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Twitchpotentiationcapacityofplantarflexormusclesinwomenwithincreasingage

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Biology of Sport, Vol. 19 N°3, 2002

TWITCH POTENTIATION CAPACITY OF PLANTARFLEXOR MUSCLES IN WOMEN WITH INCREASING AGE

M.Pääsuke, J.Ereline, H.Gapeyeva, P.Sander, S.Sirkel Institute of Exercise Biology, University of Tartu, Estonia

Abstract. Twitch potentiation capacity of the plantarflexor (PF) muscles among the groups of women of the 3rd (n=12), 5th (n=12) and 8th (n=12) decade of their life was compared. The posterior tibial nerve in popliteal fossa was stimulated by supramaximal square-wave pulses of l ms duration at rest and after brief isometric maximal voluntary contraction (MVC). The 3Td decade group had significantly greater MVC force, and twitch post-activation potentiation and maximal rate of force development (RFDmax,) as well as shorter twitch contraction time (CT) than the 5th and 8th decade group. Two younger groups of women produced significantly greater iwitch maximal force (Fmax) and rate of relaxation (RRmax) than the 8th decade group. The potentiated twitch Fmax, RFDmax and RRmax were significantly greater, and twitch CT shorter than resting twitch only for the 3rd decade group. These data suggest that the mechanisms responsible for twitch potentiation are markedly influenced by ageing.

(BioLSport 19:213-223. 2002)

Key words: Human plantarflexor muscles - Contractile properties - Twitch potentiation - Women - Ageing

Introduction

Ageing is associated with decreased muscle strength and speed of contraction [4,6,10.24,34]. There have been numerous studies assessing the age-related differences in maximal voluntary isometric or isokinetic muscle strength. Less attention has been paid to the age-related changes in the electrically evoked twitch contractile properties of the human skeletal muscles- However, the stimula ted twitch contraction characteristics can be used for measuring muscle force-generating and force-potentiation capacity, speed of contraction and relaxation independently from skill and motivation.

Reprint request to: Prof. Mati Pääsuke, Institute of Exercise Biology, University of Tartu, 5 Jakobi Street. 51014 Tartu, Estonia; Tel./fax: +37 27 376 286; E-mail: matip@ut.ee

It is well documented that there is an enhancement of twitch amplitude after maximal voluntary contractions. The increases in twitch maximal force, which result the prior activity, are referred to as post-activation potentiation. Postactivation potentiation of human skeletal muscles has been measured in respect to childhood and adolescence [5,26] and the adaptation of the subjects at different to strength and power training [1,24,28,35]. A decrease in twitch potentiation capacity with ageing has been reported [23,25,33], yet it is not clear at which age this impairment begins.

The aim of this study was to measure differences in twitch potentiation capacity of skeletal muscles in women with increasing age. The maximal isometric contraction force and supramaximal twitch characteristics in resting and potentiated state were compared among the groups of female subjects of the 3rd, 5th and 8th decade. Recordings were performed from the plantarflexor muscles which are important in posture and movement and are invo lved many working and sport activities.

Material and Methods

Table l Age and physical characteristics of the subjects (mean±SE)

Groups n Age (years)

Height (cm)

Body mass (kg)

3rd decade 12 20.5±0.3 170.2±1.6 59.8±1.5 5th decade 12 44.2±0.9 164.1±1.4* 63.8±3.3 8th decade 12 71.4±0.9 160.8±1.9* 64.3±2.0# P<0.01 compared with 3rd decade group; #P<0.05 compared with other groups A total of 36 female subjects agreed to participate in the present study. The subjects were distributed into 3 age groups: 3rd decade group (aged 20-24 years, n=l2), 5th decade group (aged 40-49 years, n=l2) and 8th decade group (70-77 years, n=l2). The subjects were screened by questionnaire to exclude those with diagnosed musculoskeletal disorders. They were habitually physically active. Man of the subjects performed recreational physical activities such as walking, jogging swimming and aerobics. However, none of them had any background in competitive sports of any kind. The elderly people all lived at home and performed activities of daily life independently. All the subjects were informed of the

