Twitch Contractile Properties Research Articles

Influence of maximal muscle strength and intrinsic muscle contractile properties on contractile rate of force development

'Explosive' muscle strength or contractile rate of force development (RFD) is a term to describe the ability to rapidly develop muscular force, and can be measured as the slope of the torque-time curve obtained during isometric conditions. Previously, conflicting results have been reported regarding the relationship between contractile RFD and various physiological parameters. One reason for this discrepancy may be that RFD in various time intervals from the onset of contraction is affected by different physiological parameters. The aim of the present study was to investigate the relationship between voluntary contractile RFD in time intervals of 0-10, 0-20, ..., 0-250 ms from the onset of contraction and two main parameters: (1) voluntary maximal muscle strength and (2) electrically evoked muscle twitch contractile properties. The main finding was that voluntary RFD became increasingly more dependent on MVC and less dependent on muscle twitch contractile properties as time from the onset of contraction increased. At time intervals later than 90 ms from the onset of contraction maximal muscle strength could account for 52-81% of the variance in voluntary RFD. In the very early time interval (<40 ms from the onset of contraction) voluntary RFD was moderately correlated to the twitch contractile properties of the muscle and was to a less extent related to MVC. The present results suggest that explosive movements with different time spans are influenced by different physiological parameters. This may have important practical implications when designing resistance training programs for specific sports.

Age-Related Changes in Twitch Properties of Plantar Flexor Muscles in Prepubertal Children

The twitch of the triceps surae muscle (TS), which characterizes the contractile properties independently of volition, differs in amplitude, but not in time course, when evoked in pre or postpubertal children. The aim of the present study was to compare the TS twitch contractile properties in prepubertal children (7 to 11 y). M-wave and twitch were recorded at rest by supramaximal electrical stimulations of the posterior tibial nerve. Twitches were characterized by peak torque (Pt), contraction time (CT), half relaxation time (HRT), and rate of torque development (dPt/dt). Electromechanical delay (EMD) was quantified with regard to the TS M-wave onset. Pt values increased significantly with the age of the prepubertal children but remained lower than that for adult subjects. CT and HRT values did not change with age. Thus, dPt/dt increased significantly between the 7-year-old and the 11-year-old children but remained significantly lower than that for adults. Despite EMD values decreased with age, they remained significantly higher than those of adult subjects. These results confirmed the link between growth processes and the increase in twitch torque for prepubertal children within a limited range of age. However, the time-course characteristics were not affected by age. The increase in dPt/dt and the decrease in the EMD could be used as indirect indicators of changes in contractile kinetics and in musculo-tendinous stiffness with the age of the prepubertal children. The age-related relationships established by this study will serve as reference values for clinical testing of the TS performances in relation to muscle disease or disuse.

How Central is Central Fatigue?

Fatigue is defined as the exercise-induced reduction in the ability of muscle to exert maximal force or power. Some of the failure occurs because of the inability of the central nervous system to drive the muscle. This is termed central fatigue. It is unclear whether central fatigue induced by contracting a muscle on one side of the body can alter the neural drive to the unfatigued contralateral, homologous muscle. PURPOSE Our purpose was to determine whether central fatigue induced by repeated contractions of one quadriceps group alters the activation of the unfatigued, homologous, contralateral muscle. METHODS Ten healthy, untrained males (age = 23.3, height =1.79m, weight = 76.5kg) volunteered as subjects in a repeated measures experiment. The subjects participated in a familiarization session and two separate days of data collection. Maximal isometric force, surface electromyographic activity and twitch contractile properties were measured on the right and left quadriceps and hamstring muscles before and after a standard fatigue protocol. On Day 1 the subjects made repeated contractions of 30% maximal voluntary contraction (MVC) until they could no longer hold the target force. Day 2 was a control day with no fatigue protocol. Central fatigue was assessed with the interpolated twitch technique. RESULTS These low intensity submaximal contractions induced central fatigue, as demonstrated by a 20% decline in voluntary activation and 38% decline in MVC in the experimental limb. In the unfatigued limb there was a small (7%) but significant (p<0.03) decline in MVC that was accompanied by a 4% decline (p<0.02) in percent activation. There was no decline in the MVC and percent activation from pre to post in the control experiment. CONCLUSIONS These data suggest that fatigue is somehow transferable from an active muscle on one side to the contralateral, homologous muscle. We are unable to determine whether the mechanism involves cortical or spinal regions. Supported by NSERC grant to EC.

