Electromyographic Analysis of Lower Limb Muscles During Multi-Joint Eccentric Isokinetic Exercise Using the Eccentron Dynamometer

Abstract

Eccentric muscle actions are integral to human movement, rehabilitation, and performance training due to their characteristic high force output (overload) and low energy cost and perceived exertion. Despite the growing use of eccentric devices, a gap in the research exists exploring multi-muscle activation profiles during multi-joint eccentric-only, isokinetic exercise. This study aimed to quantify and compare surface electromyographic (EMG) activity of four leg muscles—vastus lateralis (VL), tibialis anterior (TA), biceps femoris (BF), and medial gastrocnemius (GM)—during a standardized (isokinetic) submaximal eccentric multi-joint exercise using the Eccentron dynamometer. Eighteen healthy adults performed eccentric exercise at 40% of their maximal eccentric strength. Surface EMG data were analyzed using root mean square (RMS) and integrated EMG (iEMG) variables. Repeated-measures ANOVAs and effect sizes (ES) were used to evaluate within-subject differences across muscles. Results showed significantly greater activation in the VL compared to all other muscles (p < 0.05; and ES of 1.28–3.17 versus all other muscles), with the TA also demonstrating higher activation than the BF (p < 0.05). The BF exhibited the lowest activation, suggesting limited hamstring engagement. These findings highlight the effectiveness of the multi-joint isokinetic eccentric leg press movement (via an Eccentron machine) in targeting the quadriceps and dorsiflexors, while indicating the possible need for supplementary hamstring and plantar flexor exercises when aiming for a comprehensive lower body training routine. This study provides important insights for optimizing eccentric training protocols and rehabilitation strategies.

1. Introduction

Eccentric muscle actions, where a muscle lengthens while under load, are essential to human movement and commonly implemented across rehabilitation, injury prevention, resistance training, and general fitness applications. Because they produce higher force with lower energy cost, eccentric exercises are well-suited for rebuilding strength, preventing atrophy, and promoting muscle growth and enhancing function [1]. Consequently, eccentric-based resistance training has seen rapid growth in both research and real-world practice [2].

Eccentric exercise is often performed using isokinetic dynamometers, which have become increasingly common in both research and clinical settings for assessing and training eccentric muscle function under controlled conditions [3,4]. By maintaining a constant velocity during movement, these devices enable isolation of eccentric loading and provide consistent, reproducible neuromuscular loading profiles during the exercise task. When combined with surface electromyography (EMG), isokinetic protocols offer valuable insights into which lower limb muscles are most active during specific movements, information essential for designing targeted rehabilitation programs, optimizing performance training, and minimizing the risk of overload-related injuries. Additionally, while the present study focuses mainly on larger lower limb muscle groups, it is noted that intrinsic foot musculature also plays an important role in postural control, stability, and efficient force transfer during complex multi-joint movements [5]

Early EMG research on eccentric muscle actions primarily focused on single-joint movements. For example, Westing et al. [6] demonstrated that quadriceps activation was lower during eccentric versus concentric knee extensions. Similarly, Duchateau and Enoka [7,8] have described distinct neural control strategies (including motor unit firing patterns) during eccentric versus concentric loading of the ankle dorsiflexors. While informative, these studies do not reflect the complexity of real-world or functional movements, which typically involve the coordinated activation of multiple joints and muscle groups. The unique biomechanical properties of eccentric actions—including higher force output, distinct force–velocity relationships, and altered neural control—directly influence how multiple joints and muscle groups are recruited and coordinated in complex tasks.

To address the complexity of functional movements, recent efforts have explored multi-joint tasks, but with notable limitations. EMG investigations of lower limb resistance exercises have often focused narrowly on specific muscles or exercise variations. In a comparative study of leg press configurations, Da Silva et al. [9] showed that muscle activation varies substantially with changes in foot and hip position. The gluteus maximus was most active in the horizontal high-foot leg press, while the rectus femoris, vastus lateralis (VL), and gastrocnemius (GM) exhibited higher activation in low foot versus high foot placement variations. Stien et al. [10] extended this work by comparing multi-joint (leg press) and single-joint (knee extension, kickback) exercises. They found the VL was more active in the leg press, while the rectus femoris and biceps femoris (BF) were preferentially recruited in the isolated (single-joint) tasks. Together, these studies emphasize the role of joint configuration in influencing muscle activation across different muscles within and between muscle groups, but these are limited in that they do not isolate the eccentric loading phase or include broader (e.g., synergist, antagonist, stabilizer) muscles across multiple joints during multi-joint eccentric movement tasks.

