D. G. Sale

Gender differences in strength and muscle fiber characteristics

SummaryStrength and muscle characteristics were examined in biceps brachii and vastus lateralis of eight men and eight women. Measurements included motor unit number, size and activation and voluntary strength of the elbow flexors and knee extensors. Fiber areas and type were determined from needle biopsies and muscle areas by computerized tomographical scanning. The women were approximately 52% and 66% as strong as the men in the upper and lower body respectively. The men were also stronger relative to lean body mass. A significant correlation was found between strength and muscle cross-sectional area (CSA; P≤0.05). The women had 45, 41, 30 and 25% smaller muscle CSAs for the biceps brachii, total elbow flexors, vastus lateralis and total knee extensors respectively. The men had significantly larger type I fiber areas (4597 vs 3483 μm2) and mean fiber areas (6632 vs 3963 μm2) than the women in biceps brachii and significantly larger type II fiber areas (7700 vs 4040 μm2) and mean fiber areas (7070 vs 4290 μm2) in vastus lateralis. No significant gender difference was found in the strength to CSA ratio for elbow flexion or knee extension, in biceps fiber number (180 620 in men vs 156 872 in women), muscle area to fiber area ratio in the vastus lateralis 451 468 vs 465 007) or any motor unit characteristics. Data suggest that the greater strength of the men was due primarily to larger fibers. The greater gender difference in upper body strength can probably be attributed to the fact that women tend to have a lower proportion of their lean tissue distributed in the upper body. It is difficult to determine the extent to which the larger fibers in men represent a true biological difference rather that a difference in physical activity, but these data suggest that it is largely an innate gender difference.

Postactivation potentiation: role in human performance.

SALE, D.G. Postactivation potentiation: Role in human performance. Exerc. Sport Sci. Rev., Vol. 30, No. 3, pp. 138–143, 2002. Postactivation potentiation (PAP) is the transient increase in muscle contractile performance after previous contractile activity. This review describes the features and mechanism of PAP, assesses its potential role in endurance and strength/speed performance, considers strategies for exploiting PAP, and outlines how PAP might be affected by training.

Strength training effects in prepubescent boys.

Possible changes in muscle size and function due to resistance training were examined in prepubertal boys. Thirteen boys (9-11 yr) volunteered for each of the training and control groups. Progressive resistance training was performed three times weekly for 20 wk. Measurements consisted of the following: 1 repetition maximum (RM) bench press and leg press; maximal voluntary isometric and isokinetic elbow flexion and knee extension strength; evoked isometric contractile properties of the right elbow flexors and knee extensors; muscle cross-sectional area (CSA) by computerized tomography at the mid-right upper arm and thigh; and motor unit activation (MUA) by the interpolated twitch procedure. Training significantly increased 1 RM bench press (35%) and leg press (22%), isometric elbow flexion (37%) and knee extension strength (25% and 13% at 90 degrees and 120 degrees, respectively), isokinetic elbow flexion (26%) and knee extension (21%) strength, and evoked twitch torque of the elbow flexors (30%) and knee extensors (30%). There were no significant effects of training on the time-related contractile properties (time to peak torque, half-relaxation time), CSA, or %MUA of the elbow flexors or knee extensors. There was, however, a trend toward increased MUA for the elbow flexors and knee extensors in the trained group. Strength gains were independent of changes in muscle CSA, and the increases in twitch torque suggest possible adaptations in muscle excitation-contraction coupling. Improved motor skill coordination (especially during the early phase of training), a tendency toward increased MUA, and other undetermined neurological adaptations, including better coordination of the involved muscle groups, are likely the major determinants of the strength gains in this study.

Influence of exercise and training on motor unit activation.

