Exercise synchronized with audible cues preserves motor unit action potential: a randomized control trial study

Abstract

Background

Motor training with audible cues can improve motor performance; however, few studies have examined the effect of synchronized audible cues during exercise. This study aimed to determine the immediate effects of terminal knee extension (TKE) exercise with audible cues on motor unit behavior, including motor unit action potential (MUAP) and motor unit firing rate (MUFR). The test–retest reliability and minimal detectable change (MDC) were evaluated.

Method

Thirty healthy adults were randomly enrolled to two groups: TKE exercise with audible cues and without. A Trigno–Galileo sensor of electromyography (Delsys, Inc.) was used to collect vastus medialis (VM) muscle activity during a single-leg squat test before and after TKE exercise. Metronome beats (60 beats/minute) were assigned to provide rhythm during TKE exercise in audible-cue group. In the control group, participants performed TKE exercises at a self-paced speed. After 3 days, 15 participants voluntarily returned to test VM activity again. A two-way mixed ANOVA (2 × 2; group × time) was used to examine main and interaction effects, and independent t-tests were used to compare mean changes between groups. ICC and Spearman correlation tests were used to analyze test–retest reliability of MUAP and MUFR parameters.

Results

After TKE exercise, the final data from 27 participants (13 experimental, 14 control) were analyzed. Both groups exhibited a decrease in peak and average MUAP. A significant reduction was observed (p = 0.03) in group without audible cues. The MUFR did not significantly change following TKE exercise in either group. Moderate test–retest reliabilities were 0.70 and 0.58 and MDC were 0.82 mV and 0.65 mV for the peak and average MUAP, respectively.

Conclusion

Incorporating rhythm may promote neuromuscular control and cause preserves to the MUAP of VM muscle, but not to MUFR. TKE exercises synchronized with metronome beats may be an effective strategy to promote VM function in individuals with knee dysfunction because of its safety, low cost, and ease of use during rehabilitation. Evaluating TKE with audible cues in individuals with knee dysfunction also warrants further investigation.

Trial registration

NCT06565325 (21 August 2024, first available on ClinicalTrials.gov).

Background

Therapeutic exercise is a widely used clinical intervention. It is often used as a standalone treatment or combined with other modalities, such as physical devices and manual techniques for physical therapy and rehabilitation [1–3]. Resistance training for 4–8 weeks can increase movement [4] and prevent the risk of injury [5]. Terminal knee extension (TKE) exercise is a widely used intervention that promotes muscle function and increases quadricep muscle strength, particularly the vastus medialis (VM) muscle. The peak force of voluntary VM contraction is recorded at the most knee-extended position during the closed-kinetic chain of knee extension exercise [6]. Targeted VM strengthening is often incorporated to improve knee function and control patellar alignment in individuals with knee joint dysfunction [7]. Improving outcomes through therapeutic exercise is challenging and requires an integrated knowledge of musculoskeletal and neurological systems.

Previous studies [8, 9] demonstrated that exercise synchronized with metronome rhythms altered the function of the primary motor cortex. Higher activity of the basal ganglia, prefrontal area, and supplementary motor area was also observed. Leung et al. [9] found that metronome-paced strength training results in muscle strength gains after 2- and 4-weeks of training and also showed a significant increase (40%) of corticospinal excitability compared with self-paced strength training. This indicates multi-area brain involvement of exercise synchronized with rhythmic sound and may attenuate the motor drive signal to the motor unit behavior that controls muscle contraction, as measured by decomposed electromyography (dEMG). During soccer training, improved movement accuracy and reduced cerebellar activation were observed after 4 weeks of skill training with a metronome, suggesting enhanced motor processing [10]. In 2022, a systematic review evaluated the effects of combining motor training programs with interactive metronome training. The results suggested that training with metronome beats improves in motor function by strengthening and optimizing motor timing [11].

