The core is a group of muscles surrounding your trunk that stabilize your spine, transfer force between your upper and lower body, and support nearly every movement you make. It includes far more than the abdominal muscles most people picture when they hear the word. The deep stabilizers, the diaphragm, the pelvic floor, and the muscles running along the spine all work together in a coordinated system that matters as much for picking up groceries as it does for throwing a ball or recovering from childbirth.
More Than Just Abs
When people say “core,” they tend to think of a six-pack. The actual core is better understood as a cylinder of muscles wrapping around the midsection, with a top (the diaphragm) and a bottom (the pelvic floor). Research describes two broad muscle systems within the core. A local system made up of deeper muscles coordinates and controls movement at individual spinal segments, keeping vertebrae stable relative to each other. A global system of larger, more superficial muscles provides stability across multiple segments and produces the bigger forces needed for dynamic movement like running, lifting, or twisting.1Multidisciplinary Digital Publishing Institute (MDPI). The Multi-Component Structure of Core Strength There is also an axial-appendicular transfer system that links the trunk to the limbs, which is the piece that lets your core contribute to the power of a throw or a kick.
The paraspinal muscles running along the back of the spine deserve special attention. These muscles, including the multifidus and erector spinae, contain a high proportion of slow-twitch fibers suited for sustained low-level activity, the kind of work needed to hold your posture upright all day. When those muscles weaken or fatigue more easily than they should, back pain often follows.2PubMed Central. Fibre type characteristics and function of the human paraspinal muscles: normal values and changes in association with low back pain
How the Core Keeps Your Spine Stable
Your lumbar spine, on its own, is not particularly sturdy. Without the muscles around it, it would buckle under surprisingly small loads. Two mechanisms keep it from doing so. One is coactivation: opposing muscles on the front and back of the trunk contract at the same time, bracing the spine like guy-wires on a tent pole. The other involves intra-abdominal pressure, generated when the deep abdominal muscles contract and the diaphragm descends, creating a pressurized column of support in front of the spine. Both mechanisms raise the spine’s tolerance for load, and they work both independently and together.3PubMed. Intra-abdominal pressure mechanism for stabilizing the lumbar spine
What makes this system work is not raw strength in one muscle but highly coordinated activation patterns across many muscles at once. Those patterns have to shift constantly depending on the task, because the stability demands of sitting at a desk, picking up a child, and sprinting are all different.4PubMed Central. Coordination of muscle activity to assure stability of the lumbar spine This is why core training that only strengthens muscles in one position or one plane misses the point. Your core needs to be responsive and adaptable, not just strong in a plank.
The Central Link in Every Movement
In biomechanics, there is a concept called the kinetic chain: the idea that force travels through a series of connected body segments during any athletic or everyday movement. The core, specifically the lumbopelvic-hip complex, sits at the center of that chain. It acts as the transfer station where forces generated in the legs get passed up to the arms and vice versa.5PubMed Central. Role of kinetic chain in sports performance and injury risk: a narrative review
Consider throwing. Your legs push off the ground, your hips rotate, and that rotational energy travels through the trunk to the shoulder and arm. If the core is weak or poorly coordinated, energy leaks out at the transfer point, and the throw loses speed. A study of handball players found that adding core training to their regular program increased throwing velocity, specifically because stronger muscles in the lumbopelvic region improved the stability needed for more efficient power transfer between the lower and upper body.6PubMed Central. Effect of Core Training on Male Handball Players’ Throwing Velocity The same principle applies to kicking a soccer ball, swinging a golf club, or even pushing a heavy door open. The limbs produce the visible motion, but the core is what turns individual joint movements into a coordinated whole-body effort.
Your Brain Fires the Core Before You Move
One of the more striking findings about core function is that your brain activates trunk muscles before you consciously move a limb. These are called anticipatory postural adjustments: the nervous system pre-stiffens the trunk to brace against the destabilizing effect of, say, raising your arm. Research using brain stimulation techniques has shown that excitability in the motor cortex’s control of the erector spinae muscle increases about 40 milliseconds before a rapid shoulder movement begins, driven by a reduction in the brain’s normal inhibition of that motor pathway.7PubMed Central. Cortical contributions to anticipatory postural adjustments in the trunk In practical terms, your brain is tightening your back muscles before your arm even starts to move, and this happens automatically.
