Resistance training rewires the nervous system before it ever changes the muscle itself — and everything it builds, from tendon stiffness to bone density, unwinds again the moment loading stops.
What actually shows up on the exam
No sport runs on a single energy system in isolation — the phosphagen, glycolytic, and oxidative systems are always contributing something simultaneously. What changes from sport to sport is which one is doing most of the work at the moment force actually needs to be produced. Ranking a sport's primary demand is really a question about effort duration and repeatability: a single explosive action favors the phosphagen system, a sustained hard effort of a couple of minutes forces a shift toward glycolysis, and anything measured in tens of minutes or hours has to lean on oxidative metabolism because nothing else can keep the ATP supply going that long.
| Sport / event | Primary system | Secondary system | Why |
|---|---|---|---|
| Shot put, vertical jump, 1RM lift | Phosphagen | Glycolytic | Single maximal effort under ~10 s |
| 100 m sprint | Phosphagen | Glycolytic | Near-maximal effort, ~10 s, ATP-PC dominant then fading fast |
| 400 m sprint, 100 m swim | Glycolytic | Phosphagen | 45–60 s all-out — too long to run on stored ATP-PC alone |
| Wrestling / judo / MMA (single period) | Glycolytic | Phosphagen | Repeated high-force bursts over a 2–5 min period |
| Boxing round | Glycolytic | Oxidative | Intermittent high-intensity work across a 2–3 min round |
| 800 m run | Glycolytic | Oxidative | ~2 min maximal effort — the classic glycolytic-ceiling event |
| Basketball, soccer, hockey, tennis | Glycolytic | Oxidative | Short explosive bursts stacked across a long, intermittent match |
| Rowing 2,000 m | Glycolytic | Oxidative | ~6–7 min sustained near-maximal — glycolytic and oxidative share the load almost evenly |
| 1,500 m swim, 5,000 m run | Oxidative | Glycolytic | 15–20 min hard effort, past the point glycolysis alone can sustain |
| Marathon, triathlon, cross-country skiing (distance) | Oxidative | — | Hours of sustained submaximal work; phosphagen contribution is negligible |
Where people lose points
Don't read "primary system" as "only system." Even a marathon has phosphagen and glycolytic contribution at the start line and at a finishing kick — the exam is testing what meets most of the ATP demand for most of the effort, not what's present at all.
Chronic resistance training reshapes five systems at once — neural, muscular, connective tissue, endocrine, and cardiovascular — but not all of them move in the same direction, and not all of them move at the same speed. The pattern worth memorizing: almost everything related to force production goes up, while a few variables tied to oxidative capacity per unit of muscle either don't keep pace with fiber growth or actually shrink relative to it.
| System | Variable | Direction |
|---|---|---|
| Neural | Motor unit recruitment (access to high-threshold units) | ↑ |
| Neural | Rate coding (motor unit firing rate) | ↑ |
| Neural | Motor unit synchronization | ↑ |
| Neural | Antagonist co-contraction during the lift | ↓ |
| Muscular | Fiber cross-sectional area | ↑ |
| Muscular | Whole-muscle cross-sectional area | ↑ |
| Muscular | Pennation angle | ↑ |
| Muscular | Capillary density (per unit of fiber area) | ↓ / no change |
| Muscular | Mitochondrial density (per unit of fiber area) | ↓ / no change |
| Muscular | Anaerobic enzyme activity (e.g. PFK, LDH) | ↑ |
| Connective | Tendon stiffness | ↑ |
| Connective | Ligament tensile strength | ↑ |
| Connective | Bone mineral density (loaded sites) | ↑ |
| Endocrine | Acute post-exercise testosterone, GH, IGF-1 | ↑ (transient) |
| Endocrine | Resting testosterone / GH levels | no change / minor ↑ |
| Endocrine | Androgen receptor content in trained muscle | ↑ |
| Cardiovascular | Left ventricular wall thickness | ↑ (mild, concentric pattern) |
| Cardiovascular | Resting heart rate | no change / slight ↓ |
| Cardiovascular | Resting blood pressure | no change / slight ↓ |
A shortcut for the "decrease or no change" row
Three things don't scale up as fast as the fiber does: capillary density, mitochondrial density, and resting cardiovascular numbers. A fiber can nearly double in cross-sectional area while its capillary and mitochondrial count stays roughly where it was — so density, which is a ratio, falls even though the raw count didn't shrink at all.