Human MUs vary considerably in twitch force, contractile speed, axonal conduction velocity, fatigue resistance, recruitment thresholds, firing rates, and firing patterns. These functional properties, together with the corresponding morphological characteristics such as soma size, axon diameter, and muscle fiber size, are interrelated. The smallest (soma size, axon diameter, muscle fiber size) MUs have the smallest twitch force, the slowest contraction speed, the slowest conduction velocity, the greatest resistance to fatigue, the lowest recruitment thresholds, and the lowest minimum and maximum firing rates. The converse applies to the largest MUs. Between the extremes are MUs with intermediate characteristics. MUs are generally recruited in order of size in voluntary contraction of increasing force or effort. Thus, units are recruited in order of increasing twitch force and contractile speed and decreasing resistance to fatigue. In some muscles MU recruitment occurs throughout the range of contraction force, whereas in other muscles most if not all MUs are recruited by about 50% of maximum contraction force. The latter pattern is characteristic of small muscles that perform precise movements. The recruitment order of MUs according to size is based on the inverse relation between susceptibility to discharge and motoneuron size. Thus, for evenly distributed and increasing excitatory synaptic input to a pool of motoneurons, smaller motoneurons will begin to fire before larger motoneurons. This arrangement ensures, for example, that the small, fatigue-resistant MUs will be preferentially activated in prolonged, low-intensity exercise, to which these units are most suited. In brief, intense exercise, the associated greater excitatory input will also recruit the large MUs, taking advantage of their greater strength and contractile speed. A frequent question is whether rapid, ballistic or explosive contractions and movements are associated with selective or preferential recruitment of large, fast twitch MUs. There is evidence of synaptic input systems that preferentially excite large, fast twitch MUs and inhibit small twitch MUs; however, the majority of evidence from human experiments indicates that the recruitment order is not reversed in ballistic contractions. For technical reasons, most studies have used isometric contractions, but recently successful recordings of single MUs have been made during locomotion. Future research must develop a successful recording arrangement for the study of recruitment and discharge properties of single MUs in large proximal muscles during activities such as kicking, jumping, and throwing.(ABSTRACT TRUNCATED AT 400 WORDS)

Effects of strength training and immobilization on human muscle fibres

SummarySeven healthy male subjects were studied under control conditions and following 5–6 months of heavy resistance training and 5–6 weeks of immobilization in elbow casts. Cross-sectional fibre areas and nuclei-to-fibre ratios were calculated from cryostat sections of needle biopsies taken from triceps brachii. Training resulted in a 98% increase in maximal elbow extension strength as measured by a Cybex dynamometer, while immobilization resulted in a 41% decrease in strength. Both fast twitch (FT) and slow twitch (ST) fibre areas increased significantly with training by 39% and 31%, respectively. Immobilization resulted in significant decreases in fibre area by 33% for FT and 25% for ST fibres. The observed nuclei-to-fibre ratio was 10% greater following the training programme. However, this change was non-significant. There was also a nonsignificant correlation between the magnitude of the changes in fibre size and the changes in maximal strength following either training or immobilization.

Velocity Specificity of Resistance Training

SummaryVelocity specificity of resistance training has demonstrated that the greatest strength gains occur at or near the training velocity. There is also evidence that the intent to make a high speed contraction may be the most crucial factor in velocity specificity.The mechanisms underlying the velocity-specific training effect may reside in both neural and muscular components. Muscular adaptations such as hypertrophy may inhibit high velocity strength adaptations due to changes in muscle architecture. However, some studies have reported velocity-specific contractile property adaptations suggesting changes in muscle kinetics. There is evidence to suggest velocity-specific electromyographic (EMG) adaptations with explosive jump training. Other researchers have hypothesised neural adaptations because of a lack of electrically evoked changes in relation to significant voluntary improvements. These neural adaptations may include the selective activation of motor units and/or muscles, especially with high velocity alternating contractions. Although the incidence of motor unit synchronisation increases with training, its contribution to velocity-specific strength gains is unclear. However, increased synchronisation may occur more frequently with the premovement silent period before ballistic contractions. The preprogrammed neural circuitry of ballistic contractions suggests that high velocity training adaptations may involve significant neural adaptations. The unique firing frequency associated with ballistic contractions would suggest possible adaptations in the frequency of motor unit discharge. Although co-contraction of antagonists increases with training and high velocity movement, its contribution is probably related more to joint protection than the velocity-specific training effect.

Exercise and bone mineral density.

SummaryA decrease in physical activity may lead to an increased loss of bone and an increase in the incidence of osteoporotic fractures. Studies have demonstrated increases in bone formation in animals and increases in bone mineral density in humans. Studies of animals show that bone has enhanced physical and mechanical properties following periods of increased stress. Strains which are high in rate and magnitude, and of abnormal distribution, but not necessarily long in duration, are best for inducing new bone formation, resulting in the strengthening of bone by increased density. Cross-sectional studies show that athletes, especially those who are strength-trained, have greater bone mineral densities than nonathletes, and that strength, muscle mass and maximal oxygen uptake correlate with bone density. Longitudinal training studies indicate that strength training and high impact endurance training increase bone density.Strain induction, the deformation that occurs in bone under loading, may cause a greater level of formation and an inhibition of resorption within the normal remodelling cycle of bone, or it may cause direct activation of osteoblastic bone formation from the quiescent state.Various mechanisms have been proposed for the transformation of mechanical strain into biochemical stimuli to enhance bone formation. These include prostaglandin release, piezoelectric and streaming potentials, increased bone blood flow, microdamage and hormonally mediated mechanisms. These mechanisms may act on their own or in concert, depending on the loading situation and the characteristics of the bone.