Changes in neural factors affect the development of muscle strength [12, 13]. Six to twelve weeks of resistance exercise stimulate the activity of the motor unit behavior, and the dEMG analysis revealed that a higher firing rate and a larger motor unit were activated to produce greater muscle force [9]. In clinical practice, health professionals anticipate improvements in muscle performance following a single session of therapeutic exercise; however, the immediate effects on motor unit behavior following a single session of exercise remain unclear. Typically, patients perform exercises independently following an exercise prescription; however, variations in patient compliance and preferred exercise speed may limit the effectiveness of exercise [14, 15]. Therefore, determining the effect of incorporating audible cues by synchronizing exercise with metronomes is important. Identifying methods to enhance the effectiveness of exercise is valuable for improving clinical outcomes.

To our knowledge, this is the first study that explores the mechanisms underlying motor unit behavior following immediate exercise synchronized with audible cues. This study aimed to determine the immediate effects of TKE exercise with audible cues on motor unit behavior, including motor unit action potential (MUAP) and motor unit firing rate (MUFR). The test–retest reliability and minimal detectable change (MDC) were measured to assess the psychometric properties of the dEMG measurement. We hypothesized that incorporating rhythm through a metronome would enhance neuromuscular control and potentially lead to changes in the motor unit behavior of the VM muscle. These findings provide insight into the underlying mechanisms of the immediate effects of TKE exercise synchronized with audible cues on VM motor unit behavior and may be incorporated into health practice.

Methods

The present study was designed and conducted at the Faculty of Physical Therapy, Mahidol University. The sample size was estimated using G*Power software (version 3.1.9.7). A total of 30 participants was determined based on an ANOVA statistical test, assuming a large effect size of 0.14, an alpha error of 0.05, a power of 0.95, and an anticipated dropout rate of 20%. During exercise and testing, a licensed physical therapist monitored participants for any discomfort, pain, or adverse symptoms.

Participants

Thirty healthy individuals were enrolled and randomly assigned to two groups, including TKE exercise with and without audible cues by a researcher (Fig. 1). Stratified randomization was conducted using a free web-based tool. The participants were blinded to their group assignment. All participants exhibited a light physical activity level (inactive lifestyle or exercising < 60 min/day for ≤ 3 days/week) [16]. Individuals were excluded if he/she had a history of severe lower extremity injury or musculoskeletal problems within the past 6 months. They were asked to avoid consuming any drinks that may affect muscle function, such as coffee or tea, within 24 h of testing. The study protocol was approved by the Mahidol University Central Institutional Review Board (MU-CIRB 2024/021.0802), which fully complied with the Declaration of Helsinki. The study adhered to the CONSORT guidelines and the Clinical Trial Registry was NCT06565325 (21 August 2024, first available on ClinicalTrials.gov).

Fig. 1.

Study procedure and allocation of the participants

Before testing, a four-channel Trigno–Galileo sensor of the electromyography system (Delsys Inc., Boston, USA) was used to collect the VM muscle activity of the dominant leg during a single-leg squat test. Electromyography (EMG) signal was sampled at 1000 Hz at 16-bit resolution using an analog bandwidth of 20–450 Hz. The skin was shaved and cleaned with alcohol before attaching the dEMG sensor, and the location of the dEMG sensor was at the 80% line of the anterior superior iliac spine and medial joint space, based on SENIAM recommendations (Fig. 2A) [17]. The signal quality was assessed during full knee extension in a sitting position. The 10 µV baseline noise and green zone of the quality meter were accepted before EMG data collection.

Fig. 2.

An example of electrode location (A) and motor unit behavior analysis in a single-leg squat test by NeuroMap software (B)

Single-leg squat test before and after the TKE exercises

Motor unit behavior of the VM muscle in the dominant leg was assessed during single-leg squat tests performed before and after TKE exercise. The starting position was seating on an adjustable chair with 90° knee flexion, an upright trunk, and arms folded across the chest. Each participant performed repetitive single-leg squats for 45 s (Fig. 3A–C). To standardize movement speed between pre- and post-exercise tests, a metronome (30 beats/min) was used during the squat tests. Each beat corresponded alternately to knee extension (concentric phase) and knee flexion (eccentric phase) of the single-leg squat. The EMG data for the VM muscle was captured.

