Poor correlations between isometric tests and dynamic performance: relationship to muscle activation

Functional performance, maximal strength, and power characteristics in isometric and dynamic actions of lower extremities in soccer players

The purposes of the present study were to determine muscle strength and power output characteristics in a group of professional soccer players and to identify their relationships with 2 functional performance tests (vertical jumping height and 15-m sprint time). Maximal strength and power indices attained against different loads in barbell back squat exercise, isometric maximal force of the knee extensor and plantar flexor muscles, isokinetic peak torque of the knee extensors muscles, vertical jumping height in squat and counter-movement jumps, and 15-m sprint time tests were assessed in 21 semiprofessional soccer players (age 20 +/- 3.8 years). Correlation analyses were performed to examine the relationship between each of these measures. The main results of the present study were that (a) maximal power in concentric half-squat exercise was attained with a load of 60% of 1 repetition maximum, representing 112% of body weight; (b) the performance in the functional tests selected was significantly related with all the half-squat variables measured, especially with loads of 75-125% of body weight; and (c) low to nonsignificant correlations were found between functional tests performance and isometric and isokinetic muscle strength measures. It was concluded that in semiprofessional soccer players (a) isometric and isokinetic muscle strength assessed in an open kinetic chain were not movement-specific enough to predict performance during a more complex movement, such as jump or sprint and (b) concentric half-squat exercise was principally related with the functional tests selected when it was performed against external loading within the range of the load in case of which the maximal power output was attained.

Evaluation of the torque-velocity test of the BTE-Primus as a measure of sincerity of effort of grip strength

An inverse linear relationship exists between torque and velocity in the mid-ranges of an isotonic maximal contraction in a single joint movement (such as the elbow and knee). We hypothesized that submaximal effort does not produce a linear torque-velocity relationship because replicating a submaximal isotonic contraction requires an enormous amount of proprioceptive feedback and the nervous system may not be able to accurately replicate both force and speed of contraction. If this hypothesis is true, the torque-velocity test of the BTE-Primus would be an effective method for assessing sincerity of effort. The purpose of this study was to examine if differences exist in the linear torque-velocity relationship between maximal and submaximal grip strength effort. Due to the fact that a test is not valid unless it is reliable, an additional purpose was to calculate the test-retest reliability of velocity during isotonic contraction using the torque-velocity test of the BTE-Primus' grip tool. Participants included 32 healthy, right-hand dominant (16 male, 16 female) persons, aged 20-50 years (mean age 25+/-8.0), with no history of upper-extremity injury. The subjects participated in two days of grip-strength testing (approximately two weeks apart) and were instructed to exert maximal effort with both hands on one day, and to feign injury with one hand on the other day. Each day included two sessions of testing, which consisted of performing the "torque-velocity test" on the BTE-Primus grip attachment (#162). We randomly assigned the feigning hand (dominant vs. nondominant) and the effort (maximal vs. submaximal). The test administrator was blinded to the level of effort. On each day, four isotonic grip-strength tests were performed at loads of 20%, 30%, 40%, and 50% of isometric test scores. Three repeated isotonic grip strength trials were performed at each load and the average was plotted. One plot was generated for the maximal effort and another for the submaximal efforts. Average torque was plotted against the average velocity at each of the four loads and for each level of effort (maximal vs. submaximal). The linear relationship of the torque-velocity curve was examined by performing regression analysis, calculating the intercept, slope, correlation coefficient (r), and the coefficient of determination (r(2)) for each curve. Paired t-tests were used to compare the intercept, slope, and r(2) between maximal and submaximal efforts. Bonferroni correction set the alpha level at 0.0167. Sensitivity and specificity values were calculated for linearity (r(2)) and a Receiver Operator Characteristic (ROC) curve was constructed to obtain the optimal sensitivity and specificity combination. In addition, test-retest reliability was determined for velocity of maximal isotonic effort using Intraclass Correlation Coefficient. Significant differences between maximal and submaximal efforts were found for the intercept (t=5.069; p<0.001) and for linearity as expressed by r(2) (t=5.414; p<0.001). Mean r(2) was 0.89 for maximal effort and 0.53 for submaximal effort. The slopes of maximal and submaximal efforts were not significantly different (t=0.14; p=0.888). The ROC curve revealed the optimal combination of sensitivity (0.69) and specificity (0.72) values. Test-retest reliability of maximal isotonic grip effort for velocity was r=0.843. The differences in intercepts suggested that velocity was greater during maximal effort. Greater r(2) values indicated greater linearity for maximal efforts than submaximal efforts. These findings suggest that the torque-velocity test of the BTE-Primus can distinguish between maximal and submaximal efforts during grip-strength testing. However, the test misclassified 31% of submaximal effort and 28% of maximal error, for a total error of 59%. Therefore, this test does not possess adequate sensitivity and specificity values to justify its use in the clinic.

