Exercise Training and Sports Performance - Electrotherapy Application for the Treatment of Delayed Onset of Muscle Soreness (DOMS)

Physical exercise is associated with improved health, wellness and physical performance. The fitness and health related benefits derive from acute physiological responses and chronic adaptations in response to regular physical training. A progressive increase in the workload is required to elicit more pronounced physiological responses and adaptations.

Athletes engage in specific training programs in order to improve performance in their respective sports. ?This often requires athletes to perform high intensity and high volume exercise regimens, which are necessary for enhancing physical performance. However, high workload training regimens place a great deal of stress on the body, and can lead to delay onset of muscle soreness (DOMS). Muscle soreness is an exercise-induced phenomenon that occurs several hours to days after unaccustomed or strenuous physical exercise.

DOMS is classified as a type I muscle strain injury, and the symptoms include muscle stiffness and pain to palpation and/or movement, which usually peak between 24 and 72 hours after exercise. The etiology of DOMS has not been fully elucidated, but it is believed that high-force muscle contractions cause micro-damage to the muscle architecture and to the sarcolemma, resulting in calcium homeostasis disruptions, immune and inflammatory responses, with accumulation of inflammatory mediators leading to the activation of nociceptors and pain development. There is evidence to suggest that eccentric contractions are more likely to cause DOMS. This because, during eccentric contractions the muscle lengthens while generating force, and the cross-bridges are separated with greater force due to disruption of the actin-myosin bonds before relaxation. Consequently, a greater force is developed within the active motor units, which increases the risk of injury to the myotendinous junction. In addition to causing discomfort, DOMS limit the ability to train and perform.

Generally DOMS is a transient phenomenon, however, if not managed appropriately it may lead to more serious injuries. The causes and mechanisms of DOMS development are intricate. There is evidence to suggest that DOMS occurs following physical exercise, and is the result of complex interactions involving various factors such as muscle damage, inflammation and calcium accumulation.?

Skeletal muscle tissue includes type I muscle fibers, also known as slow twitch, and type II fibres, also called fast twitch, and the content and composition of their connective tissue appears to be different. Type II muscle fires are more susceptible to injury, as they exhibit a less robust structure compared to type I. Consequently, physical activities requiring high-force muscle contractions, may result in excessive strain of type II fibers connective tissue and lead to DOMS. Moreover, performing eccentric exercise can result in microscopic lesions of contractile components, or even myofibrillar disruption of the z-line, as well as more widespread disruption of the sarcomere architecture. This damage is more likely to occur during eccentric contractions because there is a decrease in active motor units, leading to an augmented tension per unit area. Besides, type II muscle fibers exhibit the narrowest and weakest z-lines, and consequently they are more prone to mechanical disruption to their structural elements. The sensation of pain arises from nociceptors activation, which are present in the musculotendinous junction, connective tissue and local vasculature.

The notion of muscle damage is further supported by the high plasma levels of the enzyme creatine kinase (CK). This enzyme is present in skeletal and cardiac muscles, and is considered an indicator of membrane permeability and tissue damage. Following eccentric exercise, circulating CK rise dramatically above normal levels, suggesting the presence of damage to the muscle tissue and increased sarcolemma permeability. Even though there is a discrepancy between peak muscle soreness and peak in serum CK levels, however, micro-lesions and structural damage to the muscle tissue are important factors associated with DOMS.

Besides, repetitive eccentric exercise can lead to edema and inflammatory cells infiltration, which are indicators of an inflammatory response. This occurs as a result of damage to the muscle tissue structures, and accumulation of inflammatory mediators, including bradykinin, prostaglandins and histamine at the injury site, which attract the immune cells neutrophils and monocytes. The increased permeability of the local vasculature leads to an influx of exudate and edema development. The accumulation of this protein rich fluid in the affected area, increases the osmotic pressure, activating the nociceptors. It appears that there is a relationship between peak edema levels and peak muscle soreness. Also, this can be exacerbated by the conversion of monocytes into macrophages, which accumulate at the injured site and produce compounds that sensitize pain receptors.?

In a healthy muscle tissue, calcium is stored in the sarcoplasmic reticulum, but as a result of muscle structure and sarcolemma damage, calcium ions leak out and accumulate at the injured site, which can affect cellular respiration and ATP synthesis, and may also activate the enzymes proteases and phospholipases, resulting in further injury to the sarcolemma and production of the inflammatory mediators prostaglandins and leukotrienes, which also can contribute to pain development.

In addition, there are other factors that can be involved in DOMS development, and these include lactic acid accumulation and muscle spasm. However, lactic acid may be involved in pain occurring shortly after high-intensity exercise, but it is unlikely that it plays a significant role in DOMS, because muscle soreness can last from several hours up to few days, while lactic acid levels drop considerably within one hour after the cessation of exercise. A muscle spasm is associated with increased muscle tension, augmenting the intro-muscular pressure and compressing the local vasculature, leading to ischemia with accumulation of metabolites and other molecules that can cause pain. This, in turn, can increase the spasm, aggravating the ischemia. However, various studies used electromyography (EMG) to evaluate skeletal muscles activity during DOMS and the results were mixed. Some of these investigations showed no increase in EMG activity in sore muscles, and other investigations showed increased EMG activity, but no relationship between its magnitude and perception of soreness.

