Neural Adaptations to Resistive Exercise

Abstract

It is generally accepted that neural factors play an important role in muscle strength gains. This article reviews the neural adaptations in strength, with the goal of laying the foundations for practical applications in sports medicine and rehabilitation. An increase in muscular strength without noticeable hypertrophy is the first line of evidence for neural involvement in acquisition of muscular strength. The use of surface electromyographic (SEMG) techniques reveal that strength gains in the early phase of a training regimen are associated with an increase in the amplitude of SEMG activity. This has been interpreted as an increase in neural drive, which denotes the magnitude of efferent neural output from the CNS to active muscle fibres. However, SEMG activity is a global measure of muscle activity. Underlying alterations in SEMG activity are changes in motor unit firing patterns as measured by indwelling (wire or needle) electrodes. Some studies have reported a transient increase in motor unit firing rate. Training-related increases in the rate of tension development have also been linked with an increased probability of doublet firing in individual motor units. A doublet is a very short interspike interval in a motor unit train, and usually occurs at the onset of a muscular contraction. Motor unit synchronisation is another possible mechanism for increases in muscle strength, but has yet to be definitely demonstrated. There are several lines of evidence for central control of training-related adaptation to resistive exercise. Mental practice using imagined contractions has been shown to increase the excitability of the cortical areas involved in movement and motion planning. However, training using imagined contractions is unlikely to be as effective as physical training, and it may be more applicable to rehabilitation. Retention of strength gains after dissipation of physiological effects demonstrates a strong practice effect. Bilateral contractions are associated with lower SEMG and strength compared with unilateral contractions of the same muscle group. SEMG magnitude is lower for eccentric contractions than for concentric contractions. However, resistive training can reverse these trends. The last line of evidence presented involves the notion that unilateral resistive exercise of a specific limb will also result in training effects in the unexercised contralateral limb (cross-transfer or cross-education). Peripheral involvement in training-related strength increases is much more uncertain. Changes in the sensory receptors (i.e. Golgi tendon organs) may lead to disinhibition and an increased expression of muscular force. Agonist muscle activity results in limb movement in the desired direction, while antagonist activity opposes that motion. Both decreases and increases in co-activation of the antagonist have been demonstrated. A reduction in antagonist co-activation would allow increased expression of agonist muscle force, while an increase in antagonist co-activation is important for maintaining the int...

The Influence of Resistance Training Experience on the Between-Day Reliability of Commonly Used Strength Measures in Male Youth Athletes

“…This study provides scientists and practitioners with between-day reliability reference data to determine real and practical changes for youth athletes with different resistance training experience. Additionally, this study also provides confirmation that improvements in strength, commonly seen in adolescent research (4,11), and practical nature. 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64…”

“…It appears that when healthy young adults who have no previous resistance training experience complete strength testing, maximal strength between sessions may vary due to a lack of familiarization with the movement (25). However, as seen in the current study, 6 months resistance training experience appears to be a sufficient period of time to minimize any effects of familiarization and off-set the initial rapid adaptation of the central nervous system to external loading (8,11,25). This 6 month time frame has also been shown to be sufficient in promoting reliability in other resistance training exercises in young adults and adolescents (4,9).…”

“…This 6 month time frame has also been shown to be sufficient in promoting reliability in other resistance training exercises in young adults and adolescents (4,9). Additionally, this familiarization to learning a movement appears to be permanent to an extent, with periods of abstinence (>6 months) from resistance training in experienced individuals not appearing to affect the reliability of their maximal lifting capability (11,25 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64This study, is not without its limitations. In the current study, depth in the squat was assessed by the top of the thigh being parallel to the floor (which has previously been used in adolescent literature (7)), however this could lead to systematic bias with interrater reliability, as this criterion is yet to be established, especially in between-day studies.…”

“…Previous work from Ritti-Dias et al (25) explored this phenomenon, demonstrating that significant changes (p= 0.01) in strength occur between resistance training sessions in subjects with no prior resistance training experience while their well-trained counterparts (>2 years) do not. These differences were attributed to modification in motor unit recruitment and rate coding, which is more prone to occur if athletes have little to no prior resistance training experience (11). As such, scientists and practitioners should acknowledge this when interpreting strength changes in athletes with differing training experience.…”

Preliminary framework for Familiar Auditory Sensory Training (FAST) provided during coma recovery

“…The concept of muscle fatigue, related to performing too many of the same physical exercises, illustrates our concern. That is, there is evidence of maladaptive responses to highly intense motor treatment programs involving overuse of a muscle [28].…”