How Central is Central Fatigue?

Fatigue is defined as the exercise-induced reduction in the ability of muscle to exert maximal force or power. Some of the failure occurs because of the inability of the central nervous system to drive the muscle. This is termed central fatigue. It is unclear whether central fatigue induced by contracting a muscle on one side of the body can alter the neural drive to the unfatigued contralateral, homologous muscle. PURPOSE Our purpose was to determine whether central fatigue induced by repeated contractions of one quadriceps group alters the activation of the unfatigued, homologous, contralateral muscle. METHODS Ten healthy, untrained males (age = 23.3, height =1.79m, weight = 76.5kg) volunteered as subjects in a repeated measures experiment. The subjects participated in a familiarization session and two separate days of data collection. Maximal isometric force, surface electromyographic activity and twitch contractile properties were measured on the right and left quadriceps and hamstring muscles before and after a standard fatigue protocol. On Day 1 the subjects made repeated contractions of 30% maximal voluntary contraction (MVC) until they could no longer hold the target force. Day 2 was a control day with no fatigue protocol. Central fatigue was assessed with the interpolated twitch technique. RESULTS These low intensity submaximal contractions induced central fatigue, as demonstrated by a 20% decline in voluntary activation and 38% decline in MVC in the experimental limb. In the unfatigued limb there was a small (7%) but significant (p<0.03) decline in MVC that was accompanied by a 4% decline (p<0.02) in percent activation. There was no decline in the MVC and percent activation from pre to post in the control experiment. CONCLUSIONS These data suggest that fatigue is somehow transferable from an active muscle on one side to the contralateral, homologous muscle. We are unable to determine whether the mechanism involves cortical or spinal regions. Supported by NSERC grant to EC.

Changes in the human muscle force-velocity relationship in response to resistance training and subsequent detraining

Previous studies show that cessation of resistance training, commonly known as "detraining," is associated with strength loss, decreased neural drive, and muscular atrophy. Detraining may also increase the expression of fast muscle myosin heavy chain (MHC) isoforms. The present study examined the effect of detraining subsequent to resistance training on contractile performance during slow-to-medium velocity isokinetic muscle contraction vs. performance of maximal velocity "unloaded" limb movement (i.e., no external loading of the limb). Maximal knee extensor strength was measured in an isokinetic dynamometer at 30 and 240 degrees/s, and performance of maximal velocity limb movement was measured with a goniometer during maximal unloaded knee extension. Muscle cross-sectional area was determined with MRI. Electromyographic signals were measured in the quadriceps and hamstring muscles. Twitch contractions were evoked in the passive vastus lateralis muscle. MHC isoform composition was determined with SDS-PAGE. Isokinetic muscle strength increased 18% (P < 0.01) and 10% (P < 0.05) at slow and medium velocities, respectively, along with gains in muscle cross-sectional area and increased electromyogram in response to 3 mo of resistance training. After 3 mo of detraining these gains were lost, whereas in contrast maximal unloaded knee extension velocity and power increased 14% (P < 0.05) and 44% (P < 0.05), respectively. Additionally, faster muscle twitch contractile properties along with an increased and decreased amount of MHC type II and MHC type I isoforms, respectively, were observed. In conclusion, detraining subsequent to resistance training increases maximal unloaded movement speed and power in previously untrained subjects. A phenotypic shift toward faster muscle MHC isoforms (I --> IIA --> IIX) and faster electrically evoked muscle contractile properties in response to detraining may explain the present results.

Modulation of motor unit discharge rate and H-reflex amplitude during submaximal fatigue of the human soleus muscle.

