Introduction
Smooth muscle is the muscle that is found lining our hollow organs. That is to say, our intestines, bladder, blood vessels, uterus ( if applicable). It is also the muscle that closes sphincters (entry-exit to and from stomach, anus, urethra) and in the eye operates our iris and focuses the lens.
Smooth muscle, like cardiac muscle is not under our direct nervous control (as is skeletal muscle), it’s actions are, for the most part, autonomous and we are either unaware of its action (except during childbirth or intestinal or renal colic) Smooth muscle is also very strong, gramme for gramme when compared to skeletal muscle. Smooth muscle cells have mitochondria as do skeletal and cardiac muscle cells, and cell-death occurs in all these tissues using the same mechanisms.
Smooth muscle is not studied in any way comparable to skeletal or cardiac muscle but what has been carried out on the physiological and biochemical level does not suggest any fundamental difference in the tissues on the sub-morphological-histological level. It follows that it is reasonable to propose that smooth muscle will age as both cardiac and skeletal muscle ages.
It is also reasonable to propose that smooth muscle will be adaptive in a way analogous to hear and skeletal muscle. By this I mean that smooth muscle should respond to physiological demand in an adaptive sense, ie it can be conditioned, improved, by reasonable demand and use... ie exercised.
In old age a phenomenon with skeletal muscle called sarcopenia occurs when muscle mass is lost as mitochondria 'decommission’ old cells. This happens for a variety of reasons. The cells may be damaged or simply unused and have been marked ‘parked’ and have not been used for a long time, maybe due to illness, starvation or incapacitation.
For skeletal muscle we know that regular exercise, especially load bearing exercise, coupled with a good diet mitigates early sarcopenia. For skeletal muscle the 'use it or lose it' principle applies as old age encroaches. Cardiomyopathy does not exhibit sarcopenia with age but instead shows a thicken of non-functional muscle in the heart's cell walls. Again though physical activity benefits heart muscle seemingly through improving its uptake of oxygen and reducing stress hormones. In both cases the point is that these muscles respond to stressors* in a good way and adapt and condition as a result.
It seems, to repeat myself, unlikely to say the least, that smooth muscle cannot also be conditioned and de-conditioned through its physiological activity in respect to physical challenges.
The De-conditioning life-style
Let's take smooth muscle function tissue by tissue choosing four major groups..
1) Eyes. Pupil dilation and lens focus are controlled by the iris sphincter muscle and the lens' ciliary muscle, both are smooth muscles and not under our conscious control. A modern office worker will typically experience only regulated artifical light which will be similar if slighly less at home. In both cases the ambient light will be unchanging. In term of focal distance 'Work' and leisure today is heavily concentrated on screen focal distances be it phone, computer or TV. In other words these muscles are not called upon to do much work.
2) Guts. Motion through the gut and retention within the gut are carried out by smooth muscle. Three sphincters seal off the gut into compartments. Two enclose the stomach sealing it off while its muscles churn and emusifies food in a highly acidic environment. The third, the anal sphincter prevents waste from digestion being evacuated inconvenienty before water has been recalimed into the body. A 'mexican wave' of smooth muscle contraction known as peristalsis, moves liquids and semi-liquids and solids through the gut. De-conditioning these muscles is easy. Simpy put, food ingested approximates to pre-mastcated, pre-digested baby food. This is pretty much a description of many modern diets.
For example, a burger pattie is the norm today as the primary intake of meat. A pattie is basically macerated and emulsified meat and fat, saving the stomach substantial amounts of work. Carbohydrate intake is biased away from high fibre complex carbohydrates to oligosacharrides and simple sugars, again saving the gut work. Fat intake is overwhelmingly in what is termed 'hidden fat' rather than solid fat, obviating the need for active emulsfication by the gut's muscles.The modern gut has little to do when comapared to a hunter-gatherers gut.
Finally sphincters! Reflux and incontinence are common afflictions of age. These imperfectly closed and easily overhwelmed valves also need conditioning to retain their power in old age.
3) Lungs. Lungs have delicate envelopes of smooth muscle, they are inflated as a result of the lifting of the rib cage causing a reduction in pressure allowing air to enter the lungs. Positive pressures will damage the lung. Normal breathing is what is called 'tidal'. Only a small amount of air is ventilated when sedentary or even walking. The point is, deep breathing is reserved to hard exercsie or deliberate full inhalation. Sedentary workers do not make demands on the lungs smooth muscle.
4) Arteries. Arteries have muscular walls to their vessels. Major arteries such as the aorta have thick powerful layers of smooth muscle. Heavy work such as lifting objects increase blood pressure. It does so transiently but for instance it is not uncommon fo weightlifters to develop arterial pressures in excess of 400mmHg. Modern medicine and lifestyle works hard to maintain a near constant ideal blood pressure of 120/80mmHg ( systolic/diastolic) or even lower.
Discussion
I think that the assertion that smooth muscle in modern urban man/woman is given a lot less work to do as a result of a carefully controlled environment and life style.
The question is does it matter? To answer this question we can look at some of the age-related woes that beset the examples above. All examples given will be multi-factorial, that is they cannot be attributed to smooth muscle alone even so it's worth a quick canter through some common disorders.
Night-vision acomodation slows with age; time to focus at distance increases; constipation or IBS increases with age; sphincters lose effectiveness as in acid reflux and incontinence;lung capacity, and shortness of breath are typical as we age; arteries bulge with aneurisms and even rupture.
On the flip side is there any suggestion that placing demands on smooth muscle ( in the Goldilocks zone) has a beneficial effect?
For eyes, I can only think of advice given to heavy screen time users to get away from screen and re-focus every twenty minutes; for guts we have the proposed benefits of fibrous food, whole foods and good fecal-biome; for lungs deep breathing exercises are highly recommended and finally for arteries resistance training is very popular.
Of course in the above list of 'good for you' we do not expect to see 'good for your smooth muscles' but indeed all of them would exercise just those muscles.
It's time for a new set of health guidlines, 'Look after your smooth muscle' Below is a program of 'exercise'forsmooth muscle health.
1) Daylight for eyes
2) Fibrous and raw food ( eg vegetables, mushrooms, whole meat, beans, pulses) for guts
3) Resistance work ( weights ) for arteries and sphincters.
4) Aerobic exercise for lungs
*As an aside it is important to point out that such stressors as exercise need to be 'not too much and not too little' ie they are in the Goldilocks Zone.
