This blog post delves into the usefulness of impulse in the Nordic hamstring exercise, exploring its definition and how it can be used in sports science.
In my athlete testing series on YouTube in collaboration with VALD, we've previously discussed the importance of NordBord force metrics and asymmetry. Today, we focus on the impulse metric, stemming from a question raised by Christi Edson, a sports scientist and strength and conditioning coach at Orlando Pride. While impulse may be of interest in jump testing, Christi asked about its utility during the Nordic hamstring exercise when measured via a NordBord.
What is Impulse?
Impulse is defined as the product of force and time. This concept was previously discussed in the context of the jump height calculation that uses the impulse momentum method. Essentially, impulse measures the area under the force-time curve. Whether we’re examining a jump takeoff or the NordBord exercise, impulse in Newton-seconds is determined by summing all the force created over the given time period.
Measuring Torque and Impulse with the NordBord
In the video below, I use NordBord data exported from the VALD Hub to illustrate impulse. The example data includes multiple athletes performing three repetitions of the Nordic hamstring exercise on both the left and right sides. The NordBord metrics give us peak force, left vs right asymmetry, and impulse measurements for each trial.
Here are some clarifications to assist with understanding:
Torque: This is the rotational force generated around the knee, calculated by force times length.
Maximum Impulse: The highest impulse recorded from the three repetitions on each side, expressed in Newton seconds.
Comparing Impulse Between Athletes
Given that impulse is a function of force and time, when comparing impulse between athletes, the difference could be due to the variation in the time each athlete takes to perform the exercise. In the example provided in the video above (from ~2:44 onwards), two athletes have similar force measurements but significantly different impulse outcomes due to the time taken to complete their repetitions.
Athlete 1 performed each rep approximately over 10-11 seconds, leading to a much higher impulse compared to Athlete 2, who completed their reps in about 4 seconds. This clearly demonstrates that varying time under tension can result in significant discrepancies in impulse values.
In practical terms, comparing impulse between athletes might not be particularly useful unless the time for the exercise is standardised. As impulse is a function of both force and time, large differences can occur purely due to time variations. Therefore, looking into the force traces and analysing the duration of each repetition is crucial for accurate comparisons.
Comparing Impulse Within an Athlete
Impulse can be more insightful when comparing within an athlete, particularly when analysing left vs. right side performance. For example, discrepancies in force-time curves can explain why one side generates more impulse than the other.
In one case study, Athlete 3 (see below) showed a significant force asymmetry but relatively lesser impulse asymmetry because their weaker side engaged earlier during the rep, producing higher force (and therefore, impulse) initially.
Conversely, Athlete 4 exhibited similar maximum force on both sides, yet showed significant impulse asymmetry due to one side generating more consistent force throughout the exercise, leading to a higher area under the curve. Watch the video above for more detail of each of these cases.
Summary and Practical Advice
Overall, while between-athlete comparisons of impulse may not be highly practical without standardising exercise time (which is then just essentially comparing force anyway), within-athlete comparisons (such as left vs. right side) can offer valuable insights. It is essential to look beyond the raw metrics and understand the context and the force-time curves to appreciate how the impulse is generated.
For further understanding of the NordBord and the Nordic hamstring exercise, view other videos in our athlete testing series. Special thanks to VALD Performance for sponsoring this series.
Frequently Asked Questions (FAQs)
What is impulse?
Impulse is a measure of force over time, representing the area under the force-time curve, measured in Newton seconds.
Why might comparing Nordic hamstring impulse between athletes be problematic?
Differences in impulse between athletes could stem from variations in the time taken to perform the exercise, making such comparisons potentially misleading.
Is Nordic hamstring impulse useful for within-athlete comparisons?
Yes, impulse can be valuable for within-athlete comparisons, particularly for analysing left vs. right side performance during the Nordic hamstring exercise.
This article is support by VALD Performance. For more information, about their technology, visit their website.
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