Declining motor unit discharge rates and H-reflex amplitude have been observed in separate experiments during fatiguing submaximal contractions in humans. The purpose of this experiment was to investigate motor unit discharge rate, H-reflex amplitude, and twitch contractile properties concurrently during a fatiguing submaximal isometric contraction of the ankle plantarflexors. Eleven healthy subjects performed fatiguing contractions of low force (25% maximal voluntary contraction (MVC)) or high force (42-66% MVC). Hoffmann (H)-reflexes, muscle compound action potentials (M-waves), twitch contractile properties, and motor unit discharges were recorded from the soleus muscle. In the low-force fatigue task, motor unit firing rate increased gradually over time, whereas the resting H-reflex was significantly depressed at 15% of endurance time and remained quasiconstant for the rest of the task. This suggests that the processes mediating the resting H-reflex depression are relatively independent of those modulating the motor unit firing rate during a low-force fatigue task. In the high-force fatigue task, a decline in the average motor unit discharge rate was accompanied by a decrease in the resting H-reflex amplitude and a prolongation of the twitch half-relaxation time (HRT) at the completion of the fatigue task. Overall, motor unit firing rate was modulated in parallel with changes in the twitch HRT, consistent with the muscle wisdom hypothesis.

Comparison of Twitch Contractile Properties of Plantar Flexor Muscles in 9–10-Year-Old Girls and Boys

Twitch contractile properties of plantar flexor muscles were compared in 9- to 10-year-old girls and boys. No significant gender differences (p &gt; .05) in isometric maximal voluntary contraction force and twitch peak force, contraction and relaxation times, and twitch maximal rate of force development in either resting or potentiated state have been observed. However, boys had significantly greater (p &lt; .05) twitch postactivation potentiation and potentiated twitch maximal rate of relaxation than girls. These results indicated that twitch force-potentiation capacity of skeletal muscles prior to puberty is more highly developed in boys than girls.

Effect of force level and training status on contractile properties following fatigue.

This study examined the effects of fatigue on the contractile properties of the twitch contraction evoked by the H-reflex and the maximal M-wave. Untrained subjects demonstrated more pronounced slowing of half relaxation time (HRT) in the H-reflex twitch than in the M-wave twitch and also more slowing of HRT in high force vs. low force contractions. Endurance-trained subjects, however, demonstrated a shortening of the H-reflex twitch. The twitch contractile properties following fatigue are dependent on force level and training status.

Comparison of Twitch-Contractile Properties of Plantar-Flexor Muscles in Young and 52- to 63-Year-Old Men

This study compared maximal voluntary-contraction (MVC) force and twitch-contractile properties of the plantar-flexor muscles in resting and postactivation potentiation slates of 2 groups of men matched for similar levels of physical activity: young (19- to 22-year-olds. n = 13) and 52–63 years old (n = 12). MVC force, twitch peak force (PT), maximal rates of force development and relaxation, and postactivation potentiation were higher (p &lt; .05) in young than in 52- to 63-year-old men. In young men. potentiated-twitch PT was 23.3% higher (p &lt; .01) than resting twitch. Resting- and potentialed-twitch-contraction times were 16.7% and 18.3% shorter, respectively (p &lt; .001), in young than in 52- to 63-year-old men. These Findings suggest that late middle age is characterized by reduced capacity for evoked twitch-force generation and potentiation and slowed speed of contraction of the plantar-flexor muscles.

The Effect of 5, 10, and 20 Repetition Maximums on the Recovery of Voluntary and Evoked Contractile Properties

Maximal strength training has been reported to emphasize neural adaptations. The main objective of this study was to detect differences in muscle activation between 5, 10, and 20 repetition maximum (RM) sets. Fourteen subjects performed elbow flexion with 5, 10, and 20RM. Subjects were tested for maximum isometric force (maximal voluntary contraction [MVC]), twitch amplitude (peak twitch [Pt]), time to peak twitch (TPT), half relaxation time ((1/2) RT), electromyography (EMG), and muscle activation (interpolated twitch). Subjects were tested preexercise and 30 seconds, 1, 2, and 3 minutes postexercise. There were no significant differences in MVC, muscle activation, or antagonist/agonist EMG after 5, 10, or 20RM. However, greater RM did have a greater detrimental effect on twitch properties than fewer RM. Peak twitch was significantly (p = 0.004) less (32.08%) for the 20 than for the 5RM, whereas TPT shortened (p < 0.05) by 7.3 and 11.1% with 10 and 20RM vs. 5RM, respectively. Half relaxation time at 20RM was shortened (p < 0.05) by 20.6 and 25.4% compared with that at 5 and 10RM, respectively. MVC, muscle activation, and temporal twitch properties did not recover within 3 minutes of recovery. In conclusion, whereas 5RM did not produce greater muscle inactivation, twitch contractile properties were affected to a greater degree by a higher number of RM.