Fig. 3.

An example of the single-leg squat test (A–C) and terminal knee extension exercise (D–F)

TKE exercise

All participants performed the TKE exercise with a resistance load. Before exercising, an appropriate intensity of exercise for the tested or dominant side was individually estimated from the non-dominant leg. Initially, the 1RM was determined, and participants were asked to perform the TKE exercise with maximum load until fatigue. Any compensatory movement from the ankle, hip, or trunk was permitted during the TKE exercise. The 1RM was calculated from the number of TKE repetitions and the applied resistance load using the O’Connor equation as follows: 1RM = resistance load × (1 + 0.025 × number of repetitions to failure) [18]. Subsequently, 60% of the 1RM was estimated and used as the load intensity for the TKE exercise of the tested limb in this study [19].

The TKE session involved 3 sets of 10 repetitive knee bends and extensions (Fig. 3D–F). A three-minute rest was allowed between TKE sets. For the experimental group, participants performed the TKE exercise with audible cues. The audible cue was a metronome beat (60 beats/min) to provide a rhythm during the TKE exercise. During the TKE exercise, one metronome beat corresponded to knee extension (concentric phase) and the subsequent beat to knee flexion (eccentric phase). Participants were instructed to initiate and complete each knee movement in synchronization with the metronome beats to maintain consistency of TKE exercise program. In the control group, the participants performed the TKE exercise without audible cues or at a self-paced speed.

Test–retest reliability test

To determine the test–retest reliability of the dEMG measurement, 15 individuals (7 from the TKE with audible-cues group and 8 from the TKE without audible-cues group) returned to provide EMG data for the VM muscle 3 days later. The protocol for the single-leg squat test was the same as the TKE exercise on the initial visit.

Data acquisition and statistical analysis

After EMG data collection, NeuroMap software v1.2.1 (Delsys, Inc., Boston, USA) was used for processing the signal decomposition (Fig. 2B). The MUAP and MUFR were determined. Peak and average values of MUAP and MUFR were compared between the two groups to identify any change in motor unit behavior of the VM muscle.

SPSS software (IBM, version 29) was used to statistically analyze the data. The Shapiro–Wilk test was used to evaluate data normality. The MUAP and MUFR data exhibited a normal distribution. Therefore, a two-way mixed ANOVA (2 × 2; group × time) was conducted to evaluate the main and interaction effects on motor unit behavior. Independent t-tests were used to compare mean changes in parameters between groups. A p-value < 0.05 was considered statistically significant. An intraclass correlation coefficient and Spearman’s correlation test were used to analyze the test–retest reliability of the MUAP and MUFR parameters, respectively. The ICC level was interpreted as poor (< 0.50), moderate (0.5–0.75), good (0.75–0.90), or excellent (> 0.90) [20].

Results

Incomplete EMG data from three participants was excluded. Therefore, 27 participants were included in the final analysis. Participant characteristics are listed in Table 1, with no significant differences observed between the TKE with and without audible cues groups. Table 2 shows the interaction and main effects of group and time, while Fig. 4 shows the changes in motor unit behavior before and after training in both groups. The mean change following the TKE exercise and the psychometric properties of the dEMG measurements are listed in Tables 3 and 4, respectively.

Table 1.

Participant characteristics (mean ± SD)

Characteristics	TKE with audible-cues group (n = 13)	TKE without audible-cues group (n = 14)	p-value
Age (years)	22.15 ± 2.15	21.29 ± 2.09	0.361b
Weight (kg)	55.84 ± 8.44	54.79 ± 7.25	0.287a
Height (m)	1.62 ± 0.08	1.61 ± 0.07	0.774a
BMI (kg/m2)	21.11 ± 2.05	21.02 ± 1.77	0.988b
Gender (female/male)	9/4	11/3	-

^a tested with an independent t-test

^b tested with Mann–Whitney U test

Table 2.

