Muscle Fatigability

Muscle fatigability refers to how quickly muscle performance declines when the muscle is repeatedly or continuously activated. Unlike grip strength, which reflects maximum force, fatigability measures endurance capacity which is how long a muscle can sustain activity before performance decreases.¹

This concept is central to the broader idea of vitality capacity, which describes the body’s ability to maintain physical and physiological function over time.² Reduced muscle endurance has been linked to slower gait speed, mobility limitations, and elevated inflammatory markers in older adults.^3,4

Recent studies show that measuring fatigability through repetitive or sustained grip tests provides valuable insights into frailty risk and physical resilience.³ These findings highlight muscle fatigability as a reliable functional biomarker for healthy aging.^3-5

Muscle fatigability is most accurately assessed through a sustained grip test. In this test, the participant maintains maximal grip effort until force falls below 50% of their starting value. The duration of sustained force defines Fatigue Resistance (FR), while the total area under the force–time curve reflects Grip Work (GW).⁴

To account for body size differences, GW can be normalized to body weight " a measure known as GW_rel"⁴ This allows fair comparison across individuals and populations.

The Eforto device automates these calculations, providing an objective and reproducible assessment of muscle fatigability and endurance.⁵

The test results in two key measures:⁶

Fatigue Resistance (FR): This indicates how long (in seconds) you can maintain your grip above (50%) half your maximum strength.⁶

Grip Work (GW), which represents the total amount of "work" your hand muscles performed during the test. Think of it as the overall effort (labor) your muscles put forth, measured by the area under the force-time curve captured during your test.⁶

Note: The validated Eforto test includes built-in safeguards to ensure you squeeze at maximum effort, preventing inaccurate results from submaximal squeezing.

Adapted from Bautmans, I., et al. (2011, The Journal of nutrition, health and aging, 731-736).

GW_rel follows the same age-related patterns as grip strength but provides a more nuanced understanding by integrating endurance and body weight.⁴

Validated studies established that Grip Work (GW) and Fatigue Resistance (FR) can be normalized for body weight (GWrel) to allow comparison across individuals.^3,5

The Eforto app displays Fatigue Resistance in seconds using the more informative parameter GW_rel for the classification into three “traffic light” zones for easy interpretation:

• Green (Normal): Healthy muscle endurance

• Yellow (At Risk): Below-expected endurance, possible pre-frailty

• Red (Very Low): Significantly reduced endurance, often associated with frailty.

The Eforto® app presents GW_rel in three color-coded zones reflecting validated trends in published literature.^3-5

Low GW_rel is strongly linked to physical frailty in older adults. Even in people over 80 who felt okay, those with lower grip work were far more likely to be in a pre-frail or frail state compared to robust peers. In one study, older adults with both low muscle endurance and high fatigue were much more likely to be frail than those with good endurance.³

Frailty means reduced reserve and resilience, making it harder to recover from illnesses or injuries. It’s also associated with loss of independence.^7-9

• Higher frailty and disability risk in older adults⁷

• Higher levels of systemic inflammation (e.g., IL-6)⁸

• Poor mobility, recovery, and perceived vitality⁹

Emerging research suggests the Capacity to Perceived Vitality (CPV) ratio, measured by comparing objective muscle performance (GW) with self-reported fatigue, as a promising marker of overall vitality capacity. ¹⁰  Persistent low endurance signals a loss of physiological reserve and may precede functional decline.³

The good news is that muscle endurance is modifiable at any age. Evidence-based strategies include:^11-16

• Progressive resistance and endurance training: Regular grip exercises, resistance bands, and weightlifting significantly improve grip work and fatigue resistance in older adults.

• Adequate protein and nutrient intake: Supports muscle repair and reduces inflammation.

• Balanced recovery: Proper rest prevents overtraining and promotes adaptation.

• Addressing underlying health issues: Managing inflammation, hormonal imbalances, or chronic conditions enhances training benefits.

Meta-analyses confirm that multimodal exercise programs combining strength and endurance training produce the greatest improvements in grip performance and muscle stamina among aging populations.¹⁶

If muscle endurance remains very low or continues to decline despite regular activity, medical evaluation is recommended. Persistent fatigue, frequent falls, or loss of daily function may indicate underlying causes such as:^11,15,16

• Nutritional deficiencies

• Hormonal disorders

• Chronic inflammation

• Neuromuscular disease

In clinical practice, metrics like FR, GW, and CPV are being explored as tools to monitor recovery trajectories and detect frailty progression. ⁷ Tracking GWrel over time offers an objective view of a patient’s vitality capacity and helps tailor personalized interventions.