Motor Unit Recruitment

Motor units, consisting of a motor neuron and the muscle fibers it innervates, are recruited following Henneman’s size principle: motor units are activated in an orderly progression from low-threshold to high-threshold units as force demands increase. High-threshold motor units primarily innervate Type II muscle fibers, which are recruited only during high force output or near muscular failure.

This principle was first demonstrated by Henneman (1957): PubMed .

Type II fibers have greater hypertrophic potential due to their superior force production capacity and heightened responsiveness to mechanical overload. Schoenfeld (2010) reviewed hypertrophy mechanisms and highlighted mechanical tension as the primary driver of muscle growth, with metabolic stress and muscle damage as secondary contributors. PubMed .

Carbohydrates, Glycogen & High-Threshold Recruitment

High-intensity resistance training relies heavily on intramuscular glycogen as a key energy substrate. Glycogen depletion reduces force capacity and accelerates fatigue, limiting the ability to recruit high-threshold motor units effectively.

Leveritt & Abernethy (1999) showed that reduced muscle glycogen impairs resistance exercise performance, particularly during repeated high-intensity efforts. PubMed .

Similarly, Burke et al. (2011) emphasized that carbohydrate availability is crucial for sustaining high-intensity exercise capacity, especially when performance depends on glycolytic energy pathways. PubMed .

Adequate glycogen stores indirectly support maximal recruitment of Type II fibers by enabling sustained force production and delaying fatigue.

Glycogen-Water Association

Glycogen is stored in muscle cells together with water, with each gram of glycogen binding approximately 2.7-3.0 grams of intracellular water.

Olsson & Saltin (1970) observed that increases in muscle glycogen correlate closely with proportional increases in muscle water content, confirming the approximate 3:1 water-to-glycogen ratio. PubMed .

Maintaining adequate systemic hydration is therefore essential to support optimal glycogen storage capacity. Insufficient total body water may limit glycogen accumulation and impair sustained high-force output during resistance training.

Hydration and Neuromuscular Function

Dehydration exceeding roughly 2% of body mass impairs neuromuscular performance and strength. Sawka et al. (2007) reported that such levels of body water deficit negatively affect physiological function and exercise capacity, including motor unit recruitment. PubMed .

Optimal hydration preserves plasma volume, thermoregulation, and neuromuscular function, all vital for maximal force production and effective high-threshold motor unit recruitment.

MUR Optimization Calculator

This calculator estimates pre-exercise carbohydrate and fluid intake based on upper evidence-supported recommendations for maximizing high-intensity performance and physiological readiness.

These targets align with established sports nutrition position stands and represent the upper end of intake ranges to support glycogen availability, hydration, and neuromuscular performance.

Carbohydrates: 1.0-1.2 g/kg 1-2 h pre-exercise (Burke et al., 2011; Thomas et al., 2016).
Hydration: 5-7 mL/kg 4 h pre-exercise (Sawka et al., 2007; ACSM Position Stand).
Glycogen-Water Ratio: ~3 g water per g glycogen (Olsson & Saltin, 1970).