University in San Diego. He serves as the Head of Sport Performance and Training for the USA Cycling National Track Sprint Program. Since 2023, Antonio has been a member of the NSCA Board of Directors. In addition to his academic and professional roles, Antonio is a published author and a highly sought-after international speaker and lecturer.
