Malachy P. McHugh

Recent advances in the understanding of the repeated bout effect: the protective effect against muscle damage from a single bout of eccentric exercise

The repeated bout effect refers to the adaptation whereby a single bout of eccentric exercise protects against muscle damage from subsequent eccentric bouts. While the mechanism for this adaptation is poorly understood there have been significant recent advances in the understanding of this phenomenon. The purpose of this review is to provide an update on previously proposed theories and address new theories that have been advanced. The potential adaptations have been categorized as neural, mechanical and cellular. There is some evidence to suggest that the repeated bout effect is associated with a shift toward greater recruitment of slow twitch motor units. However, the repeated bout effect has been demonstrated with electrically stimulated contractions, indicating that a peripheral, non‐neural adaptation predominates. With respect to mechanical adaptations there is evidence that both dynamic and passive muscle stiffness increase with eccentric training but there are no studies on passive or dynamic stiffness adaptations to a single eccentric bout. The role of the cytoskeleton in regulating dynamic stiffness is a possible area for future research. With respect to cellular adaptations there is evidence of longitudinal addition of sarcomeres and adaptations in the inflammatory response following an initial bout of eccentric exercise. Addition of sarcomeres is thought to reduce sarcomere strain during eccentric contractions thereby avoiding sarcomere disruption. Inflammatory adaptations are thought to limit the proliferation of damage that typically occurs in the days following eccentric exercise. In conclusion, there have been significant advances in the understanding of the repeated bout effect, however, a unified theory explaining the mechanism or mechanisms for this protective adaptation remains elusive.

Exercise-Induced Muscle Damage and Potential Mechanisms for the Repeated Bout Effect

Unfamiliar, predominantly eccentric exercise, frequently results in muscle damage. A repeated bout of similar eccentric exercise results in less damage and is referred to as the ‘repeated bout effect’. Despite numerous studies that have clearly demonstrated the repeated bout effect, there is little consensus as to the actual mechanism. In general, the adaptation has been attributed to neural, connective tissue or cellular adaptations. Other possible mechanisms include, adaptation in excitation-contraction coupling or adaptation in the inflammatory response.The ‘neural theory’ predicts that the initial damage is a result of high stress on a relatively small number of active fast-twitch fibres. For the repeated bout, an increase in motor unit activation and/or a shift to slow-twitch fibre activation distributes the contractile stress over a larger number of active fibres. Although eccentric training results in marked increases in motor unit activation, specific adaptations to a single bout of eccentric exercise have not been examined.The ‘connective tissue theory’ predicts that muscle damage occurs when the noncontractile connective tissue elements are disrupted and myofibrillar integrity is lost. Indirect evidence suggests that remodelling of the intermediate filaments and/or increased intramuscular connective tissue are responsible for the repeated bout effect.The ‘cellular theory’ predicts that muscle damage is the result of irreversible sarcomere strain during eccentric contractions. Sarcomere lengths are thought to be highly non-uniform during eccentric contractions, with some sarcomeres stretched beyond myofilament overlap. Loss of contractile integrity results in sarcomere strain and is seen as the initial stage of damage. Some data suggest that an increase in the number of sarcomeres connected in series, following an initial bout, reduces sarcomere strain during a repeated bout and limits the subsequent damage.It is unlikely that one theory can explain all of the various observations of the repeated bout effect found in the literature. That the phenomenon occurs in electrically stimulated contractions in an animal model precludes an exclusive neural adaptation. Connective tissue and cellular adaptations are unlikely explanations when the repeated bout effect is demonstrated prior to full recovery, and when the fact that the initial bout does not have to cause appreciable damage in order to provide a protective effect is considered. It is possible that the repeated bout effect occurs through the interaction of various neural, connective tissue and cellular factors that are dependent on the particulars of the eccentric exercise bout and the specific muscle groups involved.

The association of hip strength and flexibility with the incidence of adductor muscle strains in professional ice hockey players.

This prospective study was conducted to determine whether hip muscle strength and flexibility play a role in the incidence of adductor and hip flexor strains in National Hockey League ice hockey team players. Hip flexion, abduction, and adduction strength were measured in 81 players before two consecutive seasons. Thirty-four players were cut, traded, or sent to the minor league before the beginning of the season. Injury and individual exposure data were recorded for the remaining 47 players. Eight players experienced 11 adductor muscle strains, and there were 4 hip flexor strains. Preseason hip adduction strength was 18% lower in the players who subsequently sustained an adductor muscle strain compared with that of uninjured players. Adduction strength was 95% of abduction strength in the uninjured players but only 78% of abduction strength in the injured players. Preseason hip adductor flexibility was not different between players who sustained adductor muscle strains and those who did not. These results indicate that preseason hip strength testing of professional ice hockey players can identify players at risk of developing adductor muscle strains. A player was 17 times more likely to sustain an adductor muscle strain if his adductor strength was less than 80% of his abductor strength.