In the first several weeks of any new resistance program, almost none of the strength gain is coming from bigger muscle. It's coming from a nervous system that is learning to use the muscle it already has more completely and more efficiently. That's why an untrained lifter's strength curve looks steep early and then flattens — the easy neural gains get banked first, and hypertrophy has to catch up and take over from there.
Motor units are always recruited in a fixed order: smallest, lowest-threshold, most fatigue-resistant units first, and progressively larger, higher-threshold, more powerful units added on top as force demand rises. Nothing about resistance training changes that order — what it changes is how much of the pool an athlete can actually reach and how quickly.
The neuromuscular junction itself remodels with chronic training, not just the muscle downstream of it. The junction's contact area with the muscle fiber tends to enlarge, branching of the motor axon terminal increases, and the number of acetylcholine receptors and synaptic vesicles available for a rapid-fire sequence of action potentials goes up. Functionally, that means less chance of transmission failure during a demanding set — the junction can keep faithfully passing along a fast train of signals instead of missing beats as fatigue sets in.
It's not just the junction — the spinal cord adapts too
Motoneuron excitability rises with training, and descending drive from the motor cortex down the corticospinal tract increases. Reflex inhibition from the Golgi tendon organ also gets partially overridden with heavy training experience, letting a trained lifter express more of the force their muscle is actually capable of producing rather than having it clamped down defensively.
Three EMG-documented phenomena show, better than almost anything else, that strength has a real neural component separate from muscle size.
| Phenomenon | What happens | What EMG shows |
|---|---|---|
| Cross-education | Training one limb produces a measurable strength gain in the opposite, untrained limb | Increased activation in the homologous untrained-limb muscle, with little to no hypertrophy there — the transfer is central, not local |
| Bilateral deficit | Maximal force produced by both limbs contracting together is less than the sum of each limb's individual maximal force | Lower total EMG amplitude during the bilateral effort than the sum of the two unilateral efforts — reduced descending neural drive, not reduced muscle capacity |
| Instability training effect | Lifting on an unstable surface raises demand on stabilizer muscles but typically lowers the force and power a lifter can produce at a given load | Higher EMG in core/stabilizer musculature; lower EMG and lower output in the prime mover compared with the same lift performed on a stable surface |
A common mix-up
The bilateral deficit isn't universal — some athletes who specifically train bilateral lifts (many strength/power athletes) show bilateral facilitation instead, where the two-limb effort matches or exceeds the summed unilateral efforts. The deficit is a trainable, task-specific pattern, not a fixed law of physiology.
Muscle protein is in constant turnover — synthesis and breakdown are both happening all the time, and a training bout temporarily raises both rates. What actually produces hypertrophy is a net positive balance, accumulated over repeated bouts with adequate recovery and protein intake, where synthesis outpaces breakdown for long enough to add contractile and structural protein faster than it's lost.
Three stimuli are generally credited with driving that positive balance, and most real training sessions combine all three to some degree:
| Stimulus | Typical trigger | What it mainly builds |
|---|---|---|
| Mechanical tension | Heavy loads, near-maximal motor unit recruitment, high force per cross-sectional area | Myofibrillar protein — the actin/myosin machinery that directly produces force |
| Metabolic stress | Moderate loads, short rest, high reps, metabolite buildup (H⁺, lactate, inorganic phosphate), cell swelling | Sarcoplasmic volume and a stronger acute hormonal response; a secondary contributor to overall size |
| Muscle damage | Novel movements, emphasized eccentric loading, disruption at the Z-line | Triggers the inflammatory and satellite-cell repair response; its direct contribution to added size is smaller and more debated than the other two |
Mechanical loading doesn't build muscle by itself — it has to get converted into a biochemical signal first, and that conversion follows a fairly consistent chain of events inside the fiber.