Martin-Fuentes et al. [11] systematically reviewed EMG activity in different leg press variants and found the vastus medialis and VL consistently exhibited the highest activation, particularly at 90° of knee flexion. Notably, muscle-specific recruitment could be modified by foot placement and hip adjustments—such as high foot position increasing gluteus maximus activation, or adduction squeezing recruiting adductor longus. However, most reviewed studies emphasized the concentric phase, and few included comprehensive muscle group comparisons. The review concluded that a major limitation in the literature is the lack of standardized, multi-muscle EMG analysis during eccentric-only tasks, particularly in protocols that simulate real-world multi-joint loading.

A small amount of research has examined EMG activity across various muscles during multi-joint tasks with greater emphasis on isolating the eccentric phase. Luera et al. [12] investigated EMG–force relationships during squat ramping and found differing activation patterns under eccentric conditions; however, their study focused on a small number of thigh muscles and lacked eccentric-only phase isolation (e.g., where the eccentric was not preceded by a concentric action). Armstrong et al. [13] used a motorized isotonic system to investigate EMG responses during accentuated-eccentric barbell squats. They observed reduced EMG activity in the knee extensors and gluteus maximus during the eccentric phase of the accentuated-eccentric condition compared to the concentric phase of the traditional squat, despite increased external loading. However, the isotonic nature of the system and the lack of precisely controlled joint angular velocities limit the internal validity required for highly confident interpretation.

Taken together, these findings highlight several persistent gaps in the literature: (1) muscle selection bias, as most studies prioritize prime movers over synergists and stabilizers and thigh muscles over lower limb muscles; (2) combined eccentric-concentric cycles, where eccentric actions are embedded within concentric-eccentric cycles; and (3) a lack of investigation into eccentric-only loading under tightly controlled (e.g., velocity) multi-joint conditions. Despite the increasing availability of isokinetic and isotonic dynamometers capable of delivering pure eccentric loads, there is still little known about how various lower limb muscles compare in their activation magnitude under these specific conditions.

Park et al. [14] and Petrofsky et al. [15] partially address this gap by providing the most relevant insights to date, having examined lower limb muscle activation using a lower body multi-joint eccentric isokinetic dynamometer (Eccentron BTE Technologies, Hanover, MD, USA). Park et al. [14] studied stroke patients undergoing virtual reality-based eccentric training on an Eccentron dynamometer and found that slow-velocity eccentric actions produced significantly higher EMG activation in the VL, vastus medialis, and GM compared to faster movements. Petrofsky et al. [15] examined EMG activity across four traditional eccentric exercises and the Eccentron isokinetic device in healthy adults. The Eccentron elicited the highest muscle activation across all major lower limb groups (average EMG = 60.4% of max), highlighting its efficiency as an eccentric loading modality. Yet, neither study focused largely on the eccentric-based lower limb multi-muscle comparison aspect as they were more primarily interested in comparing across eccentric-based exercises [15] and training programs [14].

Taken together, this body of work underscores the need for a lower limb multi-muscle activation comparison profile under eccentric-only, multi-joint, standardized (isokinetic) submaximal loading conditions. Such a profile would help clinicians identify over- or under-recruited muscles during eccentric protocols, providing evidence to optimize eccentric exercise schemes and reduce injury risk in diverse populations.

Therefore, the purpose of this study was to compare neuromuscular activation (surface EMG) of four lower limb muscles during a submaximal, eccentric-only, multi-joint isokinetic resistance exercise. The muscles selected for evaluation include the tibialis anterior (TA), GM, BF, and VL, key contributors to both primary movement and joint stabilization of the lower body joints. We hypothesize that there will be significant differences in the activation levels across the muscles, with the VL exhibiting the greatest activation and the BF the lowest activation. By providing a comparative lower limb muscle activation profile under standardized eccentric-only conditions, this study will (1) address key limitations in the multi-joint eccentric-exercise EMG literature, (2) inform targeted rehabilitation and performance training recommendations, and (3) help guide the development of safer, more effective eccentric exercise prescriptions.

