Measuring performance in spinal cord injury rehabilitation: New possibilities?

Introduction

Smartwatches quantify performance

Many of us today walk around or exercise with products that sense many aspects of our daily activity. Smartphones and sensor developments mean that we can be more in tune with what is happening to our health and fitness. My Suunto watch monitors my heart rate and heart rate variability, blood oxygenation, skin temperature and activities in many ways. It can nudge me to exercise more, tell me how I recovered from exercise and alert me to how well or how badly I am sleeping.. I personally do like having these insights, but of course, some people will hate the idea of this.

In our work at Anatomical Concepts, we deal with many people who wish to recover from a neurological insult. It might be a stroke or a spinal cord injury and they seek rehabilitation to get their lives back on track. Often therapy exercises are used, augmented with different types of technology such as FES bikes. We see some clients who work at their therapy as if they are training for the Olympics. They look for more information and any insights that help them strive for recovery. We also see some clients that focus more on just accepting how they are now and make little extra effort to recover.

It’s just human nature. What we know from the domain of elite sport, is that those athletes who strive for more knowledge about how their bodies work will tend to outperform those that dont take this extra step. Individual psychology leads some of us to embrace this kind of self knowledge and others to avoid it. I contend that our clients who seek to learn more will achieve more in their rehabilitation.

The smartwatch and related sensor technologies have opened up some new possibilities. In this article, im going beyond the smartwatch of today and take a look at what additional sensor technologies might assist us with spinal cord injury rehabilitation in the near future. In particular, we take a look at two sensor areas that have grown in popularity with elite athletes - Muscle oxygen sensing and Lactate sensing. If you want to know why, then read on.

Measure what matters

My original training as a Control Systems Engineer taught me that if you want to control something well, you must measure what matters. Measuring a system's output performance lets us know if we are "on track" or need to take corrective action. Just hold onto these thoughts, and we will return to them momentarily.

When someone has survived a neurological insult such as a spinal cord injury, the initial imperative might be survival and then medical stability, but then rehabilitation can take place. We can then ideally (resources permitting) strive to recover as much function as possible and compensate with assistive technology only for any function that seems lost.

For this reason, I see rehabilitation as more like the physical training an athlete might undergo to achieve a performance goal. When this makes sense, I like to draw ideas and inspiration from the general domain of enhancing human performance. Of course, goal setting in rehabilitation is highly individual and complex, and there are differences in how an injured body responds to exercise. But we can say the same about training an elite athlete or a special forces operative. The reason that looking at training in these areas might be productive is down to the fact that there is much more resources committed to these areas than to rehabilitation.

“If you want to control something well, measure what matters”

Let's imagine a perfect world in which we consider a human subject along with the intervention we make (the input), which produces a change in the person - the output of the system.

A control engineer discusses open-loop and closed-loop systems. In an open-loop situation, no information about the state of the output is available at the input. In a closed-loop system, there is feedback. A "controller" considers the state of the output, and changes the input based on the difference between the desired output and the present output.

Systems can be controlled automatically or manually. When a therapist is training a client, they are "closing" the loop manually. They intervene (the input), and over time, they monitor some meaningful measure of performance (the output). Based on their observations, they might refine their intervention until the desired performance is achieved. The "loop" is closed by the therapist measuring something and taking action based on that. There is almost certainly a time lag and other complications that engineers describe as non-linear or fuzzy behaviours in control terms.

The above is a long-winded way of saying that performance measurement matters to our ability to control something.

In rehabilitation, many types of performance output measurements are routinely used. These might include simple or validated questionnaires or instrumented approaches. For many years, we have had simple and complex gait analysis systems, force platforms, pressure measurement products, power output measurements from cycling or arm cranking, and much more. These are useful in the right context but indicate what the body is producing as an output rather than measuring the physiological state.

There is a second type of performance measurement that can be of interest, though. That is, measurements that indicate what is happening inside the body.

