All fibers contract via the same process, but not all fibers are created equal. Fibers are commonly divided into fast twitch and slow twitch, but there are more differences than just contraction speed. Fibers vary in force production, energy production, and fatigue resistance. While there is a genetic component to the distribution of fibers by type, training can change muscle fiber characteristics.⁽¹⁾ In addition to pure fast and slow, hybrid fibers blend the characteristics of slow and fast fibers, ⁽¹⁾ so that an individual’s musculature exhibits properties along a continuum from slow to fast. This chart summarizes some characteristics of muscle fibers.

We don’t use every muscle fiber on every rep. For example, a 30% deadlift recruits fewer fibers than 90%. Toes to bar recruits different fibers than box jumps. During sustained activity, the brain cycles fibers on and off. As demand increases (heavier weight, more reps, increased intensity) additional fibers are recruited, and the interval between contractions decreases.

The sequence of recruitment is unclear; early research supported ascending size order of motor unit recruitment,⁽²⁾ but newer findings indicates that larger, faster motor units be recruited first in response to mechanical demands of a task.⁽³⁾ Regardless of the sequence, motor unit recruitment is task specific.

Now that we’ve reviewed the basics of muscle fiber recruitment and contraction, let’s turn to the energy side. ATP provides energy to cellular processes, but very little ATP is stored in cells. Instead, cells produce ATP to meet increased energy demands. The immediate source of ATP is phosphocreatine (PCr). Cells can generally store 4 to 5 times more PCr than ATP. Experiments dating back to 1970 showed that muscle contraction was interrupted when PCr was depleted, though ATP concentration remained high.⁽⁴⁾

More recently, Imaging shows that during a contraction, ATP levels remain constant while PCr declines.⁽⁵⁾ During the contraction period, ATP is consumed. PCr immediately resynthesizes ATP⁽⁶⁾, ensuring a steady supply of energy. During the relaxation period, PCr is resynthesized from glycogen breakdown and glycolysis.^{(7, 8)} Glycolytic processes and lactate production begin with the first muscle contraction. This has been observed repeatedly; experiments have shown significant lactate accumulation from intense exercise lasting only 5 seconds⁽⁹⁾ or 10 seconds.⁽¹⁰⁾ Between contractions, glycogen is resynthesized from plasma glucose.⁽⁷⁾ The energy for this re-synthesis of glycogen is provided by the oxidation of lactate.

Ongoing exercise increases the concentration of ADP, signaling mitochondria to increase oxidative phosphorylation.⁽⁶⁾ Energy produced in mitochondria is shuttled to myofibrils by phosphocreatine. We now have a clearer view of energy production:

• PCr is a store and shuttle of phosphates. During a muscle contraction, PCr instantly replenishes ATP. PCr also shuttles energy from the mitochondria to working muscles. Strictly speaking, PCr is not an energy system, as it does not extract energy from macronutrients, but simply moves and stores energy.

• Glycogen supplies ATP beginning immediately upon the commencement of exercise^{(7, 8, 10)} and continuing for the duration of exercise.⁽¹¹⁾ Glycogen breakdown refills PCr pools during the relaxation phase of muscle contraction. Glycolysis has been shown to provide up to 20% of ATP for muscles working at sustainable intensity⁽¹¹⁾ even during longer duration activities.

• Mitochondria refill ATP and PCr pools in the interval between contractions. The concentration of ADP is a signal for the mitochondria to oxidative phosphorylation.

Phosphocreatine re-synthesis

During intense exercise, PCr concentration declines. Most of us have experienced this; it’s that feeling of hitting the gas, but the engine won’t go. But it’s not just a feeling. Experiments have shown that during intense exercise, performance is more closely related to the re-synthesis of PCr than to the level of lactate accumulation. There is also evidence of a strong relationship between the decline in PCr and a decline in force during high intensity exercise. ⁽⁶⁾ The extent of PCr breakdown is primarily dependent on the intensity of muscle contraction. Recovery of PCr during non-fatiguing exercise is almost exclusively oxygen dependent, but during very intense exercise, PCr recovery occurs in two stages

• An oxygen-dependent fast phase, beginning immediately upon cessation of exercise. Athletes with substantially depleted PCr have been shown to recover ~65% of their pre-exercise PCr after 90 seconds. ⁽⁶⁾

• A pH dependent slow phase, which completes the recovery. After exercise of sufficient intensity, in which pH is substantially decreased, full recovery of PCr can take 10-15 minutes.

The likely mechanism of delayed PCr recovery after intense in reduced-pH environments is because low pH inhibits the ability of skeletal muscle to resynthesize ATP and therefore PCr. ^(6,11)

pH equilibrium

If you’ve pushed very hard and perhaps collapsed to the floor after a difficult workout, you’re familiar with reduced pH. Sometimes we call this being over the redline. Intense exercise recruits more glycolytic fibers, and glycolytic reactions produce lots pf protons, lowering pH if you cannot buffer or transport them. Low pH adversely impacts capacity due to:

• Decreased contraction velocity and ATPase activity. As pH decreased from 7.0 to 6.0, contraction velocity and ATPase activity decreased 27% and 25%, respectively. Isometric force production decreased 45%. ⁽¹²⁾

• Reduced oxidative activity. A decline in pH inhibits the rise in ADP concentration, weakening the signal for increased mitochondrial activity, and limiting oxidative phosphorylation during exercise ⁽¹¹⁾ resulting in lower sustainable work rates.

Possible decreased force production. One experiment showed that a drop in pH from 7.0 to 6.5 reduced isometric force by about 35%. ⁽⁶⁾ Another experiment showed that force decreased up to 44%, depending on muscle fiber type. ⁽¹³⁾ However, recent evidence has challenged the relationship between reduced pH and decreased force production, ⁽¹⁴⁾ so this remains somewhat unclear.

While effective training and athlete development programs should be individualized to an athlete’s goals and starting point, there are several principles which apply to training generally:

Effective strength and power training stresses a broad spectrum of motor units with bias toward the faster end. As athletes progress and technique improves, the bias toward intensity may increase. Training should also develop explosiveness (rate of force development) and utilize accessory work to correct imbalances resulting from previous athletic activities.

Skills practice should begin by prioritizing acquisition, retention or transference of a skill. Less experienced athletes generally benefit from low-stress repetition, whereas advanced athletes should encounter more randomized practice and contextual interference ^{(15, 16)}

Conditioning focuses on sustainable work rates. After correcting any imbalances, oxidative capacity across the broadest possible spectrum of motor units will enhance PCr recovery during any task. This can be accomplished with Polarization and interval work. High intensity intervals increase buffer capacity significantly,⁽¹⁷⁾ enabling cells to maintain higher pH, which delays fatigue and facilitates more complete replenishment of PCr. Advanced athletes should encounter zero rest, mixed intensity and variable work-rest intervals.