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  • Writer's pictureJo Clubb

Objective Muscle Fatigue Assessment: A Missing Puzzle Piece in Athlete Monitoring

Updated: May 21

In athlete monitoring, the pursuit of objective measures for assessing readiness has long been a challenge. This post explores low frequency fatigue and Myocene - a neurostimulation technology that offers objective muscle fatigue assessment.

Assessing athlete fatigue often resembles assembling a jigsaw puzzle with missing pieces. While we have subjective measures of wellness and fatigue, along with physical performance tests and biochemical markers, a direct and objective measure of muscular fatigue – that is practical for the applied setting – has been an elusive puzzle piece.

This is why I’m intrigued by Myocene. This technology leverages neurostimulation to quantify a fatigue index based on submaximal muscular contractions, circumventing issues of voluntary effort and motivation. This article delves into the science behind Myocene and its potential application in athlete monitoring programmes.

The Challenge with Neuromuscular Fatigue Monitoring

To gauge readiness in athletes, we frequently target measures of neuromuscular fatigue (NMF): a reduction in maximal voluntary force induced by exercise. It is classified into central or peripheral based on its origin, but it is the larger magnitude and slower recovery of peripheral fatigue that primarily explains the recovery of NMF (Thomas et al., 2017).

Peripheral fatigue itself has two components: short-lasting fatigue related to metabolic factors and long-lasting fatigue in which the force-frequency relationship is impacted, predominantly by impaired calcium release. It is therefore, this peripheral NMF and specifically the long-term component that we try to capture through physical assessments.

A flowchart shows muscle fatigue at the top which breaks down into Central fatigue and Peripheral fatigue. Peripheral fatigue then breaks down further into short-term fatigue and long-term fatigue.
Components of Muscle Fatigue

While physical performance tests (most commonly, jump testing) offer insights into physical capacity, performance is influenced by effort and motivation. A reduced effort can of course still be telling of an athlete’s fatigue status, but the challenge remains in distinguishing between motivation and true physical limitations. Whereas, peripheral nerve or muscle electrical stimulation offers a method to capture alterations in muscle contractility, which is representative of peripheral fatigue without involving athlete’s contribution, removing the motivational aspect from the equation.

Low Frequency Fatigue: How Myocene Works

Low Frequency Fatigue (LFF), also known as prolonged low-frequency force depression, represents a persistent form of muscle fatigability characterised by diminished force output at low stimulation frequencies. This phenomenon, attributed to decreased calcium ion release within muscle fibres, has significant implications for muscle function following various fatiguing tasks, and offers a potential avenue for fatigue assessment.

The muscle force-frequency is a sigmoid curve (see below), and when fatigued the curve moves to the right but also importantly, the slope changes as well. The slope at low frequency (approx. 10-50 Hz) is altered to a greater extent than at high frequencies (above 80 Hz). As such, the gold standard for assessing LFF is the ratio of low to high frequency force responses to peripheral nerve electrical stimulation, often specifically at frequencies of 20 Hz and 80 Hz (The P20-to-P80 ratio). For further detail on electrical stimulation for testing NMF, read this review by Millet and colleagues (2011).

On the left a chart shows force against frequency, with two sigmoid curves demonstrating how the curve from a fatigued muscle shifts down and left. On the right the muscle fatigue index equation of force at low frequency over force at high frequency is displayed, and showing how the rested muscle is greater than the fatigued muscle
Measuring Muscle Fatigue Index in Rested vs Fatigued Muscles

Based on this understanding, Myocene calculates a novel metric, Powerdex, which represents the low to high frequency force response ratio at different stimulation intensities. One study has evidenced the validity of the tool and the Powerdex value to assess LFF after strenuous exercise, in this case a series of drop jumps (Ridard et al., 2022). So let’s next explore how exactly this data is captured.