Fast unilateral isometric knee extension torque development and bilateral jump height

PURPOSE

We hypothesized that the initial rate (first 40 ms) of unilateral knee extensor torque development during a maximally fast isometric contraction would depend on the subjects' ability for fast neural activation and that it would predict bilateral jumping performance.

METHODS

Nine males (21.8 +/- 0.9 yr, means +/- SD) performed unilateral fast isometric knee extensions (120 degrees knee angle) without countermovement on a dynamometer and bilateral squat jumps (SJ) and countermovement jumps (CMJ) starting from 90 and 120 degrees knee angles (full extension = 180 degrees ). The dynamometer contractions started either from full relaxation or from an isometric pre-tension (15% maximal isometric torque, Tmax). Torque time integral for the first 40 ms after torque onset (TTI-40, normalized to Tmax) and averaged normalized rectified knee extensor EMG for 40 ms before fast torque onset (EMG-40) were used to quantify initial torque rise and voluntary muscle activation.

RESULTS

TTI-40 without pre-tension (range: 0.02-0.19% Tmax per second) was significantly lower than TTI-40 with pre-tension, and both were significantly (r = 0.81 and 0.80) related to EMG-40. During jumping, similar significant positive relations were found between jump height and knee extensor EMG during the first 100 ms of the rise in ground reaction force. There also were significant positive linear relations between dynamometer TTI-40 and jump height (r = 0.75 (SJ 90), 0.84 (SJ 120), 0.76 (CMJ 90), and 0.86 (CMJ 120)) but not between dynamometer Tmax and jump height (-0.16 < r < 0.02).

CONCLUSION

One-legged TTI-40 to a large extent explained the variation in jump height. The ability to produce a high efferent neural drive before muscle contraction seemed to dominate performance in both the simple single-joint isometric task and the complex multijoint dynamic task.

Relationship between side medicine-ball throw performance and physical ability for male and female athletes

The purpose of the study was to examine the factors contributing to performance of a side medicine-ball throw (S-MBT) and a fast side medicine-ball throw (FS-MBT) and to analyze some of the factors which account for the difference in side medicine ball throw performance between the sexes. Sixteen males and ten females were evaluated by S-MBT, FS-MBT, isometric maximal trunk rotation torque (IMTRT), One repetition maximum of Parallel Squat (1RM(PS)) and Bench Press (1RM(BP)), Bench Press peak power (BP(PP)), Static Squat Jump peak power (SSJ(PP)) and vertical jump height. Males demonstrated significantly greater scores than females in all measurements. Significant correlations were observed in males, but not in females, between the distances during S-MBT and the IMTRT values (r = 0.596-0.739, P < 0.05-0.01) and the 1RM(PS) values (r = 0.683-0.725, P < 0.01). In FS-MBT performance, significant correlations were observed in males, but not in females, between the ball velocity values during FS-MBT and the IMTRT values (r = 0.611-0.687, P < 0.05-0.01), 1RM(BP) values (r = 0.596-0.655, P < 0.05-0.01) and 1RM(PS) values (r = 0.679-0.718, P < 0.01). These results suggested that the contributing factors of S-MBT and FS-MBT performance were deferent in males and females. Hence, the side medicine-ball throw test would be useful to examine the trunk rotation power of male athletes, but may have a limited potential as a predictor of trunk rotation power for female athletes.

Techniques and considerations for determining isoinertial upper‐body power

Power is an integral aspect of many sports. Although power output of the lower body is often measured during jumping and cycling movements, much less is known about power as pertains to the upper body musculature. Recently, isoinertial methods--with constant gravitational load--of power testing have become common, but little is known of the reliability and criterion validity of these tests as they pertain to sport performance. In addition, the varied methodology makes a lucid model more evasive. The aims of this review are to examine the various methods of assessing upper body power, to establish its role in predicting athletic performance, and to assess the body of literature that has assessed power output of the upper extremities by isoinertial methods. To our knowledge, only two studies on isoinertial upper-body power have shown a direct correlation to sporting ability (Baker, 2001; Baker et al., 2001); therefore, many unanswered questions exist as to the efficacy of these tests as predictors of athletic ability or as a method to track athletes' training over time. From this review we hope to allow the sport coach to assess the overall utility of these tests in terms of availability, safety and external validity.