Comparison of the motor unit behaviors during single-leg squat tests

Parameters	Group	Pre-training		Post-training		Group effect			Time effect			Group x time effect
		Mean ± SD	95% CI (lower, upper)	Mean ± SD	95% CI (lower, upper)	F	p-value	Partial η²	F	p-value	Partial η²	F	p-value	Partial η²
Peak MUAP (mV)	TKE with audible cues	1.50 ± 0.59	1.15, 1.86	1.41 ± 0.53	1.09, 1.73	0	0.97	0	15.05	<0.001	0.38	5.36	0.03	0.18
	TKE without audible cues	1.62 ± 0.33a	1.43, 1.82	1.28 ± 0.29a	1.11, 1.44
Average MUAP (mV)	TKE with audible cues	1.30 ± 0.32	1.11, 1.49	1.22 ± 0.22b	1.09, 1.35	3.16	0.09	0.11	18.95	<0.001	0.43	5.39	0.03	0.18
	TKE without audible cues	1.24 ± 0.20a	1.12, 1.36	0.98 ± 0.23a,b	0.84, 1.11
Peak MUFR (pps)	TKE with audible cues	13.66 ± 0.87	13.13, 14.19	13.26 ± 1.72	12.22, 14.30	1.96	0.17	0.07	0.3	0.59	0.01	0.72	0.40	0.03
	TKE without audible cues	14.26 ± 2.16	13.04, 15.49	14.34 ± 1.88	13.26, 15.43
Average MUFR (pps)	TKE with audible cues	4.26 ± 0.66	3.86, 4.66	4.49 ± 1.32	3.70, 5.29	0	0.96	0	2.25	0.15	0.08	0.07	0.79	0
	TKE without audible cues	4.19 ± 1.22	3.49, 4.90	4.52 ± 1.30	3.77, 5.27

Bold values indicate statistically significant difference (p ≤ 0.05) tested with two-ways mixed ANOVA

MUAP motor unit action potential, MUFR motor unit firing rate, mV millivolts, pps pulse per second

^a Significant difference between pre- and post-training

^b Significant difference between groups

Fig. 4.

Changes in motor unit behavior during pre- and post-training with and without audible cues. The bars represent 95% confident intervals

Table 3.

Comparison of the mean differences between groups

Parameters	Group	Mean difference		p-value	Effect size
		Mean ± SD	95% CI (lower, upper)		Cohen’s d	95% CI (lower, upper)
Peak MUAP (mV)	TKE with audible cues	−0.09 ± 0.25	−0.24, 0.06	0.03	0.89	0.09, 1.68
	TKE without audible cues	−0.35 ± 0.33	−0.54, −0.16
Average MUAP (mV)	TKE with audible cues	−0.08 ± 0.21	−0.21, 0.05	0.03	0.89	0.09, 1.68
	TKE without audible cues	−0.26 ± 0.20	−0.38, −0.15
Peak MUFR (pps)	TKE with audible cues	−0.40 ± 1.58	−1.35, 0.55	0.41	−0.33	−1.08, 0.44
	TKE without audible cues	0.09 ± 1.40	−0.73, 0.90
Average MUFR (pps)	TKE with audible cues	0.23 ± 1.05	−0.41, 0.87	0.79	−0.1	−0.86, 0.65
	TKE without audible cues	0.33 ± 0.89	−0.18, 0.84

Bold values indicate statistically significant difference (p ≤ 0.05) tested with an independent T-test

MUAP motor unit action potential, MUFR motor unit firing rate, mV millivolts, pps pulse per second

Table 4.

