Infuence Of Age On Force And Re‑lengthening Dynamics After Tetanic Stimulation Withdrawal in The Tibialis Anterior Muscle Part 1

Abstract

Purpose During alternate movements across a joint, the changeover from one direction of rotation to the opposite may be influenced by the delay and rate of tension reduction and the compliance to the re-lengthening of the previously active muscle group. Given the aging process may affect the above-mentioned factors, this work aimed to compare the dynamics of both the ankle torque decline and muscle re-lengthening, mirrored by mechanomyogram (MMG), in the tibialis anterior because of its important role in gait.

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Methods During the relaxation phase, after a supramaximal 35 Hz stimulation was applied at the superficial motor point, in 20 young (Y) and 20 old (O) subjects, the torque (T) and MMG dynamics characteristics were measured.

Results The T and MMG analysis provided: (I) the beginning of the decay after cessation of stimulation (T: 22.51±5.92 ms  [Y] and 51.35±15.21 ms [O]; MMG: 27.38±6.93 ms [Y] and 61.41±18.42 ms [O]); (II) the maximum rate of reduction (T: −110.4±45.56 Nm/s [Y] and − 52.72±32.12 Nm/s [O]; MMG: − 24.47±10.95 mm/s [Y] and−13.76±6.54 mm/s [O]);  (III) the muscle compliance, measuring the MMG reduction of every 10% reduction of torque (bin 20–10%: 15.69±7.5[Y] and 10.8±3.3 [O]; bin 10–0%: 22.12±10.3 [Y] and 17.58±5.6 [O]).

Conclusion Muscle relaxation results are different in Y and O and can be monitored by a non-invasive method measuring physiological variables of torque and re-lengthening dynamics at the end of the electromechanical coupling previously induced by the neuromuscular stimulation.

Keywords Aging · Relaxation process · Rate of torque reduction · Stimulation · MMG

Abbreviations

D Delay

MC Muscle compliance 

MMG Mechanomyogram 

MMG0T MMG at the end of torque reduction 

N Normalized 

RR Rate of reduction 

SERCA Sarcoendoplasmic reticulum calcium transport  ATPase 

T Torque 

TR Time of reduction 

US Ultrasound

Introduction

The alternated movement of a joint, such as an ankle, knee, or elbow, is related to the coordinated, alternated activation of the flexor and extensor muscles acting across the hinge. While the muscles providing an angular momentum toward the joint rotation direction are referred to as agonists, the muscle group generating an opposite angular momentum is defined as antagonists. Every time the joint rotation is reversed, there is a swap between the roles of the antagonist and agonist muscles. The changeover from one direction of rotation to the opposite as well as the re-lengthening phase of the past agonist is influenced by (a) the tension reduction of the previously active muscle group, (b) the compliance of this last to re-elongation by the new active agonist. Consequently, the assessment of biomechanical parameters during these two aforementioned processes could provide functional data to characterize the muscular features affecting the agonist–antagonist sequential activity.

Gait can be considered as a global alternating movement resulting from the combination of several joints alternating fexion–extension sequences. According to  Westerblad et al. (1997), “slowed relaxation of antagonist's muscle might counteract the desired movement during rapid, alternating movements”. Thus, during normal locomotion, the slowing of relaxation of the previously active muscle group may greatly affect the dynamic of the joint transition from a rotational direction to the following. This may influence the locomotion parameters, particularly in aged subjects. Indeed, data about gait analysis suggest that age influences the gait in length stride, and phase duration (Mulas et al. 2021; Fukuchi et al. 2019). Changes in gait in the elderly have been also associated with an increased risk of institutionalization and death. For instance, a reduction in walking speed has been demonstrated to be predictive of life expectancy (Studenski et al. 2011). Furthermore, disturbances in balance and gait have been implicated in an increased risk of falls (Osoba et al. 2019).