A 1-year combined weight-bearing training program is beneficial for bone mineral density and neuromuscular function in older women

Forty-eight community living women 66-87 years old volunteered to participate in a 12-month prospective, randomized, controlled, trial. The aim was to determine if a combined weight-bearing training program twice a week would be beneficial to bone mineral density and neuromuscular function. The participants were pairwise age-matched and randomly assigned to either an exercise group (n=24) or a control group (n=24). Twenty-one subjects in the intervention group and 19 in the control group completed the study. The exercise program lasted for 50 min and consisted of a combination of strengthening, aerobic, balance and coordination exercises. The mean percentage of scheduled sessions attended for the exercise group was 67%. At the completion of the study, the intervention group showed significant increments in bone mineral density of the Ward's triangle (8.4%, P<0.01) as well as improvement in maximum walking speed (11.4%, P<0.001) and isometric grip strength (9.9%, P<0.05), as compared to the control group. The conclusion was that a combined weight-bearing training program might reduce fracture risk factors by improving bone density as well as muscle strength and walking ability. This program could be suitable for older community living women in general, and might, therefore, have important implications for fracture prevention.

Assessing Voluntary Muscle Activation with the Twitch Interpolation Technique

The twitch interpolation technique is commonly employed to assess the completeness of skeletal muscle activation during voluntary contractions. Early applications of twitch interpolation suggested that healthy human subjects could fully activate most of the skeletal muscles to which the technique had been applied. More recently, however, highly sensitive twitch interpolation has revealed that even healthy adults routinely fail to fully activate a number of skeletal muscles despite apparently maximal effort. Unfortunately, some disagreement exists as to how the results of twitch interpolation should be employed to quantify voluntary activation. The negative linear relationship between evoked twitch force and voluntary force that has been observed by some researchers implies that voluntary activation can be quantified by scaling a single interpolated twitch to a control twitch evoked in relaxed muscle. Observations of non-linear evoked-voluntary force relationships have lead to the suggestion that the single interpolated twitch ratio can not accurately estimate voluntary activation. Instead, it has been proposed that muscle activation is better determined by extrapolating the relationship between evoked and voluntary force to provide an estimate of true maximum force. However, criticism of the single interpolated twitch ratio typically fails to take into account the reasons for the non-linearity of the evoked-voluntary force relationship. When these reasons are examined, it appears that most are even more challenging to the validity of extrapolation than they are to the linear equation. Furthermore, several factors that contribute to the observed non-linearity can be minimised or even eliminated with appropriate experimental technique. The detection of small activation deficits requires high resolution measurement of force and careful consideration of numerous experimental details such as the site of stimulation, stimulation intensity and the number of interpolated stimuli. Sensitive twitch interpolation techniques have revealed small to moderate deficits in voluntary activation during brief maximal efforts and progressively increasing activation deficits (central fatigue) during exhausting exercise. A small number of recent studies suggest that resistance training may result in improved voluntary activation of the quadriceps femoris and ankle plantarflexor muscles but not the biceps brachii. A significantly larger body of evidence indicates that voluntary activation declines as a consequence of bed-rest, joint injury and joint degeneration. Twitch interpolation has also been employed to study the mechanisms by which caffeine and pseudoephedrine enhance exercise performance.

'Psyching-up' and muscular force production

Psyching-up refers to self-directed cognitive strategies used immediately prior to or during skill execution that are designed to enhance performance. This review focuses on research that has investigated the effect of psyching-up on force production; specifically, strength, muscular endurance and power. Although firm conclusions are not possible, the research tentatively suggests that psyching-up may enhance performance during dynamic tasks requiring strength and/or muscular endurance. However, more research is required. Power has received scant empirical attention and there are not enough data to support any conclusions. Preparatory arousal appears to be the most effective strategy although other strategies like imagery, self-talk and attentional focus also have empirical support. The range of tasks that have been used to measure force production have been limited to movements such as handgrip, leg extension, bench press, sit-ups, press-ups, pull-ups, and the standing broad jump. Additionally, most studies have used undergraduate and/or untrained samples. Only a very small number of studies have examined well-trained individuals. Currently, no explanation for why psyching-up may influence force production has any substantive support. Although a small number of studies have examined moderating and mediating variables, few consistent patterns have emerged and knowledge in this area is somewhat restricted. Given the importance that many athletes place on their mental preparation just prior to performance this is an area that warrants further examination. Research needs to examine a range of complex sport-specific tasks and use well-trained samples. Additionally, research needs to further examine why psyching-up may enhance force production.