Test–retest reliability and the MDC of motor unit behavior measurement

Motor unit behavior	ICC	p-value	Spearman correlation	p-value	MDC
Peak MUAP	0.70	0.01	-	-	0.82
Average MUAP	0.58	0.01	-	-	0.65
Peak MUFR	-	-	0.20	0.47	-
Average MUFR	-	-	−0.15	0.58	-

Bold values indicate statistically significant difference (p ≤ 0.05)

 MUAP motor unit action potential, MUFR motor unit firing rate, ICC intraclass correlation, MDC minimal detectable change

Discussion

In the present study, we determined the immediate effects of TKE exercise with audible cues on the MUAP and MUFR of the VM muscle. In addition, test–retest reliability and the MDC of the dEMG measurements were reported. No adverse symptoms were reported during exercise and testing. To our knowledge, this is the first study to examine the effects of exercise synchronized with audible cues on motor unit behavior. Data for three participants were incomplete; therefore, EMG data of the remaining 27 participants (13 experiment and 14 control) were analyzed. Statistical analysis revealed no significant difference in the characteristics between the experimental groups (Table 1).

MUAP

An increase in MUAP is associated with the benefits of muscle hypertrophy following 8 weeks of resistance exercise [21]. The results of the present study indicated that, interaction effect of group and time significantly influenced the MUAP (Table 2). Both groups exhibited a decrease in peak and average MUAP for the VM muscle during the single-leg squat test. However, after the TKE exercise, there was a significant reduction in the group without audible cues (p = 0.001). Moreover, a significant reduction of mean difference was observed (p = 0.03) (Table 3) between groups with and without audible cues. Interestingly, TKE exercise with audible cues showed a smaller decrease in peak and average MUAP compared with the TKE exercise without audible-cues group. A previous study [9] showed that resistance training combined with a metronome rhythm showed a greater increase in corticospinal excitability compared with training at a self-paced speed. One possible mechanism is that exercise synchronized with audible cues makes the exercise task more complex by adding a cognitive load. Multitasking that involves the motor area of the brain is similar to acquiring new skills and involves the frontoparietal network with higher-order cognitive function that enhances corticospinal excitability [22]. Moreover, repetitive movements synchronized with the sound of a metronome provide a feedback mechanism that can change corticospinal excitability [9]. This indicates that in healthy adults, exercise synchronized with the beat sound of a 1 Hz metronome may promote attention and stimulate several brain motor networks, particularly in the cerebrum, by maintaining the motor drive of MUAP.

MUFR

The MUFR did not significantly change following TKE exercise in both groups (Table 2). A previous study [23] found a significant increase in MUFR after several weeks of training. At least 48 hours of strength training may increase MUFR [24], and fatigue may affect the MUFR [25]. It appears that the testing time influences the results of MUFR. It is possible that MUFR did not change in healthy adults immediately after exercise in both groups. Moreover, resting time during the TKE exercise protocol was designed to prevent a cofounding fatigue effect. Therefore, the present study indicated that the MUFR did not change immediately after TKE exercise.

Reliability and MDC

The psychometric properties of the dEMG measurement are listed in Table 4. The moderate test–retest reliabilities for the peak and average MUAP had an ICC of 0.70 and 0.58. This is the first study to report a MDC in the dEMG measurement and the MDC for the peak and average MUAP were 0.82 mV and 0.65 mV, respectively. This confirms a change in MUAP for the VM muscle with a greater value compared with that of MDC. However, the peak and average MUAP in the present study were lower compared with those of MDC. We expected a greater change in MUAP compared with MDC in individuals with knee musculoskeletal problems. The data for the peak and average MUFR were not normally distributed, and a very poor correlation was observed between the test and retest measurements. The results of the present study are in agreement with those of previous studies in which the MUFR parameter was inconsistent. Previous studies have suggested that improved efficiency in motor function is the result of enhanced cerebellar–frontal connectivity [26], improved movement accuracy, and reduced cerebellar activation [10] following training with synchronized audible cues. Applying auditory cues can facilitate sensorimotor integration and enhance neural drive through increased corticospinal excitability of the primary motor (M1) area [27, 28]. Elevated cortical excitability increases the output signal strength from the motor cortex, which results in an increase in motor unit recruitment required to complete the task [29]. Thus, audible cues may involve corticospinal excitability in the brain and send a greater output signal from the brain to the motor unit in the target muscles [22]. This results in a lower decrease of MUAP in healthy individuals after immediate TKE exercise with audible cues compared with TKE without audible cues.

