No Pain, All Gain: How EMG and Other Physiological Signals Aid Injury Avoidance

It can happen anywhere, anytime: on the pickleball court, during a pristine nature hike, or even while stepping off the front porch on the way to work. An ankle turns, a knee buckles, and suddenly you’re on the ground wondering what just happened. While such injuries often seem random, researchers who study human movement know they are rarely without warning.

Kinesiologists, scientists who study the mechanics and physiology of movement, have found that subtle changes in muscle activation, fatigue, coordination, balance, and neuromuscular control often emerge long before a strain, sprain, or overuse injury becomes apparent. To better understand these hidden danger signs, researchers increasingly rely on physiological measurements that reveal how the body’s muscular, nervous, cardiovascular, and biomechanical systems respond to activity. Such insights help identify injury risk factors and guide strategies designed to keep people moving safely and effectively.

Among the many tools used in kinesiology research, electromyography (EMG) is one of the most common because it reveals when and how muscles activate during movement. However, EMG is only one part of the story. Researchers also study force production, joint motion, movement acceleration, cardiovascular activity, respiration, muscle oxygenation, and neural activity to better understand how the body adapts to physical demands. Together, these measures provide a more complete picture of movement and help uncover patterns that observation alone may miss.

The applications of this work extend far beyond sports. Physiological monitoring helps clinicians evaluate rehabilitation progress, guide return-to-activity decisions, support fall-prevention efforts among older adults, and help design safer workplaces and training programs. Increasingly, researchers are shifting from simply treating injuries to identifying and addressing the factors that contribute to them before they occur.

Researchers in India conducted a cross-sectional study comparing elite and recreational university badminton athletes to explore whether physiological and neuromuscular characteristics could serve as injury-risk markers. The investigators collected a range of performance and neuromuscular measurements, including surface EMG recordings, to examine differences between competitive levels and establish preliminary benchmarks for injury-risk stratification. Surface muscle activity was recorded using a BIOPAC two-channel data acquisition system as part of the neuromuscular assessment protocol. The study found that elite athletes exhibited distinct physiological and neuromuscular profiles compared to recreational players, suggesting that objective physiological markers may help identify movement patterns and performance characteristics associated with increased injury vulnerability. These conclusions support the growing use of physiological profiling to identify athletes who may benefit from targeted injury-prevention strategies before injuries occur.

A team of researchers in Tunisia investigated how fatigue of the ankle evertor muscles affects force perception and neuromuscular function in healthy adults. Ankle injuries are among the most common musculoskeletal injuries in both athletic and recreational populations, making ankle stability an important area of study. Using surface EMG to measure muscle activity before and after a fatigue protocol, the researchers examined how localized fatigue affected participants’ ability to accurately reproduce force levels. Researchers used a BIOPAC EMG amplifier and data acquisition system to record electrical activity from muscles involved in ankle stabilization. The researchers found that fatigue impaired force perception and altered neuromuscular responses, suggesting that fatigue may reduce the body’s ability to control ankle movements accurately. The study results add to growing evidence that fatigue-related physiological changes can increase injury risk, particularly during prolonged exercise or competition when neuromuscular control begins to decline.

A multi-institutional team of researchers from the United States examined neuromuscular function in individuals who had undergone anterior cruciate ligament (ACL) reconstruction, comparing them with healthy controls. The researchers wanted to better understand whether the relationships between muscle performance, muscle activation, and nervous system function change after ACL surgery. Participants completed a series of strength and neuromuscular assessments, with quadriceps muscle activity recorded using a BIOPAC data acquisition system with AcqKnowledge software and an EMG amplifier. BIOPAC isolated stimulators were also used during testing to help determine how fully participants could activate their quadriceps muscles. The study found that several relationships observed in healthy knees were no longer present after ACL reconstruction, suggesting that changes in neuromuscular control can persist long after surgery and rehabilitation are complete. These findings highlight how physiological monitoring can uncover hidden deficits that may not be apparent during routine recovery assessments, helping clinicians make more informed return-to-sport decisions and potentially reducing the risk of future injury.

As these examples demonstrate, physiological measurements give researchers powerful tools to identify injury risk before injuries occur, understand how fatigue alters movement control, and evaluate recovery following musculoskeletal injury. By revealing the physiological mechanisms that influence human movement, signals such as EMG and related neuromuscular measures help bridge the gap between laboratory research and practical injury-prevention strategies. As kinesiology research continues to evolve, physiological monitoring will likely play an increasingly important role in supporting safer movement, more effective rehabilitation, and improved long-term physical performance.