The findings from all these studies suggest that various factors, including damage to the muscle architecture and connective tissue, inflammation and calcium accumulation at the site of injury, play a major role in DOMS development.?However, the contribution of other factors to DOMS, such as lactic acid accumulation and muscle spasm cannot be ruled out. Given the numerous factors involved in DOMS and their complex interactions, there is consensus among the scientific community that muscle soreness, observed following physical exercise, is likely to be the manifestation of a multifactorial phenomenon.

There is evidence to suggest that DOMS has negative repercussions on athletic performance. DOMS can be experienced by both novice and elite athletes, and the severity varies from mild, which is felt during physical activity related to daily routines, to severe pain that restricts movements. ?The structural damage to the muscle and connective tissue, associated with DOMS, can adversely affect muscular performance, both from voluntary reduction of effort and from inherent impaired muscle capacity to generate contractile force. In general, the discomfort and decrease in performance are transient, and usually permanent impairment does not occur. The transient decrease in athletic performance and training capability, associated with DOMS, can be attributed to alterations in muscle function and joints mechanics. Research has revealed that during DOMS there is a decrease in muscular strength and reduced range of motion, which represent important functional limitations. Moreover, the alteration in joints proprioception leads to an overestimation of force generation and impaired kinesthetic control, impacting on the ability to perform tasks.

Physical activity and athletic performance require the activation and coordination of different muscle groups. Depending on the task, skeletal muscles have to generate concentric, eccentric and isometric contractions in a coordinated manner, with the appropriate temporal sequencing. A number of studies showed that parameters of strength and power during DOMS are reduced in concentric and isometric contractions, but especially in eccentric contractions. Peak torque deficit occurs 24-48 hours after exercise, and appears to be more profound and persistent during eccentric contractions. The strength levels during concentric and isometric contractions return to baseline within 5 days, whereas it may take up to 8-10 days for strength during eccentric contractions to return to baseline. The altered muscle recruitment patterns and neuromuscular control, associated with DOMS, can impact on coordination, temporal sequencing and ability to perform tasks.

The severity and duration of DOMS varies among individuals, however, until full recovery from DOMS is achieved, physical performance and ability to train are compromised in both amateur and elite athletes. This because DOMS is characterized by muscle tissue damage, loss of strength and coordination, as well as functional deficiency. People striving to maintain high level of fitness, and athletes participating in sports events and competitions, need to perform high-intensity and high volume training regimens. As a consequence, many individuals tend to continue training when the muscle pain is still present, which can be detrimental. Even though DOMS is considered a sub-clinical injury, however, it needs to be managed well, and performing heavy training before adequate recovery from DOMS is achieved, can result in strain or serious and debilitating injuries.

Research has demonstrated that the application of electrotherapy improves DOMS. A double blind, placebo controlled study, evaluated the effect of microcurrent stimulation (MCS) therapy on DOMS. Eccentric exercise was used to provoke DOMS in the flexor muscles of the elbow. In all participants, the elbow flexor muscles were assessed intermittently for up to 7 days following exercise, for muscle soreness, function, inflammation, and passive shortening. The results showed that MCS therapy improved different clinical manifestations of tissue damage, including decreased serum CK levels, reduced muscle shortening and maintenance of maximum force production of the elbow flexor muscles. ?

Naclerio and co-workers, conducted a double blind, placebo controlled study to evaluate the effects of resistance training in combination with either MCS or sham treatment on DOMS. A specific exercise protocol was implemented to induce DOMS. Participants followed an identical resistance training routine, consisting of 3 sessions per week, for a total of 8 weeks, and received either a 3-hour daily MCS therapy or sham therapy, immediately post workout or during the morning on non-training days. All participants had at least 2 years of resistance training experience. The results revealed that MCS application attenuated DOMS, which was assessed over a period of 12-hours to 48-hours.

A study, conducted by Curtis and colleagues, compared the effects of MCS therapy versus sham therapy on DOMS, which was induced by implementing an eccentric contractions exercise protocol. The individuals who took part in the study included young males and females. Muscle soreness was assessed at baseline and at 24, 48 and 72 hours post-exercise. The authors reported that MCS application provided significant protection from DOMS at all time points tested.

Bioelectrical phenomena are central to numerous physiological functions, including tissue repair. Trauma or disease can alter normal bioelectricity, resulting in impaired ability to repair injured tissue. MCS is one of the electrotherapy modalities, which involves the delivery of energy in the form of an electron flow in the microamperes range (μA) to emulate the endogenous bioelectricity. The findings from various studies indicated that MCS application is capable of bringing about different health and wellness related benefits. ?The improvements of DOMS, derived from MCS application, can be attributed to its ability to replicate/activate normal endogenous current flow to the injured tissue, augment ATP and protein synthesis, and increase transport of metabolites across the plasma membrane, as well as accelerating the repair processes. These physiological changes help re-establish homeostasis in the traumatized tissue, creating an optimal environment that promotes healing. Besides, the ability of MCS to ameliorate the symptoms of muscle soreness, are derived from its electroanalgesic and anti-inflammatory properties.??

Considerations and Conclusion

Physical exercise promotes health, and ameliorates sport performance and fitness. Performing high intensity and high volume training regimens are associated with profound physiological responses and adaptations that bring about the greatest fitness and performance gains. However, these types of training programmes can result in DOMS, which may be experienced by both novice and elite athletes. The symptoms of DOMS include loss of strength and coordination, as well as functional deficiency, which can impact on physical performance and recovery time. MCS application represents an efficacious intervention that can be applied to improve DOMS.

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