On this base, it seems important to evaluate the dynamics of muscle tension reduction and re-lengthening after activation in young and old subjects. The functional parameters describing the force and re-lengthening process during the relaxation phase are not comparatively well described in the above-mentioned populations. The force decrement onset delay, from the myoelectric activity cessation, or its velocity of decay in different conditions such as pre-and post-stretching maneuver (Longo et al. 2017, 2014) or before and after fatigue (Cè et al. 2014a, 2014b), has been investigated in the literature. Only a few studies report the behavior of tension and surface mechanomyogram (MMG) signal detected by an accelerometer, monitoring the muscle re-elongation, in the relaxation phase when simultaneously recorded (Cè et al. 2014a, 2013b; Esposito et al. 2016; Longo et al. 2014). Indeed, they used the MMG as an indicator of re-lengthening onset, but not of its time dynamics.

To assess the torque and muscle length behavior at the end of muscle contraction, it is possible to use an experimental setup previously described by our group (Cogliati et al.  2020), in which the isometric torque of ankle dorsiflexion and the tibialis anterior length were simultaneously measured by a load cell and surface mechanomyography, respectively. The rationale can be summarized as follows. Since the muscle is a constant volume system, each shortening during a contraction provides an increase in the transverse diameter of the muscle. This dimensional variation can be picked up by a laser sensor. By analogy, during muscle relaxation after activity, the laser distance signal can be considered as an index of the muscle re-elongation process. The study of muscle length changes by surface MMG has already been implemented in detail (Orizio et al. 1996; Yoshitake et al. 2005; Beck et al. 2005). The rationale for adopting MMG as an indirect measure of muscle length changes, instead of the collection of ultrasound (US) images from the active muscle, is based on the following considerations: (a) at present the US technique unlikely provides more than 30–60 frames per second with a time resolution of 30–15 ms (too low for good tracking of the time behavior of re-elongation after cessation of the muscle activity), (b) the experimental setup is quite complex requiring a robotic arm sustaining the US probe immersed in a pool able to accommodate the distal leg, (c) the post-processing of each image to extract the length change is complex and time-consuming, (d) the cost of a US system is much greater than a simple laser distance sensor. Finally, the choice of the MMG signal makes the replication of the study easy.

An experimental design that can neatly provide basic data about the tension reduction and re-lengthening processes relationship of an active muscle once the activity is withdrawn must be based on stimulated contractions. In this way, it is possible to minimize the uncertainty of the individual fading pattern of the central nervous system drive suspension that may influence the outcome of the changes in the muscle contractile status during the relaxation phase.

Given the possible meaning in determining the mechanical efficiency of alternating movements, this work aimed to compare in the tibialis anterior of young and old subjects the dynamics of:

(a) the torque reduction at the ankle; 

(b) the muscle re-lengthening during the relaxation phase.

It is worth underlining how important the study of tibialis anterior mechanics is, given its major role in the gait cycle for both stabilization of the ankle joint during the early phase of stance and for elevation of the foot during the early phase of swing (Lacquaniti et al. 2012). As a consequence, its relaxation dynamics may deeply influence the timing of the transition to the following phases of the gait.

Materials and methods

Subjects and measurements

Twenty recreationally active young participants (10 males and 10 females; age 21–33 years old) and 20 recreationally active older participants (10 males and 10 females; age  65–80 years old) with no orthopedic or neurological disorders were recruited to participate in this study. After receiving a full explanation of the experiments, they provided their written informed consent. The subjects were asked to refrain from caffeine intake and intense physical activity in the 24 hours preceding the test. This study was conducted by the latest version of the Declaration of Helsinki and approved by the local ethical committee. The participants'  dominant lower limb was positioned on a specific ergometer equipped with a load cell (Fig. 1), which measured the torque generated during the electrically stimulated contractions of the tibialis anterior muscle (Cogliati et al. 2020). While the hip and the knee were, respectively, fixed at 90° and 180°, the ankle was positioned in a neutral position at 110°. The foot was strapped to the wood plate connected to the load cell (model SM-100 N, by Interface Inc., Scottsdale, US-AZ). The force signal acquired by the load cell was band-pass filtered at 0–64 Hz and amplified (MISO- OT Bioelettronica, Turin, Italy). To get the dorsiflexion torque produced by each subject, the distance between the ankle fulcrum and the load cell at the footplate was measured and used to convert the force signal in torque [T=F (N) × d (m)]. According to Orizio et al. (Orizio et al. 1999, 2008), the displacement of the tibialis anterior muscle surface was transduced as a mechanomyographic signal using an optical laser distance sensor (M5L/20, MEL Mikroelektronik, Germany). The instrument has the following features: range of measurement±10 mm, sensitivity 1 V/mm, linearity 0.6%, resolution<6 μm, bandwidth 0–10 kHz. The laser beam was pointed to the tibialis anterior muscle belly presenting the largest displacement during the tetanic stimulation. The common position was at about 1 cm from the tibial crest as reported in Fig. 1. The device provided an output DC voltage proportional to the distance between the laser beam head emitter and the reflecting muscle surface. The measure of the distance of the reflecting surface from the laser source was not affected by surface rotation within±15° and±30° concerning the short and long axis of the laser head, respectively. The force and MMG were digitized at a frequency of 1024 samples/s (CED-1401 of Cambridge Electronic Design of Cambridge).