When this anticipatory system breaks down, trouble follows. In people with chronic low back pain, the deep abdominal muscles tend to fire late, leaving the spine briefly unprotected at the start of a movement. Targeted motor training combined with neuromuscular stimulation has been shown to normalize this delayed activation, restoring proper timing to the deep core muscles.8The Clinical Journal of Pain. Peripheral Neurostimulation and Specific Motor Training of Deep Abdominal Muscles Improve Posturomotor Control in Chronic Low Back Pain This is also relevant to stroke recovery, where core stability training drives neural plasticity and helps restore these anticipatory postural responses.9PubMed. Core Stability Training for Post-Stroke Recovery: A Multifaceted Approach to Enhance Postural Control and Motor Function
The Core and Low Back Pain
Chronic low back pain is one of the most common musculoskeletal complaints worldwide, and reduced core muscle endurance is consistently linked to it. People with chronic low back pain tend to have weaker, less fatigue-resistant trunk muscles, along with impaired neuromuscular control.10International Journal of Drug Delivery Technology. Influence Of Sling-Based Neuromuscular Training On Core Muscle Endurance In Patients With Chronic Low Back Pain And the relationship works in both directions: the pain itself changes how core muscles are used, creating a feedback loop. Fear of pain and catastrophic thinking about it can lead to movement avoidance, which further deconditions the muscles and amplifies disability.11PubMed Central. Core muscle endurance and psychosocial factors affecting functional mobility in chronic low back pain: cross-sectional results
Core strength training has been shown to make measurable improvements. In one study, participants who completed a core training program showed increased cross-sectional area of the deep abdominal muscles and reduced postural sway when subjected to a sudden perturbation, meaning their trunks were better at absorbing unexpected jolts.12PubMed Central. Effects of core strength training on core stability Real-time ultrasound imaging used as biofeedback during training can help people activate their deep core muscles more effectively, by showing them the contraction on screen as it happens. Research on healthy individuals found that this approach increased both the thickness ratio and electrical activity of local core muscles compared to training without visual feedback.13PubMed Central. Effect of Core Stability Training Monitored by Rehabilitative Ultrasound Image and Surface Electromyogram in Local Core Muscles of Healthy People Using similar feedback during abdominal hollowing exercises produced more selective strengthening of the transversus abdominis relative to the external oblique, meaning people got better at activating the right muscle rather than compensating with larger, more superficial ones.14PubMed Central. Is abdominal hollowing exercise using real-time ultrasound imaging feedback helpful for selective strengthening of the transversus abdominis muscle?
The Diaphragm Does Double Duty
Most people think of the diaphragm as a breathing muscle. It is, but it also plays a significant role in postural control. When you brace your trunk to lift something heavy or catch your balance, the diaphragm descends to help build intra-abdominal pressure. This dual role means that breathing and core stability are intertwined in ways that rehabilitation programs have historically overlooked. Researchers have argued that core stabilization exercises for low back pain should start with restoring normal breathing patterns and progressively rebuilding the diaphragm’s postural control function, rather than jumping straight to planks and crunches.15PubMed. Diaphragm and core stabilization exercises in low back pain: A narrative review
People with chronic low back pain appear to have an abnormally positioned diaphragm with a steeper slope, which may contribute to their condition.16PubMed. Postural function of the diaphragm in persons with and without chronic low back pain Adding diaphragmatic breathing exercises to core stabilization work has shown some benefit: one randomized controlled trial found an improvement in transversus abdominis activation during specific tasks when breathing exercises were combined with conventional core training, though not all measured outcomes improved equally.17PubMed Central. Effect of Adding Diaphragmatic Breathing Exercises to Core Stabilization Exercises on Pain, Muscle Activity, Disability, and Sleep Quality in Patients With Chronic Low Back Pain
Pelvic Floor and the Core Cylinder
The pelvic floor forms the bottom of the core cylinder, and its function is tightly linked to the abdominal and spinal muscles above it. When the core contracts to stabilize the trunk, the pelvic floor contracts in coordination. This is why weakness or dysfunction in one part of the system often shows up as problems in another. Women with stress urinary incontinence, for instance, often benefit from training that addresses the core and pelvic floor together. A 12-week program that combined pelvic floor exercises with core stability work produced far better results than pelvic floor training alone: about 72% of the combined group were cured of leakage (less than 2 grams on a pad test), compared with 28% in the control group.18PubMed Central. The Effects of Pelvic Floor Muscle Exercise Combined with Core Stability Exercise on Women with Stress Urinary Incontinence following the Treatment of Nonspecific Chronic Low Back Pain
In postpartum recovery, breathing techniques and pelvic floor training together produced significantly greater improvements in diaphragmatic movement, pelvic floor contraction pressure, and core stability than standard care. The gains in diaphragm function correlated with improvements in both pelvic floor and core outcomes, suggesting these systems recover better as a unit than in isolation.19Journal of Engineering and Science in Medical Diagnostics and Therapy. Biomechanical Associations of Breathing Techniques in Postpartum Recovery: Effects on Diaphragm Function, Abdominal Pressure Regulation, Pelvic Floor Health, and Core Stability
Pregnancy, Postpartum, and Diastasis Recti
During pregnancy, the growing uterus stretches the abdominal wall, often causing the two halves of the rectus abdominis to separate along the midline, a condition called diastasis recti. This separation reduces the mechanical efficiency of the abdominal wall and is correlated with lower abdominal strength and endurance postpartum. At six months after delivery, postpartum women had significantly wider separation at all measured locations compared to women who had never been pregnant, along with reduced strength and endurance on every abdominal measure.20PubMed. The relationships between inter-recti distance measured by ultrasound imaging and abdominal muscle function in postpartum women
Exercise can help, both before and after delivery. A systematic review found that exercise during pregnancy reduced the occurrence of diastasis recti by about 35%, and that exercising in both the prenatal and postnatal periods may reduce the width of the separation.21PubMed. Effects of exercise on diastasis of the rectus abdominis muscle in the antenatal and postnatal periods Deep core stability exercises specifically targeting the transversus abdominis and pelvic floor have shown highly significant reductions in the separation gap and improvements in quality of life for postpartum women.22PubMed Central. Efficacy of deep core stability exercise program in postpartum women with diastasis recti abdominis The research here points toward the same principle seen throughout the core literature: targeted training of the deeper muscles, not just general abdominal work, produces the best recovery.