Clinical implications and future studies

A session of TKE synchronized with metronome beats demonstrated an immediate preservation in MUAP; however, the clinical implications should be interpreted with caution because of the inclusion of healthy adults. The present study describes a feasible strategy that may promote VM function in individuals with knee dysfunction, including those with patellofemoral pain or recovering from anterior cruciate ligament reconstruction, because of its safety, low cost, and ease of implementation during rehabilitation. Future studies should include participants with knee dysfunction and assess long-term training effects on motor unit behavior. Sample size calculations should be based on parameters obtained from this pilot project, particularly considering the sensitivity of EMG data. This study did not assess corticospinal excitability or brain connectivity in relation to auditory cues. Therefore, future research should include measurements of central nervous system function to confirm the effects of synchronized, audible cues during exercise. Moreover, it would be valuable to incorporate kinematic analyses, such as joint angular velocity, to examine knee control with and without 1 Hz metronome beats. This would help confirm the presence of peripheral control differences between conditions. EMG data of 3 participants was limited for analysis due to incomplete. Future studies should recruit a larger sample to investigate the effect of audible cues on motor unit behavior and to establish the test–retest reliability of the dEMG measurements.

Conclusion

Significant reductions in the peak and average MUAP were observed after immediate TKE exercise without audible cues compared with TKE with audible cues. Incorporating rhythm using a metronome may promote neuromuscular control, potentially leading to preserve in the MUAP of the VM muscle, but not in the MUFR. The results provide insight into the underlying mechanisms of the immediate effects of TKE exercise synchronized with audible cues. They have valuable implications for incorporating this protocol into health practice. TKE synchronized with metronome beats may represent a useful strategy to facilitate VM function in individuals with knee dysfunction because of its safety, low cost, and ease of application during rehabilitation. Evaluating TKE with audible cues in individuals with knee dysfunction also warrants further investigation.

Abbreviations

EMG

Electromyography

dEMG

Decomposed electromyography

BaS

Biomechanics and Sports

MUAP

Motor unit action potential

MUFR

Motor unit firing rate

TKE

Terminal knee extension

VM

Vastus medialis

ICC

Intraclass correlation coefficients

MDC

Minimal detectable change

RM

Repetitive maximum

mV

Millivolts

pps

Pulse per second

Authors’ contributions

PS, PK, KS, SN, AS, ST, PW, and TS conceived and designed the study; PS, SN, and KS contributed to data collection; PS, PK, and KS performed data analysis; PS, PK, and KS contributed to the interpretation and first draft of the manuscript. KS was the supervision, funding acquisition and critical revision of the article. All authors read and approved the final version of the manuscript and agree with the order of presentation.

Funding

Open access funding provided by Mahidol University. This study was supported by the Faculty of Physical Therapy, Mahidol University.

Data availability

The datasets used and/or analysed during the current study are available from the corresponding author on reasonable request.

Declarations

Ethics approval and consent to participate

The study protocol was approved by the Mahidol University Central Institutional Review Board (MU-CIRB 2024/021.0802) which is in full compliance with Declaration of Helsinki. All participants provided written informed consent. The study adheres to CONSORT guidelines and the Clinical Trial Registry was NCT06565325 (21 August 2024, first available on ClinicalTrials.gov).

Competing interests

The authors declare no competing interests.

Consent for publication

Authors confirmed that written consent for publication is obtained from participants and the participant gave written informed consent for his image to be published in this study.