Muscle physiologists often describe fatigue simply as a decline of muscle force and infer this causes an athlete to slow down. In contrast, exercise scientists describe fatigue during sport competition more holistically as an exercise-induced impairment of performance. The aim of this review is to reconcile the different views by evaluating the many performance symptoms/measures and mechanisms of fatigue. We describe how fatigue is assessed with muscle, exercise or competition performance measures. Muscle performance (single muscle test measures) declines due to peripheral fatigue (reduced muscle cell force) and/or central fatigue (reduced motor drive from the CNS). Peak muscle force seldom falls by >30% during sport but is often exacerbated during electrical stimulation and laboratory exercise tasks. Exercise performance (whole-body exercise test measures) reveals impaired physical/technical abilities and subjective fatigue sensations. Exercise intensity is initially sustained by recruitment of new motor units and help from synergistic muscles before it declines. Technique/motor skill execution deviates as exercise proceeds to maintain outcomes before they deteriorate, e.g. reduced accuracy or velocity. The sensation of fatigue incorporates an elevated rating of perceived exertion (RPE) during submaximal tasks, due to a combination of peripheral and higher CNS inputs. Competition performance (sport symptoms) is affected more by decision-making and psychological aspects, since there are opponents and a greater importance on the result. Laboratory based decision making is generally faster or unimpaired. Motivation, self-efficacy and anxiety can change during exercise to modify RPE and, hence, alter physical performance. Symptoms of fatigue during racing, team-game or racquet sports are largely anecdotal, but sometimes assessed with time-motion analysis. Fatigue during brief all-out racing is described biomechanically as a decline of peak velocity, along with altered kinematic components. Longer sport events involve pacing strategies, central and peripheral fatigue contributions and elevated RPE. During match play, the work rate can decline late in a match (or tournament) and/or transiently after intense exercise bursts. Repeated sprint ability, agility and leg strength become slightly impaired. Technique outcomes, such as velocity and accuracy for throwing, passing, hitting and kicking, can deteriorate. Physical and subjective changes are both less severe in real rather than simulated sport activities. Little objective evidence exists to support exercise-induced mental lapses during sport. A model depicting mind-body interactions during sport competition shows that the RPE centre-motor cortex-working muscle sequence drives overall performance levels and, hence, fatigue symptoms. The sporting outputs from this sequence can be modulated by interactions with muscle afferent and circulatory feedback, psychological and decision-making inputs. Importantly, compensatory processes exist at many levels to protect against p...

Physical exercise is important for people living under extreme environmental conditions to stay healthy. Particularly in space, exercise can partially counteract the loss of muscle mass and muscle strength caused by microgravity. Monitoring the adaptation of the musculoskeletal system to assess muscle quality and devise individual training programmes is highly desirable but is restricted by practical, technical and time constraints on board the International Space Station (ISS). This study aimed to test the feasibility of using myometric measurements to monitor the mechanical properties of skeletal muscles and tendons in weightlessness during parabolic flights.The mechanical properties (frequency, decrement, stiffness relaxation time and creep) of the m. gastrocnemius, m. erector spinae and Achilles tendon were assessed using the hand-held MyotonPRO device in 11 healthy participants (aged 47 +/-9 years) in normal gravity as well as in microgravity during two parabolic flight campaigns. Results showed significant (p < .05-.001) changes in all mechanical properties of both muscles and the Achilles tendon, indicating a more relaxed tissue state in microgravity. Recordings from a phantom rubber material with the device in a test rig confirmed that the device itself was not affected by gravity, as changes between gravity conditions that were too small (<1%) to explain the changes observed in the tissues.It is concluded that myometric measurements are a feasible, easy to use and non-invasive approach to monitor muscle health in extreme conditions that prohibit many other methods.Real-time assessment of the quality of a muscle being exposed to the negative effect of microgravity and also the positive effects of muscular training could be achieved using Myoton technology.

The goal of this study was to evaluate the physiological responses during incremental field tests (FT) in nordic walking (NW), walking (W) and jogging (J). Fifteen healthy middle-aged women participated in three FT. Heart rate (HR) and oxygen uptake (V(O)(2)) were monitored continuously by portable analyzers. Capillary blood lactate (La) was analyzed at rest and after every stage of the FT. The disciplines showed differences during stage 1.8 and 2.1 m s(-1) for V(O)(2) between NW and W (P < 0.05). The maximum value was measured at 1.8 m s(-1 )(8%). In accordance with La, V(CO)(2) was higher in NW compared with W during all stages (P < 0.05) and even higher in NW compared with J during 2.1 and 2.4 m s(-1). While there were higher HR for NW and W at 2.4 m s(-1) than in J (P < 0.01), there were increases for HR at fixed values of 2 (La2) and 4 (La4) mmol l(-1 )lactate for J compared with NW and W (P < 0.01). Although the speed of NW was slower than that of W at La2 and La4 (P < 0.05), there were no differences for the HR and the V(O)(2) Our results demonstrate that metabolic responses are a helpful instrument to assess the intensity during bipedal exercise. As NW speed at submaximal lactate levels is lower than in W and J, W and J test measures of HR and V(O)(2) are not suitable for NW training recommendations. Additionally, the V(O)(2) formed by performing NW is not as high as previously reported.

The Nordic Hamstring Exercise (NHE) is effective for selective hamstring strengthening to improve muscle balance between knee flexors and extensors. The purpose of this study (within subject design of repeated measures) was to determine the effects of a standardized 4-week NHE training on thigh strength and muscle balance with concomitant kinetic and kinematic monitoring. Sixteen male sprinters (22 years, 181 cm, 76 kg) performed a standardized 4-week NHE training consisting of three sessions per week (each 3×3 repetitions). Six rope-assisted and six unassisted sessions were performed targeting at a constant knee extension angular velocity of ~15°/s across a ~90-100° knee joint range of motion. Kinetic (peak and mean moment, impulse) and kinematic parameters (eg, ROM to downward acceleration, ROM DWA ) were recorded during selected sessions. Unilateral isokinetic tests of concentric and eccentric knee flexors and extensors quantified muscle group-, contraction mode-, and velocity-specific training adaptations. Peak moments and contractional work demonstrated strong interactions of time with muscle group, contraction modes, and angular velocities (η²>.150). NHE training increased eccentric hamstring strength by 6%-14% as well as thigh muscle balance with biggest adaptations at 150°/s 2 weeks after NHE training. Throughout the training period significant increases (P<.001) of peak (η²=.828) and mean moments (η²=.611) became apparent, whereas the impulse and the ROM DWA of unassisted NHE repetitions remained unchanged (P>.05). A 4-week NHE training significantly strengthened the hamstrings and improved muscle balance between knee flexors and extensors. Despite the slow training velocity, biggest adaptations emerged at the highest velocity 2 weeks after training ended.