| Pathway | Triggered by | Primary effect |
|---|---|---|
| mTOR (mTORC1) | Mechanical tension, leucine/amino acid availability, IGF-1 | The central growth signal — drives translation initiation and protein synthesis rate |
| Myostatin–Smad | Baseline regulatory tone | Normally restrains growth; resistance training reduces myostatin expression, releasing that brake |
| MAPK/ERK | Mechanical stress and growth-factor signaling | Supports gene transcription and satellite cell activity |
| Calcineurin–NFAT | Repeated rises in intracellular calcium with contraction | Influences fiber-type-specific gene expression |
Chronic resistance training doesn't convert Type I fibers into Type II fibers or vice versa in any meaningful way — that boundary is fairly fixed. What it does move, reliably, is the balance within the Type II population. Training pushes fibers toward a more oxidative, fatigue-resistant profile: Type IIx shifts toward Type IIa, often passing through a hybrid IIx/IIa fiber along the way as myosin heavy chain isoform expression gradually changes. This shift can happen surprisingly fast — meaningful changes in isoform proportion have been measured within just a few weeks of consistent training.
Detraining reverses the process: fibers drift back toward Type IIx as the oxidative pressure of training is removed, and in some cases the proportion of Type IIx fibers can temporarily overshoot past where the athlete started before training in the first place.
Bone follows Wolff's law: it remodels along the lines of the mechanical stress it regularly experiences, adding material where load demands it and resorbing material where load doesn't justify the metabolic cost of maintaining it. The catch is that bone doesn't respond to just any load — there's a threshold.
Minimal essential strain (MES)
Bone tissue needs to be strained above a certain magnitude before it shifts from a maintenance state into an actively bone-building state. That threshold, minimal essential strain, sits at roughly one-tenth of the strain that would actually fracture the bone — a large safety margin, but also a real floor. Loading below MES maintains bone; loading that regularly exceeds it is what drives new bone formation. High-magnitude, high-rate, novel loading patterns — the kind produced by jumping and heavy resistance training — clear that threshold far more effectively than repetitive, low-force loading like distance running.
| Cortical bone | Trabecular bone | |
|---|---|---|
| Location | Outer shaft (diaphysis) of long bones | Ends of long bones, vertebral bodies, pelvis |
| Share of total skeleton | ~80% | ~20% |
| Structure | Dense, low porosity | Porous, honeycomb-like lattice |
| Turnover rate | Slow | Faster — more surface area is exposed to remodeling cells |
| Primary role | Structural support and leverage | Metabolic reserve and load distribution/shock absorption |
| Response to a new loading stimulus | Detectable, but on a longer timeline | Responds faster, given its higher baseline turnover |
These four tissues share a common building block — collagen — but differ in how much they have, what type, and how it's organized, which explains why they behave so differently under load.
| Tissue | Main composition | Organization |
|---|---|---|
| Tendon | Dense Type I collagen, minimal elastin | Parallel, longitudinally aligned fibers — built for transmitting tensile force in one direction |
| Ligament | Type I collagen with more elastin than tendon | Fibers run in a less uniform pattern, reflecting multi-directional joint loads |
| Fascia | Type I and III collagen in a looser matrix | Sheet-like, multidirectional — transmits force across broader regions rather than along one line |
| Cartilage (hyaline/articular) | Type II collagen embedded in a proteoglycan-rich, water-heavy ground substance | No direct blood supply — nourished by diffusion, which is why it adapts and heals slowly |
| Adaptation | Functional effect |
|---|---|
| Increased collagen synthesis, fibril diameter, and cross-linking | Greater tensile strength for the same tissue volume |
| Increased tendon stiffness | Less energy lost to tendon stretch, so muscle force reaches the bone more directly and more quickly — relevant to rate of force development and the stretch-shortening cycle |
| Increased tendon cross-sectional area | Distributes load over more tissue, reducing strain per unit area at a given force |
Ligaments adapt through a similar collagen-remodeling process, but more slowly than tendon — one reason ligament injury risk stays elevated even after muscle strength has caught up during a return-to-training progression. Fascia responds to consistent loading by stiffening and reorganizing its fiber orientation along the dominant load line. Cartilage sits at the opposite end of the adaptability spectrum: moderate, repeated loading can support proteoglycan content and thickness over time, but because it has no direct blood supply, both adaptation and repair happen far slower than in any other connective tissue discussed here — and excessive impact loading can degrade it rather than strengthen it.