An electrical stimulator was used to deliver biphasic rectangular stimuli (100 µs duration of each phase) on the tibialis anterior muscle. The cathode electrode (5×5 cm) was placed at the skin region over the main motor point of the tibialis anterior (Fig. 1), which was identified according to Gobbo et al. (2011). The anode electrode (15×10 cm) was positioned on the gastrocnemius muscle. By increasing the amplitude of a 1 Hz stimulation train (10 pulses per each 0.1 V amplitude level), from the minimum value of 0.5 V, the maximum stimulation pulse was identified as the stimulus amplitude eliciting the largest single twitch. Three trains of 35 Hz pulses lasting 3s were administered to the motor point of the muscle with a 1 min pause between stimulations. The surface EMG evoked by the stimulation train was detected using two self-adhesive pre-gelled silver electrodes (1 cm in diameter; inter-electrode distance 30 mm). EMG was conditioned using a third-order Butterworth band-pass filter (10–512 Hz). After A/D conversion by CED-1401 (Cambridge Electronic Design, Cambridge, UK), the digitized signals were stored on a PC and sampled at 1024 samples/s.

Signal processing: analyzed parameters during the relaxation phase

To achieve the purpose of the work, the analysis described here below concerns the relaxation phase of the stimulated tetanic contraction, which has been partly already considered by several studies (Cè et al. 2013b, 2014a, 2013c; Esposito et al. 2016, 2011; Longo et al. 2016).

Out of the three stimulation trains, the one with the greatest torque value in the 100 ms time interval before the last stimulus was selected for each subject. The torque and MMG were digitally low-pass filtered at 50 Hz and subsequently normalized to their 100% referred to the average values in  100 ms time intervals. The EMG signal was used to identify the end of the electrical activity due to the tetanic stimulation train. The time at which the electrical activity was completed, after the last stimulus, was the time mark at which the evoked EMG reached its average value±3 SD calculated from a 1 s signal sample before the tetanic stimulation (see  Fig. 2).

Relaxation electromechanical delay

During the relaxation phase, a delay (D) can be observed between the end of electrical activity and the beginning of torque and MMG decay. D was calculated as the time instant when the signals decreased 3 standard deviations of their average value during stimulation, both for torque (DT) and MMG (DMMG) (Fig. 2).

Rate of torque reduction and rate of MMG reduction

The rate of reduction for torque signal (RRT) and MMG   signal (RRMMG) were calculated as the ratio between the  Δtorque or ΔMMG and Δtime (Fig. 2). Specifically, a 20 ms moving window with a step of 1 ms was used across the two signals to identify the maximum rate of reduction (Cogliati et al. 2020; Haf et al. 2015). The same calculation was performed on the normalized signals to obtain NRRT and  NRRMMG.

The time interval for 80–20% signal reduction

In addition to the discrete information provided by RR or  NRR, the time of reduction of both the normalized torque and MMG (TRT and TRMMG) in the range of 80–20% of their reductions was calculated to further characterize the dynamics of signal decay. The selected range allows us to compare the time behavior of the two signals from young and old subjects when both are dynamically changing out of the initial and final transients.