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Electrical Stimulators for Physiological Research: Compliance

All stimulators incorporate a fundamental characteristic known as “compliance.” Compliance comes in two flavors, namely voltage compliance and current compliance. Voltage stimulators emulate a voltage source, up to a certain current compliance. Current stimulators emulate a current source,

By Alan Macy, BIOPAC Systems, Inc. 

All stimulators incorporate a fundamental characteristic known as “compliance.” Compliance comes in two flavors, namely voltage compliance and current compliance. Voltage stimulators emulate a voltage source, up to a certain current compliance. Current stimulators emulate a current source, up to a certain voltage compliance. The stimulator compliance limit is important information. Here are examples:

Current Compliance

A voltage stimulator is being used to establish a field stimulation in a tissue bath. The voltage pulse is 100 volts, with a width of 1 ms. The bath establishes a 1000 ohm impedance between stimulation electrodes. Therefore, 100 volts/1000 ohms is 100 ma. In this situation, the voltage stimulator must be able to provide a current compliance up to 100 ma for at least 1 ms. If the stimulator’s current compliance is limited to 50 ma, then the output voltage will be limited to 50 ma * 1000 ohms or 50 volts, even if the stimulator is being instructed to output 100 volts.

Voltage stimulators commonly operate with the help of high voltage DC-DC converters and storage capacitors to provide the needed compliance current. If the storage capacitors can’t store enough charge or are not sufficiently supplied, then voltage stimulators may exhibit a quality known as voltage droop. Voltage droop is the quality when the stimulator output voltage can’t be maintained over a specific time interval, due to lack of sufficient current compliance. If the above stimulator can only be current compliant to 100 ma for 0.5 ms, then the output voltage pulse will be 100 volts for 0.5 ms and then will decay in direct proportion to the current compliance limit.

Voltage Compliance 

A current stimulator is being used for neuromuscular stimulation between two small electrodes. The current pulse is 10 ma, with a width of 1 ms and there is a 10,000 ohm impedance between stimulation electrodes. Therefore, 10 ma * 10,000 ohms is 100 volts. In this situation, the current stimulator must be able to provide a voltage compliance up to 100 volts for at least 1 ms. If the stimulator’s voltage compliance is limited to 50 volts, then the output voltage will be limited to 50 volts / 10,000 ohms or 5 ma, even if the stimulator is being instructed to output 10 ma.

Current stimulators commonly operate with the help of high voltage DC-DC converters and storage capacitors to provide the needed compliance voltage. If the storage capacitors can’t store enough charge or are not sufficiently supplied, then current stimulators may exhibit a quality known as current droop. Current droop is the quality when the stimulator output current can’t be maintained over a specific time interval, due to lack of sufficient voltage compliance. If the above stimulator can only be voltage compliant to 100 volts for 0.5 ms, then the current pulse will be 10 ma for 0.5 ms and then will decay in direct proportion to the voltage compliance limit.

Example

In the case of transcranial Direct Current Stimulation (tDCS), a constant current of up to 2 ma (Is) may be applied through the cranium. Assuming a combined series electrode / skin junction resistance of 20 Kohms, a cranium resistance of 100 ohms and other loop series resistances considered negligible, then the total stimulation loop impedance (Zs) is 20.1 Kohms. Accordingly, in this situation, the voltage compliance (Vs) of the tDCS current stimulator must be at least:

Is * Zs = Vs
0.002 amp * 20,100 ohms = 40.2 volts

Given the variable nature of the combined series resistances in the stimulation loop, it’s generally helpful to double the expected compliance voltage requirement of the stimulator. Accordingly, in this case, a stimulator compliance voltage of 80 volts or higher is recommended. Because the series resistance in the loop is largely a function of the electrode / skin interface junctions, the choice of electrode type and contact area has a great effect on the voltage compliance requirements of the current stimulator. Other factors contributing to the voltage compliance requirement are the equilibrium potentials and over-potentials associated with the specified electrode / electrolyte / skin junctions. Furthermore, if high resistance electrode leads are used, the potential across the leads may become material.

 

BIOPAC Systems, Inc. provides life science researchers and educators with data acquisition and analysis systems that inspire people and enable greater discovery about life. BIOPAC lets you measure physiology anywhere, with innovative, compatible wireless, wearable solutions that can be used by anyone for meaningful discovery. We make high-quality scientific tools for physiology measurement and interpretation with superior compatibility and world-class customer service and support. Visit us at www.biopac.com.

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