Resistance training's endocrine signature is mostly an acute one. A single heavy, large-muscle-mass session with short rest intervals produces a real spike in circulating testosterone, growth hormone, IGF-1, and cortisol — but resting, baseline hormone concentrations tend to shift only modestly with chronic training, if at all. What does change more reliably is sensitivity: trained muscle tends to carry more androgen receptor content, meaning it can respond more to the testosterone that's already circulating without needing the resting concentration itself to rise. Insulin sensitivity also improves with chronic resistance training, independent of any hormonal spike.
| Variable | Anaerobic/resistance training effect |
|---|---|
| Left ventricular wall | Mild thickening — a concentric pattern, adapting to intermittent pressure spikes rather than sustained volume load |
| Left ventricular chamber size | Little to no dilation — contrasts with the chamber dilation typical of aerobic/endurance training |
| Resting heart rate | Little to no change, or a slight decrease |
| Resting blood pressure | Little to no chronic change; some modest reductions reported in previously hypertensive individuals |
| Resting blood volume | Minimal change — a much smaller effect than the plasma volume expansion seen with aerobic training |
| Capillary density (relative to fiber size) | Doesn't reliably keep pace with fiber growth — same dilution pattern as mitochondrial density |
The contrast with aerobic training is the exam-relevant point here: aerobic work produces volume-overload adaptations (chamber dilation, expanded plasma volume, resting bradycardia), while resistance training produces a pressure-overload adaptation pattern (modest wall thickening, minimal dilation) — a distinction covered in more depth alongside the endurance-training adaptations in the next chapter.
Training both qualities at once is sometimes called concurrent training, and it can produce an interference effect — a blunting of the strength/power/hypertrophy gains an athlete would otherwise get from resistance training alone, without a comparable blunting of aerobic gains in the other direction.
The leading mechanistic explanation is a signaling conflict at the cellular level: sustained aerobic exercise strongly activates AMPK, an energy-sensing pathway that, among its other jobs, can suppress mTOR signaling — the same pathway that resistance training relies on to drive protein synthesis. Do enough endurance work too close to a resistance session, and the AMPK signal can partially cancel out the anabolic signal the lifting was supposed to send.
How much strength a given block of training produces depends heavily on where the athlete started.
| Status | Typical rate of gain | Primary driver |
|---|---|---|
| Novice | Largest relative gains, often week to week early on | Neural — recruitment, rate coding, and coordination improvements dominate |
| Intermediate | Gains slow noticeably and require more deliberate progression | A more even mix of neural and hypertrophy-driven adaptation |
| Advanced / elite | Smallest relative gains, often measured over months or a full training cycle | Mostly hypertrophy-driven; further neural headroom is limited, so continued progress needs increasingly precise periodization |
Peak power output is another place where the "one right load" intuition breaks down — the load that maximizes power depends heavily on the exercise itself, largely because of how technical and multi-joint the movement is.
| Exercise type | Approx. load for peak power |
|---|---|
| Jump squat / other loaded jump variations | ~0–30% 1RM (often bodyweight or light external load) |
| Squat, deadlift, and other bilateral lower-body lifts | ~50–70% 1RM |
| Bench press throw / upper-body ballistic press | ~30–45% 1RM |
| Power clean and other Olympic-style pulls | ~70–80% 1RM |
The pattern: simple, single-joint or unloaded-jump movements peak at very light relative loads, while technical, multi-joint lifts like the power clean peak at much heavier relative loads — the skill and force requirements of the lift itself push the power-optimal zone higher.