Muscle compliance

To have a detailed description of the time relationship between the torque decrement and muscle re-lengthening, the amount of relative MMG variation for each of the ten bins of relative torque decrease (from 100 to 0%: 100–90%, 90–80%, 80–70%, …, 10–0%) was calculated. This value mirrors the bin-by-bin muscle compliance  (MC) to re-elongation throughout the relaxation process.

MMG at the end of torque reduction (MMG0T)

The %MMG, the amount of re-lengthening left, when the torque reduction process was completed and reached 0% was quantifed for each subject. The parameter was identified as MMG0T and provides a measure of the whole re-lengthening process efficiency compared to tension reduction: in other words, how much the re-lengthening is incomplete once the force felt is 0.

Statistical analysis

The data were analyzed using statistical software (Sigmaplot 11). A two-way analysis of variance (ANOVA) was used to examine the main and interaction effect of age (young and old) and signals (torque and MMG) on the D, NRR,   and TR. When ANOVA was significant, pairwise comparisons were made with the Tukey post hoc test. For muscle compliance, the two factors for ANOVA were age and the relative torque decrement bin. Furthermore, an independent t-test was used to investigate differences between the groups (young and old) for maximal torque during stimulated contraction, RRT, RRMMG, and MMG0T (statistical significance p<0.05).

In the graphs, the number of asterisks (*) indicates statistically significant differences as follows: p< 0.05 (*); p<0.01 (**); p<0.001 (***).

The data reported in this work come from signals detected during the tibialis anterior tetanic response used to compare the muscle mechanics at the onset of voluntary and stimulated contractions in young and old subjects in already published works (Cogliati et al. 2020).

Results

An example of the normalized torque (red line) and MMG (black line) signals from representative young and old subjects, from which the parameters listed in the previous section have been calculated, can be found in Fig. 3. The different time at the beginning of the two signals decay between young and old subjects as well as the different slopes of the two signals is evident through the relaxation process.

Maximal stimulated contraction

The maximal torque in stimulated contraction was significantly different between young and old adults (4.9±2.5 Nm for young and 2.6±1.7 Nm for older; p<0.001). A t-test revealed a significant difference (p=0.002) between young  (3.01±1.17 mm) and old (2.01±0.73 mm) subjects for the maximal surface displacement transduced as MMG.

Relaxation electromechanical delay (DT and DMMG)

The two-way ANOVA revealed a significant effect of age (p<0.001) and signal (p=0.009) on D, but without an interaction between these factors (p=0.354). Specifically, the older subjects had a longer delay compared to younger subjects. Moreover, the beginning of relaxation for the MMG started after the torque signals.

Torque. At the beginning of the relaxation phase, DT was significantly different between young and older individuals  (22.51±5.92 ms for young and 51.35±15.21 ms for older; p<0.001) (Fig. 4).

MMG. DMMG showed the same behavior as DT, with a significant difference being observed between young and older individuals (27.38±6.93 ms for young and 61.41±18.42 ms for older; p<0.001) (Fig. 4).

Rate of torque reduction and rate of MMG reduction  (RRT and RRMMG)

During the decay phase after the stimulated contraction withdrawal, the maximal RRT in young and old was −110.4 ± 45.56 Nm/s and − 52.72 ± 32.12 Nm/s, respectively, showing a statistical difference between groups (independent t-test; p < 0.001). Accordingly, the maximal RRMMG in young (− 24.47 ± 10.95 mm/s) was significantly higher than in old (−13.76 ± 6.54 mm/s) subjects (independent t-test, p < 0.001). When considering the normalized signals, the results were similar. The two-way ANOVA revealed a significant effect of age (p<0.001) and signal (p<0.001) on the NRR, but without an interaction between these factors (p=0.508). Specifically, the NRR was higher for young subjects compared to old and the decrease of the MMG signal was slower than the torque signal (Fig. 5). Moreover, the maximal NRRT in young and old was − 1256.16 ± 333.36%/s and  − 1026.26 ± 267.76%/s, respectively, showing a statistical difference (p=0.004). Similarly, NRRMMG was statistically different between young (−867.79%/s±148.6%/s) and older (− 710.35±178.84%/s) subjects (p=0.044).

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