Low Frequency Fatigue Data Capture: The Myocene Protocol

Historically, the assessment of LFF has been limited to laboratory studies, but Myocene technology brings this methodology to the training facility. In brief, the athlete sits on a chair or table with their leg in the brace-like, custom-developed Myo-sensor, three electrodes are set up on the quad, and the pre-programmed electrical stimuli programme (called stimulation trains) runs, capturing 48 measures and totalling 2 minutes. This is then repeated on the other leg. This is demonstrated in the video below.

There is a lot to explore with this technology but from what I’ve learned about it so far, there are two key points to emphasise. Firstly, this assessment does not require voluntary contraction or movement and therefore, does not require any effort from the athlete. This is appealing as it discounts effort or motivation as a factor in the results. In addition, it could remove the need for additional loading, such as through jump testing, which may be particularly useful following intense competition and/or periods of fixture congestion.

Secondly, let’s address why the quadriceps are used for data capture. The quadriceps femoris is a key muscle for sporting performance, given its contribution to key movement demands such as jumping, kicking, and changing direction. As a primary locomotor muscle, it has also been frequently used in studies of neurostimulation (Martin et al., 2004).

In many sports, particularly football, we think a lot about the hamstrings and so might ask why this is not the location of the data capture. Yet, we shouldn’t just think of this as an isolated ‘quads test’. This approach is measuring a loss of performance caused by fatigue, a recovery assessment, which can then be used as part of the overall monitoring programme to inform decisions relating to athlete management.

From Theory to Practice

Myocene is a new, innovative technology in elite sports. With that comes limitations, caveats, and certainly the need for further understanding. Now the focus is on how this technology can be employed in the applied setting with a number of early adopters, including OGC Nice in Ligue 1, utilising this technology in professional football.

Of course, it is important to note that it is limited to one muscle (quadriceps femoris) and one contraction type (isometric). Although isometric testing has been described as the gold standard and is clearly of great interest in the applied setting given its safety and practicality, it may not be all encompassing.

Fatigue is a complex entity. Like so many things in the sports world, we try to reduce and simplify it to help quantify and manage it. In this case, Myocene is capturing a single aspect of fatigue, but offering a unique and objective assessment of it.

With those limitations in mind, there are plenty of potential avenues for applications with this technology. Clearly, the objective quantification of fatigue is the headline, therefore potentially improving recovery and training management in the days following a matchday. Yet, other avenues include asymmetry analysis, recovery time course, return to play protocols, and unbalanced fatigue, some of which we will explore in future posts.

Final Thoughts

By employing neurostimulation techniques, Myocene bypasses the limitations of voluntary muscle contractions and physical performance tests, providing a more accurate depiction of muscular fatigue. It provides a quick, non-invasive, practical solution that allows for on-field assessments and immediate feedback on an athlete's muscular fatigue status.

Like any innovation, there is much research to be done. But the unique nature of this device is appealing to early adopters trying to add valuable tools to their athlete monitoring programme. This technology might just provide us with a missing puzzle piece for athlete monitoring.

Frequently Asked Questions (FAQs)

What is Myocene and how does it work?

Myocene is a neurostimulation technology that quantifies muscle fatigue by measuring the force response ratio at different stimulation frequencies. It calculates a metric called Powerdex, which assesses low-frequency fatigue without requiring voluntary muscle contractions.

What is Low Frequency Fatigue?

Low Frequency Fatigue (LFF), also known as prolonged low-frequency force depression, represents a persistent form of muscle fatigability characterised by diminished force output at low stimulation frequencies. Measuring LFF provides insights into neuromuscular fatigue and recovery.

How does Myocene differ from traditional fatigue assessment methods?

Traditional methods often rely on physical performance tests, such as jumps or sprints, influenced by effort and motivation. Myocene uses electrical stimulation to objectively measure muscle contractility, eliminating the influence of voluntary effort on readiness measures.

This article is supported by Myocene. For more information about their technology, visit their website.


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