Every adaptation covered in this chapter is reversible, and they don't unwind at the same speed. As a rule, the qualities that were fastest to build tend to be the fastest to fade, though there are exceptions worth knowing individually.
| Quality | What happens after training stops |
|---|---|
| Oxidative/aerobic-adjacent capacity (mitochondrial enzyme activity, capillarization) | Among the fastest losses of all — measurable declines within roughly 1–2 weeks of complete inactivity |
| Power / rate of force development | Fades faster than maximal strength — it depends heavily on rate coding and fast motor unit access, both of which are neural qualities that erode relatively quickly without practice |
| Muscle cross-sectional area | Begins shrinking within roughly 2–3 weeks of full cessation, more clearly measurable by 4–6 weeks |
| Maximal strength | The most durable quality — partly protected by neural adaptations that persist even as size declines; meaningful loss typically takes several weeks to a few months of inactivity |
| Fiber type profile | Drifts back toward Type IIx, sometimes overshooting past the athlete's pre-training baseline |
| Bone mineral density | Declines the slowest of all — consistent with bone's inherently slow remodeling timescale, especially in cortical bone |
A fast way to keep the order straight
Think of it as oxidative-adjacent qualities and power fade first, size follows, and true maximal strength and bone density hang on longest. The tissues and systems with the fastest turnover in the first place (mitochondrial/capillary adaptations) are also the fastest to reverse; the slowest-remodeling tissue (bone) is the slowest to reverse too.
A lifter has been training for three weeks and has already added noticeably to their squat 1RM, with no visible change in thigh size. What best explains the gain?
B. Early strength gains, especially in the first several weeks, are dominated by neural adaptation. Measurable hypertrophy takes longer to accumulate, and Type I ↔ Type II conversion isn't a real training effect.
Which pathway is considered the central signaling hub that converts mechanical loading into increased muscle protein synthesis?
C. mTORC1 activation drives translation initiation and is the primary anabolic signal triggered by mechanical tension plus amino acid availability. Myostatin is a growth brake that training reduces, not the growth signal itself.
A researcher measures each limb's individual maximal force, sums them, and finds the total is higher than the force produced when both limbs contract together. What is this called?
C. The bilateral deficit describes bilateral output falling short of the summed unilateral outputs, attributed to reduced descending neural drive during the two-limb effort — not a muscular limitation.
Which of the following typically decreases, or fails to keep pace with fiber growth, following a hypertrophy-focused training block?
C. Capillary and mitochondrial counts don't scale up as fast as the fiber's cross-sectional area does, so their density — a ratio to fiber size — tends to fall or hold steady even as the fiber itself grows substantially.
Minimal essential strain (MES) is best described as which of the following?
B. MES sits well below fracture strain — roughly a tenth of it — and functions as the threshold above which bone shifts from maintenance into active remodeling/building mode.
Compared to cortical bone, trabecular bone is best characterized by which combination of properties?
B. Trabecular bone is porous, turns over faster because more of its surface is exposed to remodeling cells, and is concentrated at the ends of long bones, vertebrae, and the pelvis — and it makes up roughly 20%, not 80%, of total skeletal mass.
An athlete combines a high-volume distance-running program with heavy resistance training in the same block and notices their strength and hypertrophy gains stall compared to resistance training alone. What is the leading proposed mechanism?
B. High-volume aerobic work strongly activates AMPK, which can blunt mTOR-driven protein synthesis signaling — the proposed basis of the concurrent-training interference effect. The effect scales with aerobic volume/intensity and can be reduced by spacing the two sessions apart.
Which quality is generally considered the most durable — the last to decline — once structured training stops?
C. Maximal strength is protected for longer than power, size, or oxidative-adjacent qualities, largely because the neural adaptations underlying it persist even as other qualities begin to fade.
A power clean and a jump squat are compared for the load that maximizes peak power output. What would you expect?
C. Technical, multi-joint lifts like the power clean peak at heavier relative loads (roughly 70–80% 1RM) than simpler, ballistic or jump-based movements, which tend to peak at very light loads.
Which fiber type shift would you expect to see after several months of consistent heavy resistance training?
B. Training pushes the Type II population toward a more oxidative, fatigue-resistant Type IIa profile. Detraining reverses this back toward Type IIx. True conversion between Type I and Type II is not a recognized training effect.
This chapter is original study material covering topics also addressed in Essentials of Strength Training and Conditioning, 5th edition (NSCA/Human Kinetics). It is an independent work, not a reproduction or paraphrase of that text. This guide is not affiliated with, endorsed by, or sponsored by the National Strength and Conditioning Association (NSCA). CSCS® and Certified Strength and Conditioning Specialist® are registered trademarks of the NSCA.