Interoception is the sense that answers the question: "How am I feeling?" in any given moment. It is one of our lesser known senses. Interoception is the perception of bodily sensations- the tingling, throbbing, heat, coldness, pulsing, swelling, tickling, perspiration, contraction, expansion, numbness, or pain you may feel even now as you read this.
The body is performing millions of vital functions now without input from the seeming self or ego. Without thinking, the heart beats, food is digested, cells replicate, proteins are synthesized, toxins are released, bones grow, muscles are repaired, electrolytes are balanced, and you are breathed. These processes and the sensations that accompany them remain largely unconscious. You can feel this aliveness pulsing through you now.
Interoceptive awareness brings these processes to conscious awareness (Cameron, 2001). The ability to consciously monitor and feel certain physiological states (such as thirst and hunger), detect potential tissue damage or pain is essential for species survival- any species' survival. This neural circuitry is ancient and predates us.
In today's modern world, the ability to identify, access, understand, and respond skillfully to the body's signals helps us meet life's challenges and stressors (Craig, 2015). Interoceptive awareness can be trained. Simply check in: what are you experiencing in your face and head now? In your neck and shoulders? In your arms and hands? In your chest and abdomen? Back? Hips? Thighs? Legs? Feet? We are not using imagination. We are simply observing. This is a useful skill to develop in a world that manufactures so much unnecessary stress.
The term “stress,” originally introduced by Selye (1956), refers to a challenge or stimulus, psychological or physical in nature, that threatens (or that is perceived to threaten) the self. A stressor is anything that triggers a physiological response: increased heart rate, rapid breathing, muscle tension, etc. Stressors can be “real” (e.g. a category 5 hurricane heading through your neighborhood), or "imagined" (e.g the body language of colleagues during your presentation). The intensity of our reaction to a stressor is highly individual and situationally dependent (Dewe, 1993; Peters et al., 1998). One person may be terrified of a garden snake, for example, another person may be fascinated by the same snake. Storylines, implicit or explicit neurocognitive appraisals, coping strategies, social support, and past experiences are variables that affect the physiological stress response in any given situation (Anshel et al., 1997; Anderson et al., 2002; Barbenko et al., 2015; Ambeskovic et al., 2017).
How the body responds to a stressor occurs largely outside conscious awareness. (Le Doux and Pine, 2016; Ginty et al., 2017). In response to a stressor, the brain coordinates a response. The sympathetic branch of the autonomic nervous system (ANS) is activated. The ANS is divided into sympathetic (SNS) and parasympathetic (PNS) branches (Thayer and Sternberg, 2006). Although the relationship between the SNS and PNS is complex and should not be thought of as a binary “either/or” system, it is generally accepted that during a stress response, the SNS is activated and PNS, responsible for calming and stabilizing the body, is dialed down (Thayer and Sternberg, 2006). The degree of a SNS response is thought to be determined by one’s perception of how threatening the stimulus is, even if the perception is not within conscious awareness (Kalisch et al., 2015; LeDoux and Pine, 2016). Further, the physiological responses during stress can be enhanced or diminished by psychological factors, such as perceived control over the situation (McEwen, 2008).
Interoceptive training provides a degree of control over these responses when the stressor is mild to intense (depending on one's level of mastery). We begin by learning to observe sensations objectively as they arise. As I sit typing this, for example, I feel mild contraction in the trapezius muscle. The words I use to describe the sensations are not important. In fact, words are a distraction to direct experiencing. We can know something without labels; we can know something without giving it a name. If I investigate more deeply, I sense the radius and depth of the contraction, its quality, its intensity. I can relax it or let it be.
Understanding how the body and brain work can give us some leverage. SNS arousal can be beneficial and adaptive. We call this eustress or good stress. Alertness, focused attention, and improved cognitive performance are some benefits of adaptive SNS arousal or "good" stress (Jamieson et al., 2010). Low to moderate stress can improve working memory resulting in better mental performance (Oshri, et al., 2022). During adaptive SNS arousal, sensory perception including visual, auditory, and olfactory senses are also enhanced (McNish and Davis, 1997). Improved sensory/interoceptive awareness increases our ability to successfully address a stressor (Kalisch et al., 2015). In meditation, we learn to leverage this. Some meditators intentionally induce this adaptive SNS arousal prior to meditation with breath work (e.g. Wim Hof Method, bhastrika pranayama, breath holds, kapalbhati, etc), ice baths, or exercise (qigong, taichi, yoga, high intensity interval training, etc). We introduce mild stressors to prime the mind and to condition our response to stress. We learn to dial the stress response up or down. This gives us a measure of control. This sense of empowerment is called self-efficacy (Bandura, 1977, 1986, 1997).