Both athletics and rehabilitation appreciate the value of monitoring a person's physiological state. With recent developments in sensor and communication technology we see a growing number of devices, such as

• heart rate monitors

• respiration monitors

• pulse oximeters

• EMG sensors

• blood pressure sensors

• continuous blood glucose monitoring

• temperature sensors

• accelerometer-based motion sensors

Many athletes, even at the amateur level, probably use one or more of these devices themselves.

In rehabilitation, the relevance of these measurements should always be considered in relation to the individual case. For example, heart rate monitors may be misleading for many spinal cord-injured persons because the nature of the injury can lead to heart rate fluctuations not related to the effort of training.

Two devices that have been gaining popularity in elite sports are

• Muscle Oxygenation Sensors (Near-Infrared Spectroscopy - NIRS)

• Lactate Sensors

Lactate testing and muscle oxygenation measurements can be valuable tools in the rehabilitation of individuals with spinal cord injuries. However, their suitability depends on several factors, including the specific goals of rehabilitation, the condition of the person, and the practical application of these tests. Here’s an overview of how these methods can be used:

Lactate Testing

Lactate testing measures lactate concentration in the blood, indicating the anaerobic metabolism level. This could be useful in rehabilitation for monitoring exercise intensity and ensuring patients work within safe limits. We know, for example, that muscle fibre types become more fatigable following a spinal cord injury and lactate testing could be a more sensitive measure of the onset of fatigue than looking at the drop off in power produced. This is because the lactate level change would occur before the power drop-off.

In an earlier article, I wrote about the energy system that powers our muscles.

Suitability for SCI Rehabilitation:

• Monitoring Exercise Intensity: Lactate testing can help ensure the exercise intensity is appropriate for the patient’s current fitness level and recovery status. This is crucial in avoiding overexertion, which can be detrimental to recovery.

• Progress Tracking: Regular lactate testing could help track improvements in aerobic and anaerobic conditioning over time, which is beneficial for tailoring rehabilitation programs.

• Adaptations: Individuals with SCI may have different responses to exercise due to altered muscle physiology and reduced muscle mass. Lactate testing might help customise rehabilitation programs to suit these individual differences.

Blood Oxygenation (Near-Infrared Spectroscopy - NIRS)

Blood oxygenation in muscles can be measured using techniques that rely on NIRS, which non-invasively monitors tissue oxygenation levels. This approach does not rely on taking blood samples and is gaining in favour. NIRS uses near-infrared light to measure the oxygenation levels in muscle tissue. The technology relies on two main principles:

Tissue Transparency: Human tissues are relatively transparent to near-infrared light, allowing it to penetrate deep into the muscle.

Oxygenation-Dependent Absorption: Hemoglobin and myoglobin absorb near-infrared light differently depending on whether they are oxygenated or deoxygenated

Such a device has an LED light source and a detector plus signal processing and a power source. Typically, LEDs that emit light in the near-infrared spectrum (650-950 nm) are used to put light energy into the tissue. Common wavelengths include 760 nm (sensitive to deoxygenated hemoglobin) and 850 nm (sensitive to oxygenated hemoglobin). Then photodetectors capture the reflected light from the muscle tissue.

Suitability for SCI Rehabilitation:

• Assessing Muscle Oxygenation: This can provide insights into how well muscles are being oxygenated during exercise, which is important for understanding the patient’s aerobic capacity and the efficiency of their cardiovascular system.

• Preventing Overexertion: Monitoring muscle oxygenation can help prevent overexertion by ensuring that muscles receive adequate oxygen during rehabilitation exercises.

• Real-Time Feedback: NIRS can provide real-time feedback during exercise, allowing therapists to adjust the intensity or duration of exercise immediately.

Considerations for use in SCI rehabilitation

Safety: Lactate testing and NIRS are generally safe and non-invasive, making them suitable for regular use in a clinical setting.

Customisation: Rehabilitation programs must be highly individualized for SCI patients. Both methods can help customise and adjust exercise protocols based on the patient’s needs and progress.