To stretch or not to stretch: the role of stretching in injury prevention and performance

Stretching is commonly practiced before sports participation; however, effects on subsequent performance and injury prevention are not well understood. There is an abundance of literature demonstrating that a single bout of stretching acutely impairs muscle strength, with a lesser effect on power. The extent to which these effects are apparent when stretching is combined with other aspects of a pre‐participation warm‐up, such as practice drills and low intensity dynamic exercises, is not known. With respect to the effect of pre‐participation stretching on injury prevention a limited number of studies of varying quality have shown mixed results. A general consensus is that stretching in addition to warm‐up does not affect the incidence of overuse injuries. There is evidence that pre‐participation stretching reduces the incidence of muscle strains but there is clearly a need for further work. Future prospective randomized studies should use stretching interventions that are effective at decreasing passive resistance to stretch and assess effects on subsequent injury incidence in sports with a high prevalence of muscle strains.

Mechanical and physiological responses to stretching with and without preisometric contraction in human skeletal muscle

Abstract Objective: To examine electromyography (EMG) activity, passive torque, and stretch perception during static stretch and contract-relax stretch. Design: Two separate randomized crossover protocols: (1) a constant angle protocol on the right side, and (2) a variable angle protocol on the left side. Subjects: 10 male volunteers. Intervention: Stretch-induced mechanical response in the hamstring muscles during passive knee extension was measured as knee flexion torque (Nm) while hamstring surface EMG was measured. Final position was determined by extending the knee to an angle that provoked a sensation similar to a stretch maneuver. Constant angle stretch: The knee was extended to 10/dg below final position, held 10sec, then extended to the final position and held for 80sec. Variable angle stretch: The knee was extended from the starting position to 10/dg below the final position, held 10sec, then extended to the onset of pain. Subjects produced a 6-sec isometric contraction with the hamstring muscles 10/dg below the final position in the contract-relax stretch, but not in the static stretch. Main Outcome Measures: Passive torque, joint range of motion, velocity, and hamstring EMG were continuously recorded. Results: Constant angle contract-relax and static stretch did not differ in passive torque or EMG response. In the final position, passive torque declined 18% to 21% in both contract-relax and static stretch ( p p Conclusion: At a constant angle the viscoelastic and EMG response was unaffected by the isometric contraction. The variable angle protocol demonstrated that PNF stretching altered stretch perception.

Reconstruction of the Coracoclavicular Ligaments with Tendon Grafts A Comparative Biomechanical Study

Background Numerous surgical techniques have been described to address injuries to the coracoclavicular ligaments. Purpose To compare the biomechanical properties of tendon graft reconstructions with those of the native coracoclavicular ligaments and various other repair methods. Study Design Controlled laboratory study. Methods Eleven fresh-frozen human cadaveric shoulders were tensile tested to failure to compare the biomechanical properties of the native coracoclavicular ligaments, coracoacromial ligament transfer, No. 5 Mersilene suture repair, 5-mm Mersilene tape repair, and tendon graft reconstructions with gracilis, semitendinosus, and long toe extensor tendons. Results Reconstructions with semitendinosus, gracilis, or long toe extensor tendon grafts were found to have superior initial biomechanical properties compared with coracoacromial ligament transfer; failure strengths were as strong as those of the native coracoclavicular ligaments. Failure of the tendon grafts occurred through the midsubstance of the tendon graft, not at the fixation site. Conclusions Tendon graft reconstruction may be an alternative to coracoacromial ligament transfer and may provide a permanent biologic reconstruction with superior initial biomechanical properties, including that of tensile strength. Clinical Relevance Use of tendon graft reconstruction may limit the need for postoperative immobilization and lead to an accelerated rehabilitation program.

Influence of tart cherry juice on indices of recovery following marathon running

This investigation determined the efficacy of a tart cherry juice in aiding recovery and reducing muscle damage, inflammation and oxidative stress. Twenty recreational Marathon runners assigned to either consumed cherry juice or placebo for 5 days before, the day of and for 48 h following a Marathon run. Markers of muscle damage (creatine kinase, lactate dehydrogenase, muscle soreness and isometric strength), inflammation [interleukin‐6 (IL‐6), C‐reactive protein (CRP) and uric acid], total antioxidant status (TAS) and oxidative stress [thiobarbituric acid reactive species (TBARS) and protein carbonyls] were examined before and following the race. Isometric strength recovered significantly faster (P=0.024) in the cherry juice group. No other damage indices were significantly different. Inflammation was reduced in the cherry juice group (IL‐6, P<0.001; CRP, P<0.01; uric acid, P<0.05). TAS was ∼10% greater in the cherry juice than the placebo group for all post‐supplementation measures (P<0.05). Protein carbonyls was not different; however, TBARS was lower in the cherry juice than the placebo at 48 h (P<0.05). The cherry juice appears to provide a viable means to aid recovery following strenuous exercise by increasing total antioxidative capacity, reducing inflammation, lipid peroxidation and so aiding in the recovery of muscle function.