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procedures to be utilized as well as the purpose of the study and their written informed consent was obtained- The study carried the approval of the University Ethics Committee. During the measurement the subjects were seated in a specially designed chair with the dominant leg flexed to 90° ai ihe knee and mounted inside a metal frame. The fool was strapped to an aluminium footplate- The inclination of the foot could be altered by rotating the footplate about an axis that corresponded to that of the ankle joint, i.e. the medial malleolus. The ankle was dorsiflexed to 20° to ensure maxima] voluntary and stimulated torques and presumably corresponding to the "optimal" muscle length [29]. The kneecap and front side of the thigh were held down by an adjustable pad. Torques acting on the footplate were sensed by a strain-gauge transducer connected with the footplate. Signa ls from the strain-gauge transducers were linear from 10 to 1600 N. The point of application of force lo the footplate was located on articulation regions between the metatarsus and ossa digitorum pedis. The force signals were sampled at a frequency of l kHz and stored on a hard disk. The reproducibility of the torque measurements was calculated with repeated static loads on the footplate. Twenty-four to 48 h before collecting data the subjects were given instructions and the testing of isometric maximal voluntary contraction (MVC) force of the plantarflexor muscles and electrical stimulation procedures were demonstrated. This was followed by a practice session to familiarize the subjects with the procedures. The determination of the subject's dominant leg was based on a kicking preference. To measure MVC force of the plantarflexor muscles, the subjects were instructed lo push the footplate as forcefully as possible for 2-3 s. Verbal encouragement and visual feedback were used to motivate the subjects- The greatest force out of three maximal efforts was taken as the MVC force- A rest of 2 min was allowed between each of the three attempts. To determine the contractile properties of the planlarflexor muscles during an isometric twitch, the posterior tibial nerve was stimulated through a pair of surface carbon-rubber electrodes (Nemectron, Germany). The cathode (2x4 cm) was placed over the tibial nerve in the popliteal fossa and the anode (7 x 12.5 cm) was placed under the posterior- medial side of the thigh. Elastic bandages were used to keep the electrodes in place and to ensure good electrode contact. Supramaximal square wave pulses of l ms duration were delivered from an isolated voltage stimulator Medicor MG-440 (Budapest, Hungary). The evoked compound action potential (M-wave) of the soleus muscle was recorded using bipolar (20 mm interelectrode distance) electromyogram (EMG) e lectrodes (Beckman miniature skin electrodes). The electrodes were placed longitudinally on the motor point area of the soleus muscle

determined by electrical stimulation after the skin had been cleaned using alcohol swabs and abraded lightly with tine sand paper. AS a reference electrode a large carbon rubber pla te (Nemectron, 7 x 12.5 cm) was placed over the proximal part of the triceps surae muscle between the stimula ting and recording electrodes. The EMG signa ls were amplified and displayed using a Medicor MG-440 (Budapest, Hungary) preamplifier with a frequency band ranging from l Hz to 1 KHz- These signals were sampled at l kHz. During isometric twitch recording the stimulus intensity varied from approximately 25 V to supramaximal (130-150 V) in increments of3ö%-50%. Single stimuli were given at 30-s intervals and the voltage was increased in increments of 20-25 V until supramaximal twitches were reached, The maximal amplitude of the M-wave was used as a criterion for determining the supramaximal intensity of the stimulus. After the resting twitch had been recorded. the subject was instructed to make a MVC for 5 s and then to relax. A second (potentiated) twitch took place within l s after the onset of relaxation. The following characteristics of resting and potentiated twitch were calculated: twitch maximal force (Fmax:) - the highest value of isometric force production, contraction time (CT) - the time to twitch maximal force, half-relaxation time (HRT) - the time of half of the decline in twitch maxima! force. maximal rate of forces development (RFDmax) - the first derivate of the development of force (dF/dt) and maximal rate of relaxation (RRmax) as the first derivate of the decline of force (-dF/dt). The percentage increase in the potentiated twitch Fmax-. in relation to resting one was taken as an indicator of the post-activation potentiation (PAP). Standard statistical methods were used for calculation of means and standan errors (+SF-.). One-way analysis of variance (ANOVA) followed by Scheffe pos hoc comparisons were used to test for differences between groups. A level o P<0.05 was selected to indicate statistical significance.

