What’s in a Cramp?

A muscle cramp, or spasm, occurs as a result of prolonged involuntary contraction of a muscle group, a single muscle, or a group of fibres within a muscle. This may last from between a few seconds up to several minutes. A spasm may sometimes be felt in its mild form as a continuous twitching, but is usually characterised by an intense, stabbing pain that is felt through the affected muscle. Movement is restricted and the muscle usually feels hard to touch and may appear distorted while the spasm lasts. Spasms may recur a few times before going off completely.

Types of Muscles

Muscles are specialised tissues within the body of an organism that are mostly made of proteins and are largely controlled by electrical impulses from the nervous system and chemicals such as hormones (e.g. adrenaline). They are usually associated with a good capillary network that provides a constant supply of oxygen, glucose and nutrients for cellular respiration, and also removes carbon dioxide and other waste products.

There are three main types of muscles in the human body:

  • Skeletal muscles – These are also known as striated muscles, due to their striated (striped) appearance, or voluntary muscles as they are under the control of the voluntary nervous system. These muscles are attached to the bones of the skeletal system and are predominantly responsible for bone movement by means of contraction and relaxation. They usually contract rapidly in response to stimuli, and subsequently also fatigue quickly.
  • Smooth muscles – These muscles are also known as involuntary muscles as they are under the control of the involuntary nervous system. They include the muscles present in the walls of blood vessels, the gut wall, etc. and are responsible for the movement and transport of materials within these respective physiological systems. They are non-striated and usually contract, and therefore fatigue, slowly.
  • Cardiac muscle – These are specialised muscles found only in the heart. They are stimulated within the heart itself and do not fatigue throughout the organism’s lifetime. (Refer https://thebiolog.com/2021/10/22/heartbeat/ for more information on the heart.)

More often than not, it is skeletal muscles that we refer to when we talk about muscle cramps although smooth muscles may also experience cramping, albeit on rare occasion.

Anatomy of a Skeletal Muscle

The anatomy of a skeletal muscle is a somewhat complex one. However, for the purpose of this article, I shall attempt to keep things as concise as possible.

Muscle tissue comprises of many long cells known as muscle fibres which are held together by connective tissue. In skeletal muscles, these are made of long contracting fibres lying parallel to each other, known as myofibrils. The individual units present within these myofibrils are called sarcomeres, the structure of which is made up of actin and myosin proteins. These two proteins are responsible for the contracting mechanism of skeletal muscles, induced by the presence of calcium ions (Ca2+) which are released in response to impulses from the voluntary nervous system.

Detailed structure of a skeletal muscle
Pearson Edexcel International A Level Biology Student Book 2

Muscle Contraction

In brief, when Ca2+ are present in the space surrounding the proteins, they attach to specific binding sites within the muscle fibres, resulting in a series of reactions that causes the formation of cross-bridges between actin and myosin proteins, known as actomyosin. This subsequently causes the proteins to get pulled close together and slide across each other. This phenomenon is often referred to as the ‘sliding filament theory/model of contraction’, and as this occurs across all the myofibrils of a particular muscle, it results in the contraction of that muscle. Adversely, when there is no nervous stimulus for Ca2+ to be released, the actin and myosin proteins remain in their resting state, thereby allowing the respective muscle to relax.

Sliding Filament Model of Muscle Contraction

Muscle Spasms and Potential Causes

Muscle spasms, or cramps, occur when the contraction described above takes place involuntarily, often getting ‘locked’ for a while. It is not clear as to what exactly causes cramps, for which reason the condition is categorised as idiopathic.

Comparison of a muscle when relaxed, when contracted and in spasm

However, there are certain predispositions that are believed to be risk factors for the incidence of cramps. These include:

  • muscle fatigue – such as from intense exercise
  • inadequate stretching or poor conditioning – doing regular stretching may help muscles to contract more easily and efficiently during exercise or activity
  • introduction of a new activity or movements that your body is not used to
  • excess heat, dehydration – loss of body fluids may increase the likelihood of cramping
  • electrolyte depletion – salts and minerals such as magnesium, sodium, potassium and calcium are essential for the proper functioning of muscles; depletion of these due to fluid loss (through excessive sweating, dehydration, vomiting, diarrhoea, etc.) or imbalance due to a poor diet could induce cramping
  • medical conditions such as nervous disorders (e.g. involuntary nerve discharges), nerve compression (particularly in the lower back, e.g. sciatica), diabetes, hypothyroidism, kidney or liver disorders
  • impaired blood circulation – e.g. due to atherosclerosis or narrowing vessels at extremities
  • being overweight
  • sedentary lifestyle – this is thought to be particularly associated with nocturnal leg cramps
  • age – older people are more likely to experience cramping due to natural atrophy (muscle loss, thereby over-stressing the remaining muscles), reduced activity, and general slowing down of the body
  • consumption of certain medications that have a diuretic effect, such as to lower blood pressure
  • pregnancy – cramps may occur frequently during pregnancy

In general, cramps most commonly occur in the calf muscle (sometimes referred to as ‘charley horses’), hamstring, quadriceps and feet muscles. However, they are also known to occur in the abdominal muscles, arms and hands.

Treatment and Prevention of Muscle Spasms

There are a few measures that could be taken to relieve or reduce the discomfort experienced when a cramp occurs:

  • stop any activity that might have brought on the cramp
  • gently stretch and lightly massage the cramped area
  • apply heat if the muscle is tight or tense, or ice if the muscle is sore or tender

A few general measures that can be practised in order to avoid the incidence of muscle spasms include:

  • doing some form of light exercise regularly
  • warming up before intense exercise, and doing some stretching after
  • keeping hydrated, particularly during exercise and in warm climates
  • maintaining a healthy, balanced diet that includes sources of magnesium, potassium, sodium and calcium
  • taking any vitamins and supplements that your doctor might recommend for your age, gender, medical history, etc.
  • being aware of diuretic or other side effects of any medications you have been prescribed, and taking measures to balance such effects

If muscle spasms occur frequently and do not subside despite taking measures to avoid them, it is best to speak to your GP in order to determine if there might be any underlying conditions that might be contributing to this.

Cover illustration from Albuquerque Journal

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