Fall Risk in Older Adults
As people age, core muscle mass and strength decline, and the consequences extend well beyond back pain. A study of older adults with lumbar spondylosis found that trunk muscle strength was significantly correlated with fall risk in both men and women. In women at high risk of falling, both back and abdominal muscle strength predicted fall scores. In men, back muscle strength and the cross-sectional area of the erector spinae and lumbar multifidus were the relevant factors.23PubMed Central. Association between Trunk Muscle Strength and Fall Risk in Older Men and Women with Lumbar Spondylosis Falls are one of the leading causes of injury and loss of independence in older adults, which makes core strength a genuinely high-stakes issue for aging populations, not just an athletic performance concern.
Athletic Performance
For athletes, core training is often framed as injury prevention. That matters, but the performance effects are real too. A study of adolescent male soccer players found that adding core stability training to their regular seasonal program improved both ball-kicking velocity and sprint speed. Agility, on the other hand, did not improve significantly, which the researchers attributed to agility depending more on reactive decision-making and rapid direction changes than on trunk stiffness alone.24PLOS ONE. The effect of core stability training on ball-kicking velocity, sprint speed, and agility in adolescent male football players That finding is a useful reality check: a strong core helps transfer power and accelerate in a straight line, but it does not automatically improve every physical quality. The benefits are specific to tasks where trunk stability is the limiting factor.
How Core Function Is Tested
If you have seen a physical therapist or athletic trainer assess your core, they may have used one of several timed endurance tests. The McGill protocol, which includes tests like the side bridge, trunk flexor endurance, and the Biering-Sørensen test for back endurance, has shown excellent reliability in field athletes, with consistency scores between 0.80 and 0.89 across sessions.25PubMed. Trunk strength and endurance testing in field-athletes: A reliability study These tests measure how long you can hold a position against gravity, which emphasizes endurance over peak strength, reflecting how the core actually works in daily life: low-level sustained contraction, hour after hour.
Adapted versions of these tests have been validated for clinical populations as well. In people with axial spondyloarthritis, a modified endurance battery showed moderate to substantial reliability for front, side, and back trunk muscle chains, supporting its use for tracking progress in rehabilitation settings.26PubMed Central. Reliability of an adapted core strength endurance test battery in individuals with axial spondylarthritis The broader point is that core assessment is less about how much you can lift and more about how well and how long you can stabilize under modest demand.
Sitting All Day and What It Does to Your Core
Prolonged sitting has become the default posture for a huge proportion of the working population, and it changes how the core muscles behave. People who develop low back pain from prolonged sitting show altered activation patterns in their abdominal muscles specifically while sitting upright, the very position that provokes their pain. Their external oblique muscles are thicker (suggesting overactivation or compensation), while their transversus abdominis activation patterns are altered compared to pain-free individuals. These changes do not appear when the same people lie down, confirming that the dysfunction is position-specific.27Scientific Reports. Prolonged sitting-induced back pain influences abdominal muscle thickness in a sitting but not a supine position
This finding reinforces why “core strength” and “core function” are not the same thing. Someone might score well on a plank test yet have dysfunctional activation patterns in the exact posture they spend eight hours in every day. Addressing sitting-related back pain often requires retraining how the core activates during sitting, not just making it stronger in a gym setting.
An Evolutionary Footnote
Given how central the core is to human movement, you might expect that our trunk anatomy would show dramatic evolutionary adaptations to walking upright. It turns out the changes are surprisingly modest. The habitual lumbar curve (lordosis) is one clear adaptation to bipedalism, but the overall organization of the muscles along the body axis evolved in four-legged animals and proved workable for two-legged walking without major redesign.28Pathophysiology. Evolutionary aspects and muscular properties of the trunk—Implications for human low back pain In other words, we are running a bipedal operating system on hardware largely inherited from quadrupeds. This mismatch may be part of why low back pain is so staggeringly common in humans: the core muscles are doing a job they were not originally optimized for, using a spinal column that was shaped for distributing loads on all fours.
Asymmetric loading makes this worse. Carrying a bag on one side, for example, shifts the dominant trunk muscle forces to the opposite side and alters the moments at the lower lumbar spine during walking.29PubMed. Effects of asymmetric load carrying on the biomechanics of walking Over time, habitual one-sided loading can create imbalances in core muscle recruitment that compound existing vulnerabilities in the system. Something as simple as switching which hand carries a heavy bag, or using a backpack instead, reduces the asymmetric demand on the trunk muscles and the forces on the lower spine.