If a stressor does not dissipate immediately (that is, if I don't have strategies for switching it off or dialing it down), the brain will initiate an endocrine response following the activation of the SNS. The endocrine response triggers the activation of the hypothalamic-pituitary-adrenal (HPA) axis. The hypothalamus (the H in HPA-axis) releases hormones including corticotropin releasing factor (CRF) (DeRijk and de Kloet, 2005; Frasch et al., 2018). CRF in turn triggers the release of adrenocorticotropic hormone from the pituitary (the P in the HPA axis), which then reaches the cortex of the adrenal glands (the A in the HPA axis) to stimulate secretion of adrenal glucocorticoids (GC). This happens in milliseconds. Interoceptive training allows us to maintain a degree of equanimous awareness and objectivity as this process unfolds.
The increase of adrenal GCs in the blood, such as cortisol, is the physiological correlate of stress (de Kloet et al., 1998). Blood circulates GCs to reach any organ in the body. You can feel this as an adrenaline rush. With training, you can feel this flooding even whilst sitting still in meditation. If, for example, while I am sitting, I hear a sudden and unexpected noise, the startle response might trigger activation of the SNS. I'll feel a rush of adrenaline, my heart rate may accelerate, my muscles may tighten, my breath may quicken. An emotion may arise as may thoughts or commentary.
Muscles often become tense during stressful situations (Malmo et al., 1951). Lundberg et al., (1994) reported a link between trapezius myalgia (neck stiffness and pain) and increased stress. In a follow up 2002 study, he found that repetitive movements and mental stress may be related to the development of upper extremity disorders. Lundberg's study suggests that psychological stress, with or without physical load, may play a role in musculoskeletal disorders by increasing muscle tension through activation of low threshold motor units. This constant tension creates overload and degeneration over time.
The trapezius muscle runs from the back of the neck to the shoulders and down the middle of the back. Tension tends to nest here. This muscle is recruited in a fixed order. If you intentionally raise your shoulders to your ears, you are recruiting smaller and larger units. If you relax them somewhat, but not completely, smaller units may remain active. Over time, this can lead to neck, shoulder, and back pain.
It has been hypothesised that, in some cases, the pain is due to an overuse of low threshold muscle fibres- causing damage at the cellular level. Muscle fibres called low threshold motor units (MUs) are recruited at the onset of muscle activation. They fire continuously until the muscle is relaxed completely.
The Cinderella hypothesis (Hägg, 1991) postulates that if these smaller MUs remain active long enough, the strain can actually damage muscle fibers- leading to peripheral muscle pain in the neck, back, and shoulders. This can occur even during periods of "rest."
Lack of mental rest is an important risk factor for the development of muscular pain. (Lundberg, 2002) In some techniques, like progressive muscle relaxation, we practice letting go and relaxing the smallest functional units fully and completely. Meditation techniques that focus on relaxation facilitate physical and mental rest.
Stress may contribute to keeping low threshold motor units active, even in the absence of physical demands. During meditation, you may feel these muscles contract. We visit and revisit certain sites where tension tends to build- the trapezius, the masseter muscles of the jaw, the muscles around the mouth, and the frontalis, corrugator supercilii, orbicularis oculi, and procerus muscles around the eyes and eyebrows.
Changes in recruitment thresholds have been observed in the biceps brachii muscles (Christova et al., 1998) and the three heads of the triceps (Hussain, 2020). So, we relax the shoulders and arms, too. It is not uncommon for people to clench their fists when stressed. We pay attention to the hands as well. While I have yet to find studies on muscle activation in the thighs, from experience, I know these muscles are also activated.
As we scan, we find patterns. Tension does not nest everywhere. I don't pay attention to the back of my head or around the ears when scanning for tension. The muscles of the forehead, around the eyes, mouth, and jaw are more likely to be contracted. As I scan from head to toe again and again, I keep checking key areas and keep relaxing more and more, deeper and deeper, until the body is fully relaxed. It is easier to relax the mind, when the body is relaxed. Conversely, it is very difficult to still the torrent of thoughts when the mind is agitated. So, better to begin with the body.