Equipment and Training: Access to the appropriate equipment and training for both healthcare providers and patients is essential for effectively using these technologies. These technologies are relatively new but are likely to become more readily available.

Integration with Other Therapies: These tests should ideally be integrated with other rehabilitation therapies, such as physical therapy, occupational therapy, and neuromuscular electrical stimulation, for a comprehensive approach.

Related Studies

There have been investigations of muscle oxygen and lactate in studies with spinal cord injured persons. Here are some findings:

1. Muscle Oxygen Studies in SCI Individuals:

NIRS has been used to measure muscle oxygenation in SCI subjects:

- An observational study compared tissue oxygen saturation (StO2) between active and inactive muscles during incremental arm-cranking exercise (ACE) in complete SCI, incomplete SCI, and able-bodied individuals [7].

- In active muscles (biceps brachii), no differences in StO2 were observed among the groups.

- In inactive muscles (vastus lateralis), StO2 remained unchanged in complete SCI subjects, while it decreased in incomplete SCI and able-bodied individuals, suggesting differences in sympathetic vasoconstriction[7].

2. Lactate Studies in SCI Individuals:

- Research has examined blood lactate elimination in individuals with paraplegia compared to able-bodied subjects[1].

- SCI leads to adaptations like muscle mass loss and reduced oxidative capacity in paralyzed limbs, which may affect lactate metabolism.

This study suggested that Individuals with paraplegia seem to have no disadvantages in lactate elimination after exhaustive arm exercise compared with able-bodied individuals.

3. Non-invasive Sensors for Athletic Coaching:

Several non-invasive sensors have been developed for use in sports performance monitoring:

• NIRS systems, such as Humon Hex, Moxy Monitor, and PortaMon, for measuring muscle oxygenation [4].

• Wearable devices for continuous sweat lactate monitoring, which offer a non-invasive alternative to blood lactate tests[5].

While these studies and technologies demonstrate progress in non-invasive monitoring of muscle oxygen and lactate in both SCI and athletic contexts, it's important to note that the relationship between sweat lactate and blood lactate levels remains complex. Factors such as environmental conditions and the lag between exercise initiation and sweating can affect measurements[5].

In conclusion, non-invasive sensors for monitoring muscle oxygen and lactate are being used in both SCI research and athletic coaching. However, more research is needed to fully validate these technologies across various sports and clinical applications, particularly for individuals with spinal cord injuries. These developments could provide a useful tool for rehabilitation as they allow us to monitor aspects of the physiological state and loosen our dependence on physical output measures which only follow physiological changes after a time delay.

Citations:

[1] Leicht C, Perret C. Comparison of blood lactate elimination in individuals with paraplegia and able-bodied individuals during active recovery from exhaustive exercise. J Spinal Cord Med. 2008;31(1):60-4. doi: 10.1080/10790268.2008.11753982. PMID: 18533413; PMCID: PMC2435020. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2435020/

[4] Buyer’s Guide to Body Oxygenation Measurement Systems for Sports https://simplifaster.com/articles/buyers-guide-body-oxygenation-measurement-systems/

[5] Yang G, Hong J, Park SB. Wearable device for continuous sweat lactate monitoring in sports: a narrative review. Front Physiol. 2024 Apr 4;15:1376801. doi: 10.3389/fphys.2024.1376801. PMID: 38638276; PMCID: PMC11025537. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC11025537/

[6] Non invasive measurement of muscle oxygenation in elite athletes in the field. Research Grant. https://gow.epsrc.ukri.org/NGBOViewGrant.aspx?GrantRef=EP%2FF006551%2F1

[7] Horiuchi M. Effects of arm cranking exercise on muscle oxygenation between active and inactive muscles in people with spinal cord injury. J Spinal Cord Med. 2021 Nov;44(6):931-939. doi: 10.1080/10790268.2020.1754649. Epub 2020 May 7. PMID: 32379545; PMCID: PMC8725684. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC8725684/

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