ESTROGEN REPLACEMENT THERAPY DOES NOT AFFECT MUSCLE CHARACTERISTICS IN HEALTHY POST-MENOPAUSAL WOMEN

Estrogen replacement therapy (ERT) may preserve muscle function in aging post-menopausal women. This is not a universal observation, and the mechanisms of this purported effect have not been fully investigated. The purpose of this cross-sectional study was to test the hypothesis that post-menopausal women on long-term ERT would have higher leg muscle isometric force and activation compared with women not on ERT. We measured isometric dorsiflexion strength (maximal voluntary contraction, MVC), central activation (central activation ratio: CAR = MVC/[MVC + superimposed stim. force]), peripheral excitability (compound muscle action potential amplitude, CMAP), and twitch contractile properties (peak force, time to peak force); as well as isometric plantar flexion strength in 24 ERT (71.1 ± 5.0 years, mean ± SD) and 25 nonERT (71.8 ± 4.6 years) healthy women (all ≥ 10 years post-menopause). The groups were matched by age and physical activity level (measured for 7 days in all subjects using a 3-dimensional accelerometer). Analyses were by unpaired t tests. Age, height, weight, activity and number of years post-menopause were not different between groups. Muscle data (mean ± SD) are summarized in the Table.TableThere were no differences between groups in any of the muscle measures. In contrast to previous reports and our hypothesis, these data indicate that long-term ERT confers no benefit in leg muscle strength, activation and contractile properties in healthy post-menopausal women. The results suggest that ERT and nonERT women may be grouped together in future studies of muscle function in aging. Supported by NIA R21 AG1620

Age-related differences in twitch contractile properties of plantarflexor muscles in women.

This study compared maximal voluntary isometric strength and electrically evoked twitch characteristics of the plantarflexor muscles among the groups of women of the 3rd (n=14), 4th (n=13), 5th (n=11), 6th (n=12) and 8th (n=13) decade. A significant decrease (P < 0.05-0.001) has been found in isometric maximal voluntary contraction (MVC) force from the 5th decade and in twitch maximal force from the 6th decade. The 3rd decade group produced 72% greater MVC force and 43% greater twitch maximal force than the 8th decade group (P < 0.001). A prolongation (P < 0.01) in twitch contraction time was observed from the 5th decade. Twitch contraction time in the 3rd decade group was 16% shorter (P < 0.001) compared with the 8th decade group (P < 0.01). Twitch half-relaxation time did not differ significantly (P > 0.05) among the groups. A decrease (P < 0. 05-0.001) has been found in twitch maximal rate of force development from the 5th decade and in twitch maximal rate of relaxation from the 6th decade. The 3rd decade group produced 63% greater (P < 0. 001) twitch maximal rate of force development than the 8th decade group. It was concluded that in women a marked age-related reduction in maximal voluntary force-generating capacity of the plantarflexor muscles and speed of contraction of the electrically evoked twitch takes place after 40 years of age, while reduction in maximal force-generating capacity and speed of relaxation of the twitch occurs after 50 years of age.

Twitch contractile properties of plantar flexor muscles in power and endurance trained athletes.

This study compared twitch contractile properties of plantar flexor muscles among three groups of 12 subjects each: endurance and power trained athletes and untrained subjects. The posterior tibial nerve was stimulated by supramaximal square wave pulses of 1-ms duration. Power trained athletes had higher twitch maximal force, maximal rates of force development and relaxation and also maximal voluntary contraction (MVC) force. The trained subjects had a smaller twitch maximal force: MVC force ratio and shorter twitch contraction and half-relaxation times than the untrained subjects with no significant differences between the two groups. Thus, the short time for evoked twitches in the athletes compared to the untrained subjects would seem unrelated to the type of training. It is concluded that power training induces a more evident increase of muscle force-generating capacity and speed of contraction and relaxation than endurance training.