Depending on the duration and severity of the stress, the response to glucocorticoids can promote or impede brain function and adaptive behaviors. Sitting still in meditation becomes difficult or near impossible for beginners when the mind is overwhelmed. It may take some time for these chemicals to dissolve back into the body. Skilled meditators have other strategies for mitigating strong emotions. Breathwork is one way. There are many breathing techniques that can change our physiological response. Ph balance, O2 saturation, heart rate and blood pressure change. Reframing the experience as an opportunity to practice non-reactivity, as a kind of high-intensity training for the mind, is another strategy. Adjusting our attitude and approaching the stressed and agitated mind with openness, curiosity, and kindness is yet another approach. We relax into the stress. Busyness is not the obstacle; identifying with thought is the obstacle. Resisting what is is the obstacle. The idea that the mind should be still, pliable or obedient agitates and stresses the mind. Under the weight of these "shoulds" or demands the mind becomes restless. Mental stress may express as a sensation or a flood of sensations, the greater the resistance and hostility to what is. So, we approaching meditation with the intention not to engage with arising thoughts - however compelling or emotional charged. We approach with no expectation and allow whatever arises to be.
Movement is another strategy for dissipating that energy. We use the adrenaline to move: walking, intense asanas (poses), vigorous qigong or tai chi exercises or other forms of movement may help settle a restless body and agitated mind.
Interoceptive training and muscle relaxation is another strategy, and the focus of this blog post.
Stress induces muscle fatigue. Muscle sympathetic nerve activity (MSNA) is a key mechanism for maintaining tension during physiological stress. At the cellular level, motor neurons discharge bursts of action potentials to keep muscles primed to fight, flee, or freeze. You can feel this in the body. Your brain forms behavioral responses. At the most basic level, you may feel a sensation- like muscle tension. How the brain interprets this sensation may lead to an evaluation. That evaluation leads to a behavioral response.
In meditation, we interrupt this process, by simply observing and not reacting to whatever sensations arise. You realize, from your own experience, how the brain reacts to conditioned neural patterns in response to stress. These patterns inform your ANS and create habitual responses. By simply sitting, you "see" how these conditioned patterns unfold and how the tension keeps creeping in. We keep relaxing into whatever arises, intentionally leaning in, relaxing, letting go of tension.
What might this look like on the cushion? Sit long enough. At around the 30-45 minute mark, you may begin to feel discomfort and a strong urge to call it quits, stand up and leave. Our brains are conditioned to "flee" unpleasant stimuli. This is a survival response. Sitting for extended periods causes discomfort. The sensations are one thing; the interpretation is another, and the subsequent action is another. If I sit with the expectation that I will still the mind, I am setting myself up for an error response, frustration, and failure. It is challenging to work at this level of mind. So, we start with what most readers can control to a degree: the muscles. And we practice relaxing the muscles from head to toe.
(Note: for those with more severe conditions, I recommend the work of Vidyamala Burch who sustained spinal injuries and partial paraplegia, Jack Kornfield who suffers from a neuromuscular disorder, Thich Nhat Hanh, who suffered a paralyzng stroke, or SN Goenka, who was confined to a wheelchair during the final years of his life)
This is experiential learning at a subtle level. At the molecular level, a thought can trigger an action potential in motor neurons which triggers depolarisation and calcium ion release. The myosin heads form a cross-bridge with the actin filaments within the muscles. The myosin heads move the actin filaments the way an oar propels a row boat. Actin filaments are anchored to Z lines. The dragging of actin pulls the Z lines closer together, shortening the sarcomere (the muscle fibers) and causing contraction.
As I keep relaxing the body, I imagine a more subtle body that slows these signals and neural patterns. I imagine relaxing at that level. The body gives me constant feedback. I know whether the firing rate has increased or decreased by observing. Am I relaxed fully and completely? With practice, I get better at this and can fall into a relaxed state deeper and faster. Once the body is fully relaxed, I can relax the mind. Once the mind is relaxed, I can focus. Once focused, I can maintain single-pointed attention on the breath.