Page 145 - 《运动与健康科学》(英文)2024年第2期

TaggedAPTARAEndMotor units’ adaptations to disuse and active recovery 267
3 times/week with at least 24 h of recovery between sessions. 816 mA (Digitimer Ltd, Welwyn Garden, Hertfordshire,
14
Every session was composed of 3 sets of 10 repetitions of leg UK). TaggedAPTARAEnd
press and leg extension at 70% 1 repetition maximum (RM) TaggedAPTARAPBefore placing the matrix, the skin was shaved, cleaned
after a warm-up period at 30% 1RM. Both exercises were with 70% ethanol, and then with abrasive-conductive paste
executed from full knee extension (0˚) to »90˚ limb flexion. (Spes medica, Salerno, Italy). Reference electrodes were
Sets were separated by a 2-min rest. The time under tension placed on the malleolus and patella bones. After the record-
ings, the matrix border was marked with a permanent marker
was set at »2 s both in the concentric and eccentric phases.TaggedAPTARAEnd
TaggedAPTARAP1RM was estimated from the 4RM6RM during the first and emphasized by the operator at every meeting with the
exercise session 18 of each week, and the load employed was participants to allow the reproducible placement of the matrix
22
adjusted accordingly.TaggedAPTARAEnd in the following data collection points. TaggedAPTARAEnd
TaggedAPTARAPThe choice of the intensity and the decision to indirectly esti-
mate the 1RM were based on the participant characteristics (i.e., TaggedAPTARAP2.3.3. HD-EMG recordingsTaggedAPTARAEnd
not previously involved in resistance exercise and coming from TaggedAPTARAPThe HD-EMG signal was recorded during trapezoidal
contractions (Fig. 1C) at 3 different submaximal intensities
10 days of complete unloading of the lower limb).TaggedAPTARAEnd
(10%, 25%, and 50% MVC). All the contractions had a total
duration of 30 s. The ramp-up and ramp-down phases were
performed with a linear force increase/decrease set at 5%
TaggedAPTARAH22.3. MeasurementsTaggedAPTARAEnd 23
MVC/s, and the duration of the steady-state phase was
adjusted accordingly (e.g., at 25% MVC, the ramps lasted 5 s
TaggedAPTARAP2.3.1. MVCTaggedAPTARAEnd
TaggedAPTARAPMVC was assessed during MVC of the knee extensor each and the steady-state 20 s for a total 30 s contraction) (Fig.
muscles at a 90˚ knee angle using a custom-made knee dyna- 1C). All the trapezoidal contractions were repeated twice with
mometer fitted with a load cell (RS 2060290; Tedea 60 s of rest in between, and the different intensities were
Huntleigh, Selb, Germany) and attached above the ankle with proposed in random order. Participants received real-time
straps (Fig. 1B), as previously described. 4,14 TaggedAPTARAEnd visual feedback of the force produced and were instructed to
TaggedAPTARAPIn order to ensure correct assessment of MVC, 19 partici- match the trapezoidal template as precisely as possible.TaggedAPTARAEnd
pants practiced the task during the familiarization session TaggedAPTARAPEMG and force signals were sampled at 2048 Hz with the
supervised by an experienced operator. They were instructed EMG-Quattrocento (OT Bioelettronica). The EMG signal was
to “push as hard as possible” by pushing the dominant leg recorded in monopolar configuration, amplified (£ 150), and
against the load cell and maintaining the contraction for 34s. band-pass filtered (10500 Hz) at source. 23 Force was
After a standardized warm-up executed up to 70% of their recorded synchronously with the EMG signal, and the offset
perceived maximum, participants practiced the maximal task was removed before starting the recording.TaggedAPTARAEnd
until they became able to consistently reach their maximum
force values. During every contraction, participants were stabi- TaggedAPTARAP2.3.4. Force and HD-EMG signal analysesTaggedAPTARAEnd
lized to the seat by straps at the waist in order to prevent any TaggedAPTARAPThe force signal was converted to Newton (N) and low-pass
compensatory movement. Loud verbal encouragement was filtered (fourth-order, zero-lag, Butterworth, and 15 Hz cut-off). 23
provided to encourage the maximum voluntary effort. At each Force steadiness was computed as the coefficient of variation of
data collection point, the test was repeated 3 times with 60 s of force recorded during the steady-state phase (COV steady )of the
rest, and only the contraction with the maximum value was trapezoidal contractions and expressed as percentage (i.e., the
24
considered for the MVC calculation.TaggedAPTARAEnd ratio of the SD to the mean). TaggedAPTARAEnd
TaggedAPTARAPThe HD-EMG signal was band-pass filtered between 20 Hz
and 500 Hz (second-order; Butterworth) and decomposed into
TaggedAPTARAP2.3.2. HD-EMG matrix placementTaggedAPTARAEnd
TaggedAPTARAPThe HD-EMG signal was recorded from the vastus lateralis discharge times of the MUs with the validated convolutive
muscle (Fig. 1B) using a matrix of 64 equally spaced elec- blind source separation technique (Fig. 1D) (OTBioLab+; OT
trodes (GR08MM1305; OT Bioelettronica, Torino, Italy) filled Bioelettronica). 25 After the decomposition, the pattern of
with conductive cream (Ac cream; OT Bioelettronica) and discharge times for each MU was visually inspected and manu-
arranged over 5 columns and 13 rows with 8 mm interelectrode ally edited. 26 Only the identified MUs with a pulse to noise
2
distance, which corresponded to 30.72 cm of recording area.TaggedAPTARAEnd ratio (PNR) of 28 decibel (dB) (sensitivity of >85%) were
27
TaggedAPTARAPThe matrix was placed following the muscle fascicle maintained for further analyses. TaggedAPTARAEnd
orientation (detected with B-mode ultrasound; Mylab70; TaggedAPTARAPAll MUs decomposed from the 2 trapezoidal contractions
Esaote, Genoa, Italy) 20 and with the central electrodes of recorded at the same intensity during the same data collection
the last 2 rows of the matrix over the innervation zone. 21 point (for each participant) were pooled and analyzed together
The ultrasound recordings were used also to detect muscle after the removal of duplicated MUs (as explained in Section
borders and avoid the placement of the matrix across adja- 2.3.5.). This approach allowed us to increase the number of
unique MUs, to reduce variability induced by the inability of a
cent muscles.TaggedAPTARAEnd
TaggedAPTARAPThe innervation zone was detected between 35% and 20% participant to reproduce the trapezoidal path, and to avoid
of femur length 21 with low-intensity percutaneous electrical results biased by the duplication of values, which would occur
stimulation using a pen electrode with an electrical current of if duplicated MUs were not removed.TaggedAPTARAEnd