Influence of gender on post-tetanic potentiation in human dorsiflexors

Twitch contractions of the ankle dorsiflexors were evoked before and after 7 s of tetanic stimulation at 100 Hz in young women and men. Torque decreased more in men (18%) than in women (12%) during the tetanus. There was no gender difference in twitch peak torque potentiation over the 5-min post-tetanus. Potentiation was 42% (women) and 45% (men) at 5 s post-tetanus, and still present at 5 min (women 24%, men 25%). The immediate (5 s) shortening of twitch rise time was similar in women (14%) and men (13%), but during the 5-min men's rise time came to exceed whereas women's only approached pretetanus values (e.g., +9% vs. -1% at 5 min). The immediate decrease in half-relaxation time was also similar in women (24%) and men (22%); however, women's but not men's values remained less than pretetanus values for most of the 5-min period. Twitch rate of torque development increased similarly (75%) in women and men at 5 s, with no gender difference over 5 min. In contrast, rate of torque relaxation increased significantly only in men. Rate of torque development normalized to peak torque was similar in women and men pretetanus and increased similarly 5 s post-tetanus, but women had greater values through most of the 5-min post-tetanus. Normalized rate of torque relaxation was similar in women and men and not affected by tetanus. In the dorsiflexor muscles, young women and men show a similar amount and pattern of twitch force potentiation, but there are gender differences in time-related twitch contractile properties in the first 5 min after tetanus.Key words: contraction, muscle, twitch.

Age-related changes in the twitch contractile properties of human thenar motor units.

The purpose of this study was to examine the effects of aging on the contractile and electrophysiological properties of human thenar motor units (MUs). Percutaneous electrical stimulation of single motor axons within the median nerve was used to isolate and examine the twitch tensions, contractile speeds, and surface-detected MU action potential (S-MUAP) sizes of 48 thenar MUs in 17 younger subjects (25-53 yr) and 44 thenar MUs in 9 older subjects (64-77 yr). A wide range of twitch tensions, contractile speeds, and S-MUAP sizes was observed in both age groups. However, older subjects had significantly larger MU twitch tensions and slower MU twitch contraction and half-relaxation times. These changes were accompanied by increased S-MUAP sizes. These findings suggest that the human thenar MU pool undergoes significant age-related increase in MU size and slowing of contractile speed. Such adaptation may help to overcome previously reported age-related losses of thenar MUs.

Perchlorate differentially potentiates excitation-contraction coupling of diaphragm muscle from mdx and control mice.

The effects of perchlorate (6-40 mM) on contractile properties and depolarization-contraction coupling of diaphragm have been tested in muscle fibre bundles from 8 to 12-week-old mdx and control mice. Twitch contractile properties were affected by perchlorate to a similar extent in mdx and control muscles. In the absence of perchlorate, voltage-dependent parameters of both activation and inactivation were similar in mdx and control muscles. In both mdx and control muscles, perchlorate induced a shift of activation and inactivation curves toward more negative potentials. However, mdx diaphragm fibres were less sensitive to perchlorate in the low range of concentrations (6-12 mM). These results are unlikely to be due to a possible effect of the lack of dystrophin on the excitation-contraction coupling voltage sensor or ryanodine receptor. However, necrosis and regeneration occur in mdx mouse diaphragm and it is postulated that the presence of a high proportion of not fully mature fibres could contribute to the reduced sensitivity of mdx diaphragm fibres to perchlorate.

THE EFFECTS OF CLENBUTEROL ON MUSCLE MASS, CONTRACTILITY AND WORK CAPACITY IN HINDLIMB-SUSPENDED RATS 988

Twenty-three male Fisher-344 rats were used to investigate the effects of low levels of clenbuterol on retention of muscle mass, work capacity, and muscle twitch contractile properties after a 3-week hindlimb suspension. Groups consisted of normal control (XX), n = 6; suspension control (SX), n = 6; suspension placebo (SP), n = 7; and suspension clenbuterol (SC), n = 6. Clenbuterol was administered I.P., 20 μg/kg/d for 6 days, then 10μg/kg/d thereafter. After 3 weeks of hindlimb suspension, body weight (BW) was significantly less in SX (232 ± 7.6 g) and SP (257.3 ± 17.1 g) than XX (314.8 ± 12 g). There was no significant BW change in SC. Soleus muscles (SOL) of SX, SP, and SC atrophied significantly (XX: 136.1± 4.25 mg, SX:72.3 ± 2.3 mg, SP:82.7 ± 8.9 mg, and SC:85.4 ± 9.6 mg). Normalized SOL (mg SOL/g BW) significantly decreased in SX, SP, and SC (XX:44 ± 0.8%, SX:31 ± 0.8%, SP:32 ± 2% and SC:33 ± 2%). Treadmill running time to exhaustion did not change after suspension in any group. Time to peak twitch tension of SOL was less in SP and SC (XX: 61 ± 11 ms, SP:35 ± 3 ms, and SC:35 ± 5 ms). Twitch one-half relaxation time of SOL was shorter in SP and SC (XX: 109± 7 ms, SP:77 ± 3 ms, and SC:67 ± 10 ms). These results suggest that clenbuterol retards body weight loss but does not improve aerobic work capacity in response to suspension. Suspended muscles have faster twitch properties but contractile properties were not altered by clenbuterol.