Muscle strength testing: use of normalisation for body size

Assessment of muscle strength tests has been a popular form of testing muscle function in sports and exercises, as well as in other movement-related sciences for several decades. Although the relationship between muscle strength and body size has attracted considerable attention from researchers, this relationship has been often either neglected or incorrectly taken into account when presenting the results from muscle strength tests. Two specific problems have been identified. First, most of the studies have presented strength data either non-normalised for body size, or normalised using inappropriate methods, or even several different normalisations have been applied on the same sets of data. Second, the role of body size in various movement performances has been neglected when functional movement performance was assessed by muscle strength. As a consequence, muscle function, athletic profiles, or functional movement performance assessed by tested muscle strength have been often confounded by the effect of body size. Differences in the normalisation methods applied also do not allow for comparison of the data obtained in different studies. Using the following allometric formula for obtaining index of muscle strength, S, independent of body size (assessed by body mass, m) should be recommended in routine strength testing procedures: The allometric parameter should be either b = 0.67 for muscle force (recorded by a dynamometer), or b = 1 for muscle torque (recorded by an isokinetic apparatus). We also recommend using body-size-independent indices of both muscle strength and movement performance when assessing functional performance from recorded muscle strength or vice versa.

Low- or high-intensity strength training partially restores impaired quadriceps force accuracy and steadiness in aged adults

Because many daily tasks are executed at only a fraction of maximal strength, an understanding of submaximal force control may be important for improving function in aged adults. We compared the effects of low- and high-intensity (LI and HI, respectively) strength training on maximal and explosive strength and on the accuracy (force error) and steadiness (variability) of submaximal quadriceps force in elderly humans. Older subjects (age, 72 years; n = 27) had 57% lower maximal strength in comparison with young subjects (age, 21 years; n = 10). Older subjects had 190% (19 N), 50% (1 N), and 80% (4 N) more force error in matching 25 N of quadriceps force during eccentric, isometric, and concentric contractions, and had 157%, 0%, and 60% more variability in these forces compared with young subjects. Force error and force variability were correlated with each other but not with maximal strength. Thirty sessions of LI (n = 9 participants) or HI (n = 9 participants) training of equal total work increased maximal strength in the older subjects by 29%. Training also significantly reduced force error and variability--by 31% and 30%, respectively--of eccentric and concentric contractions. A control group of older subjects (n = 9) showed no significant changes in any variables. LI or HI strength training was equally effective in partially restoring elderly adults' maximal strength and control of submaximal force.

Isotonic dynamometry for the assessment of power and fatigue in the knee extensor muscles of females

Impairments in muscle power production and recovery following short-duration intense activity could lead to decreased performance and risk of injury. We developed a power test for the knee extensor muscles using torque-velocity testing and moderate isotonic loads. Twenty-eight female volunteers performed three maximal efforts at each of four isotonic loads (27.1, 40.6, 54.2 and 67.8 N. m). If the calculated regression line for the torque-velocity data had an r2 >/= 0.95 (i.e. an acceptable test), maximal power (408 +/- 56 W) was computed from the data. Immediately after torque-velocity testing, the subjects repeated maximal effort knee extensions with 33.9 N. m for three bouts of 15 repetitions with 15 s of rest to produce muscle fatigue, defined as a decrease in power output during isotonic exercise. After a 4 min rest, the torque-velocity test was repeated and power calculated (345 +/- 48 W). For the group, the recovery of maximal power after the fatigue protocol was 85%. The extremes were represented by one subject who recovered only 70% of her maximal power and another who recovered completely (>98%). Physiological differences in muscle power following repeated exercise could have an impact on the outcome of therapeutic interventions for sports injuries, fatigue syndromes and occupational over-use conditions.