Results

Mean value (±SE) of 858.1±46.8 N for MVC force of the plantarflexor muscles in the 3rd decade group was significantly greater than that of 733.8±43.9 (P<0.05) and 480.9±44.0 N (P<0-001) m the 5th and 8lh decade group, respectively. The difference in MVC force between two older groups was statistically significant (P<0.001). The 3rd and 5th decade groups had significantly greater resting and potentiated twitch Fma x compared with the 8th decade group, whereas this characteristic in t 3rd and 5th decade groups did not differ significantly (P>0.05) (Fig. l, left). The potentiated twitch F max was significantly greater compared with resting twitch only

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Twitch potentiation capacity… 217 Biol.Sport 19(3), 2002 for the 3rd decade group. PAP in the 3rd decade group was significantly greater than in the 5th and 8th decade groups (Fig. l, right). The difference in PAP between the two oldest groups was not significant (P>0.05). Fig. l Twitch maximal force (Fmax; left) and post-activation potentiation (PAP; right) in women with increased age. Values are means±SE; Rest - Resting twitch; Pot -Potentiated twitch; * P<0.05; ** P<0.01; *** P<0.001 The 3rd decade group demonstrated significantly shorter twitch CT in resting and potentiated state than that of the 5th and 8th decade groups (Fig. 2, left). The differences in twitch CT between two older groups were not significant (P>0.05). Potentiated twitch CT values were significantly shorter compared with resting twitch for the 3rd and 8th decade groups. There were no significant between-group-differences (P>0.05) in mean values of twitch HRT in either resting or potentia ted state (Fig. 2, right).

Fig.2 Twith contraction tiine (CT; left) and half-relaxation time (HRT) in women wit increasing age- For other explanations see Fig. l.

Twitch RFDmax decreased significantly with age (Fig. 3, le ft). The 3rd decade group produced greater RFDmax, in resting and potentiated stale tihan two older groups, and in the 5th decade group the corresponding characteristic was great that in the 8th decade group. Potentiated twitch RFDmax was significantly great compared with resting twitch only for the 3rd decade group. The 3rd and 5 decade groups produced significantly greater RRmax in resting and potentiated state than the 8th decade group (Fig. 3, right). No significant differences (P>0.05) twitch RRmax between two younger groups were found. Potentiated twitch RRmax was significantly greater compared with resting twitch only for the 3rd decade group.

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Fig.3 Maximal rates of twitch force development (RFD max; left) and relaxation (RR max;

right) in women with increasing age. For other explanations see Fig. l

Discussion

One of the main findings of this study is that in women a marked age-related reduction in maximal voluntary force-generating capacity of the plantarflexor muscles began earlier, i.e. after the age of 40 years, than the reduction in electrically evoked twitch maximal force-generating capacity, which did not differ significantly in 20-year-old and 40-year-old subjects. However, the resting and potentiated twitch maximal force was significantly reduced after 70 years of age- The decrease in muscle force-generating capacity with increasing age has been attributed to the reduction in muscle mass which is related to alterations in hormonal balance [10] and the decline in the quantity and intensity of physical activity [20]. It has been shown that a reduced muscle mass in elderly subjects is

associated with a decreased number and a selective atrophy of fast-twitch muscle fibres [2,3,4,18]. In the present study, elderly women had significantly higher body mass than young women. Rice et al. [27] found markedly more non-muscle tissue in plantarflexor muscles of older persons. Because of this age-related infiltration of fat and connective tissue, the reduction in muscle contractile tissue is greater than the actual reduction in muscle volume and muscle cross-sectional area. Thus, the age-related changes in muscle force-generating capacity can be explained, partly, with differences in body composition. It has been observed that a decline in maximal voluntary isometric force with ageing may be accompanied by a decrease in the capacity for neural activation of the muscles [l 1,12].