The Effects of Tapering on Strength Performance in Trained Athletes

The optimum pre-competition taper procedure for "strength athletes" is not known. We examined voluntary strength and evoked contractile properties of the elbow flexors over a 10 day rest only (ROT) and a 10 day reduced volume taper (RVT) in 8 resistance trained males (23 +/- 2.1 years). Following 3 wks of standardized training of the elbow flexors, subjects were randomly assigned to one of the tapers. Upon completion, they resumed training for 3 wks and completed the other taper. No arm training was performed during the ROT, while high intensity, low volume training was done every second day during the RVT. Maximum isometric (MVC), low (0.52 rad.s-1; LV) and high velocity (3.14 rad.s-1; HV) concentric peak torque, and evoked isometric twitch contractile properties were measured before and after each training phase and every 48 h during each taper. ANOVA comparison of the tapers revealed that MVC increased (p < or = 0.05) over pre-taper values throughout the RVT (measurement days 2, 4, 6, 8 and 10), as did LV at 2, 4, 6, and 8 d. MVC did not change over the ROT but LV was significantly higher on day 2 and lower on days 8 and 10. LV was also greater on days 4, 6, 8 and 10 during the RVT compared to the ROT. The evoked contractile properties remained largely unchanged. The data indicate that resistance-trained athletes can improve low velocity concentric strength for at least 8 days by greatly reducing training volume, but maintaining training intensity.

Strength and Contractile Adaptations in the Human Triceps Surae after Isotonic Exercise

The purpose of this study was to determine whether twitch contractile properties and strength of the triceps surae could be altered by 8 weeks of low-repetition or high-repetition isotonic exercise. Subjects were randomly assigned to either the low- or high-repetition group. Before- and after-training measurements were recorded for strength and contractile properties. The contractile variables of the muscle twitch were latency, time to peak force, peak force, half-contraction time, and half-relaxation time. Strength measurements were determined utilizing a one repetition maximal (1-RM) heel-raise testing device. A two-way ANOVA with repeated measures was used to test the effect of training on each variable. Both groups showed a significant increase in 1-RM and half-relaxation time and a decrease in electrical stimulation current after the 8-week training period. It was concluded that if high-repetition exercises develop slow-twitch Type I muscle fibers and low-repetition exercises develop fast-twitch Type II fibers, training programs must be designed specifically according to the desired outcome.

Elbow flexor evoked twitch contractile properties in untrained men and women and male bodybuilders.

The influence of elbow joint angle on elbow flexor isometric evoked twitch contractile properties was assessed in 15 young women (F), 18 young men (M) and 11 male bodybuilders (BB). Measurements were made at elbow joint angles of 1.31, 1.57, 1.83, 2.09, 2.36, 2.62 and 2.88 rad (3.14 rad = 180 degrees = full extension). The largest peak twitch torque values [mean (SE) N.m] in F [3.77 (0.20)], M [10.38 (0.68)] and BB [11.38 (1.05)] occurred at 2.88 rad. Peak torque was progressively smaller at smaller joint angles, but the decline from 2.88 to 1.31 rad was greater in M (68%) and BB (76%) than F (59%). Thus, the magnitude of intergroup differences in peak twitch torque (PT) was joint angle dependent. Twitch time to peak torque (TPT) was influenced in a complex way by joint angle in the three groups; BB had the lowest values at small joint angles but the highest values at the largest angles. Half-relaxation time (HRT) generally increased from the smallest to largest joint angles in a pattern that did not differ significantly among the three groups. Maximum rates of twitch torque development and relaxation showed the same pattern of results as PT, indicating that these time-related measures were more sensitive to joint angle effects on PT than on TPT or HRT. The results of this study indicate that careful consideration should be given to the selection of joint angles in the measurement of evoked twitch contractile properties for the purpose of making group comparisons or investigating the effects of interventions such as training.