Endurance capacity of untrained males and females in isometric and dynamic muscular contractions

SummaryThe capacity to perform isometric and dynamic muscle contractions at different forces has been measured in two separate groups of subjects: 25 men and 25 women performed sustained isometric contractions of the knee-extensor muscles of their stronger leg to fatigue, at forces corresponding to 80%, 50% and 20% of the maximum voluntary force of contraction (MVC). The second experimental model involved a bilateral elbowflexion weight lifting exercise. Eleven women and 12 men performed repetitions at loads corresponding to 90%, 80%, 70%, 60% and 50% of maximum load (lRM), at a rate of 10 · min−1 to the point of fatigue. Males were stronger (p<0.001) than females in both the static (675±120 N vs 458±80 N; mean±SD) and dynamic (409±90 N vs 190±33 N) contractions. Isometric endurance time of the males at a force corresponding to 20% of MVC was less than that of the females (180±51 s vs 252±56 s; p<0.001) but there was no difference between the sexes at 50% or 80% of MVC. Similarly, when the sexes were compared using dynamic elbow-flexion exercise, the female subjects were able to perform a greater number of repetitions than males at loads of 50% (p<0.005), 60% (p<0.001) and 70% (p<0.025) of lRM, but there was no difference between the sexes at loads of 80% or 90% of lRM. The results suggest that the endurance capacity of women is greater than that of men in both isometric and dynamic muscular exercise when the work load is relatively low compared with maximum; at higher forces, there is no difference between the sexes in endurance performance.

A comparison of strength and muscle mass increases during resistance training in young women

Abstract Strength gains with resistance training are due to muscle hypertrophy and nervous system adaptations. The contribution of either factor may be related to the complexity of the exercise task used during training. The purpose of this investigation was to measure the degree to which muscle hypertrophy contributes to gains in strength during exercises of varying complexity. Nineteen young women resistance trained twice a week for 20 weeks, performing exercises designed to provide whole-body training.The lean mass of the trunk, legs and arms was measured by dual energy x-ray absorptiometry and compared to strength gains (measured as the 1-repetition maximum) in bench press, leg press and arm curl exercises, pre-, mid- (10 weeks) and post-training. No changes were found in a control group of ten women. For the exercise group, increases in bench press, leg press and arm curl strength were significant from pre- to mid-, and from mid- to post-training (P < 0.05). In contrast, increases in the lean mass of the body segments used in these exercises followed a different pattern. Increases in the lean mass of the arms were significant from pre- to mid-training, while increases in the lean mass of the trunk and legs were delayed and significant from mid- to post-training only (P < 0.05). It is concluded that a more prolonged neural adaptation related to the more complex bench and leg press movements may have delayed hypertrophy in the trunk and legs. With the simpler arm curl exercise, early gains in strength were accompanied by muscle hypertrophy and, presumably, a faster neural adaptation.

Effect of postactivation potentiation on dynamic knee extension performance.

Abstract Six men and four women performed, in separate trials, maximal dynamic knee extensions with loads of 15%, 30%, 45% and 60% of maximal isometric knee extension peak torque (MVC). The dynamic extensions were done after postactivation potentiation (PAP) had been induced with a 10-s MVC, and in a control trial without PAP. PAP, measured as the increase in evoked twitch torque, was 53 (4)% (SE) and 43 (3)% at the time of the first and second extensions with each load. PAP failed to increase the attained peak velocity with any load; on the contrary, there was a trend for peak velocity to decrease in the first extension, which occurred ≅15 s after the 10-s MVC. The results suggest that fatigue produced by the 10-s MVC suppressed any benefit that could be derived from the induced PAP. A surface electromyogram (EMG) recorded from one muscle of quadriceps femoris gave no indication of activation failure in the first knee extension; however, activation impairment specific to the rate of force development cannot be ruled out. It is concluded that the strategy employed, namely of having knee extensions performed soon after the 10-s MVC to maximize PAP at the time of performance, was unsuccessful because there had been insufficient time for recovery from fatigue. It is possible that a longer recovery time, even at the cost of a diminished PAP, may have proved beneficial.