The present study indicated that young (20- to 24-year-old) women exhibited s significantly greater twitch force-potentiation capacity than middle -aged (40- to 49-year-old) and elderly (70- to 77-year-old) women. This agreed with the previously mentioned studies that reported decreased PAP of the ageing muscle; [23,25,33]. This study demonstrated also that potentiated twitch contraction time was significantly shorter, and maximal rates of force development and relaxation were significantly greater compared with resting twitch only for young women Two theories have attempted to explain the post-activation potentiation in the skeletal muscle: (l) increased sensitivity of the contractile apparatus to Ca2+ and/or (2) phosphorylation of the regulating light chains (R-LC) of myosin [8,22,32] Thus, the present study demonstrated that a marked decrease of twitch potentiation capacity in women began after the age of 40 years. This study agreed with the previously mentioned studies that reported slower electrically evoked twitch contraction time of the ageing muscles [19,33,34]. Th time course of isometric twitches has been found to depend on the kinetics of the excitation-contraction coupling mechanisms, including intracellular Ca2+ movements [14,15]. The prolonged twitch contraction time of older individual suggest decreased efficiency in the function of the sarcoplasmic reticulum t release Ca2+ [13]. In the present study, maximal rate of twitch force development was significantly lower in middle -aged and elderly women compared with young women. The rate of force development has rarely been used as an indicator i contraction speed, which depends largely on the rate of formation of cross-bridge between myosin and actin [17]. The decreased rate of force development has bee called early depression and the kinetics of Ca2+-release and binding to troponin have been accounted for by this phenomenon [31]. It has been shown that slower contraction speed in the elderly muscles is caused by the selective atrophy of fast twitch muscle fibres [16].

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Twitch potentiation capacity… 221 Biol.Sport 19(3), 2002 Contraversially to many earlier reports [6,19,23,34], in the present study no significant age-related differences in twitch half-relaxation time of the planlarflexor muscles in resting and potentiated state in women were observed. However, this study indicated a marked decrease of the rate of relaxation (-dF/dt) of the isometric twitch in elderly (70- to 77-year-old) women. A similar finding has also been reported for quadriceps femoris muscle [9]. Two main factors are responsible for the rate of muscle relaxation: sarcoplasmic reticulum Ca2+ uptake and the rate of cross-bridge kinetics [36]. It has been shown that the decreased rate of relaxation in elderly muscles is an indicator of reduced efficiency of the sarcoplasmic reticulum to re-uptake Ca2+ [9,13,23]. One factor which could also affect twitch contractile characteristics is age-related changes in the series elastic component (SEC) of muscles, which consists of passive (tendon) and active (cross-bridges) elements [7]. It has been shown tha t ageing is associated with changed stiffness of connective tissue [30]. A more compliant SEC in the muscles tends to decrease twitch force because the relatively brief active state associated with a twitch is not long enough for their more compliant SEC to be fully stretched so as to effectively transit force through the tendon to the bone [21]. It has been shown that increased musculo-tendinous compliance can also partly cause a slowing of the twitch contraction time [28], In conclusion, the results of this study indicated that in women a marked age-related reduction in maximal voluntary force-generating capacity of the plantarflexor muscles appears earlie r than the reduction in electrically evoked twitch maximal force-generating capacity. The mechanisms responsible for twitch potentiation after brief maximal voluntary isometric contraction are markedly influenced by ageing. References

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Accepted for publication 31.01.2002