The Role of Passive Muscle Stiffness in Symptoms of Exercise-Induced Muscle Damage

We examined whether passive stiffness of an eccentrically exercising muscle group affects the subsequent symptoms of muscle damage. Passive hamstring muscle stiffness was measured during an instrumented straight-leg-raise stretch in 20 subjects (11 men and 9 women) who were subsequently classified as “stiff” (N 7), “normal” (N 6), or “compliant” (N 7). Passive stiffness was 78% higher in the stiff subjects (36.2 3.3 N m rad 1) compared with the compliant subjects (20.3 1.8 N m rad 1). Subjects then performed six sets of 10 isokinetic (2.6 rad s 1) submaximal (60% maximal voluntary contraction) eccentric actions of the hamstring muscle group. Symptoms of muscle damage were documented by changes in isometric hamstring muscle strength, pain, muscle tenderness, and creatine kinase activity on the following 3 days. Strength loss, pain, muscle tenderness, and creatine kinase activity were significantly greater in the stiff compared with the compliant subjects on the days after eccentric exercise. Greater symptoms of muscle damage in subjects with stiffer hamstring muscles are consistent with the sarcomere strain theory of muscle damage. The present study provides experimental evidence of an association between flexibility and muscle injury. Muscle stiffness and its clinical correlate, static flexibility, are risk factors for more severe symptoms of muscle damage after eccentric exercise.

Viscoelastic response to repeated static stretching in the human hamstring muscle

The purpose of this study was (1) to evaluate the reproducibility of a new method of measuring passive resistance to stretch in the human hamstring muscle group, in vivo, using a test re‐test protocol and 2) to examine the effect of repeated stretches. Passive resistance offered by the hamstring muscle group during knee extension was measured in 10 subjects as knee flexion moment (Nm) using a KinCom dynamometer. The knee was passively extended at 5 deg/s to the final position where it remained stationary for 90 s (static phase). EMG of the hamstring muscle was also measured. The test re‐test protocol included 2 tests (tests 1 and 2) administered 1 h apart. On a separate occasion 5 consecutive static stretches were administered (stretches 1–5) separted by 30 s. Stretch 6 was administered one hour after stretch 5. In the static phase passive resistance did not differ between test 1 and test 2. Resistance declined in both tests 1 and 2, whereas EMG activity remained unchanged. The decline in resistance was significant up to 45 s. For the repeated stretches there was an effect of time (90 s) and stretch (1–5) with a significant interaction i.e., resistance diminished with stretches, and the 90‐s decline was less as more stretches were performed. Passive resistance in stretch 6 was lower than in stretch 1. The present study has demonstrated a reliable method for studying resistance to stretch of the human hamstring muscle group. A viscoelastic response of the human hamstring muscle was shown. With 5 repeated stretches, resistance to stretch diminished and each stretch exibited a viscoelastic response, albeit less with each subsequent stretch. The effect of 5 repeated stretches was significant 1 h later.

Efficacy of a tart cherry juice blend in preventing the symptoms of muscle damage

Background: Numerous antioxidant and anti-inflammatory agents have been identified in tart cherries. Objective: To test the efficacy of a tart cherry juice blend in preventing the symptoms of exercise induced muscle damage. Methods: This was a randomised, placebo controlled, crossover design. Fourteen male college students drank 12 fl oz of a cherry juice blend or a placebo twice a day for eight consecutive days. A bout of eccentric elbow flexion contractions (2 × 20 maximum contractions) was performed on the fourth day of supplementation. Isometric elbow flexion strength, pain, muscle tenderness, and relaxed elbow angle were recorded before and for four days after the eccentric exercise. The protocol was repeated two weeks later with subjects who took the placebo initially, now taking the cherry juice (and vice versa). The opposite arm performed the eccentric exercise for the second bout to avoid the repeated bout protective effect. Results: Strength loss and pain were significantly less in the cherry juice trial versus placebo (time by treatment: strength p<0.0001, pain p  =  0.017). Relaxed elbow angle (time by treatment p  =  0.85) and muscle tenderness (time by treatment p  =  0.81) were not different between trials. Conclusions: These data show efficacy for this cherry juice in decreasing some of the symptoms of exercise induced muscle damage. Most notably, strength loss averaged over the four days after eccentric exercise was 22% with the placebo but only 4% with the cherry juice.