2. Methods

2.1. Participants

An a priori power analysis was conducted using G*Power (Version 3.1.9.7) to estimate the required sample size for detecting a large effect (Cohen’s d = 0.80) with a significance level of α = 0.05 and power (1 − β) = 0.80. The analysis indicated that a minimum of 15 participants was needed to achieve adequate statistical power. Thus, we recruited a convenience sample of 18 healthy, recreationally active adults (

Table 1. Participant characteristics.

Sex	n	Age (years)	Height (cm)	Weight (kg)	BMI (kg·m−2)	Max ECC (n)
Female	8	19.9 (2.0)	174.9 (4.8)	76.7 (12.1)	25.1 (3.7)	1505.2 (447.2)
Male	10	21.6 (1.9)	167.2 (31.8)	74.5 (10.6)	23.5 (3.2)	1846.9 (540.0)

BMI = body mass index; Max ECC = maximal multi-joint lower body eccentric strength. Data are presented as mean (SD).

). All participants completed the study protocol in its entirety. Inclusion criteria required participants to be between 18 and 35 years of age. Exclusion criteria included (a) a self-reported history of neurological disorders associated with motor symptoms (e.g., stroke, multiple sclerosis, recent concussion); (b) current physical discomfort or injury that could impair the ability to perform maximal or submaximal eccentric lower limb exercises; (c) surgical intervention involving the lower limbs or trunk within the past two years; (d) a history of ligament injuries (hip, knee, or ankle) within the past two years; and (e) an eccentric isokinetic baseline strength exceeding 3225 N, which would surpass the operational capacity of the eccentric dynamometer. Written informed consent was obtained from all participants using a consent form approved by the university’s Institutional Review Board.

2.2. Procedures

Participants completed a separate familiarization session prior to experimental testing to obtain anthropometric measurements (height, weight, and BMI) and to acclimate to the multi-joint eccentric dynamometer (Eccentron, BTE Technologies, Hanover, MD, USA). During this session, a maximal eccentric strength test was conducted to determine each participant’s individualized 40% submaximal training load and this predetermined load was manually input into the Eccentron software for the subsequent exercise session. Descriptive statistics for maximal eccentric strength values are provided in Table 1. Following this, participants engaged in eccentric training at the 40% load for 3 min to simulate the protocol used during the subsequent experimental session.

Experimental testing for each participant was conducted in a single session, scheduled at least 72 h after the familiarization session. To begin, electrode sites were shaved, cleansed with an alcohol swab, and prepared for placement of adhesive waterproof electrodes over the TA, GM, BF, and VL of the dominant leg (note the dominant leg was selected in accordance with common practice in the literature, as differences in muscle activation between dominant and non-dominant limbs are unlikely, given the absence of limb-related differences observed in our prior work using force-based Eccentron measurements [16]). Electrode placement followed the standardized guidelines of the Surface Electromyography for the Non-Invasive Assessment of Muscles (SENIAM) project [17]. The intrinsic foot muscles were not included in the EMG protocol due to practical electrode placement limitations and the study’s focus on larger, extrinsic lower limb muscles primarily engaged by the Eccentron exercise movement. To minimize motion artifacts in the EMG signals, the electrodes were further secured with adhesive, hypoallergenic waterproof wrapping. Leg dominance was determined by asking participants which leg they would use to kick a ball.

Prior to the eccentric exercise bout, participants completed a 4 min treadmill warm-up at a speed of 2.5 mph (Tandem Treadmill, AMTI, Watertown, MA, USA). This was followed by a 3 min eccentric exercise session on the Eccentron. Exercise intensity was standardized across participants at a 40% submaximal load, based on each individual’s maximal eccentric strength. The dynamometer velocity was set to 23 cycles per minute, representing a moderate speed consistent with velocities used in prior studies from our laboratory that have been shown to elicit substantial strength adaptations [3,4]. The seat position on the dynamometer was adjusted to ensure a knee joint angle of 30° at the most extended position, in accordance with the manufacturer’s guidelines. During the 3 min eccentric exercise bout, muscle activity of the TA, GM, BF, and VL was recorded using a 16-channel surface EMG system (Cometa Mini Wave, Cometa SRL, Milan, Italy). Raw EMG signals were sampled at 2000 Hz, in accordance with established recommendations for high-fidelity electromyographic data acquisition [18].

2.3. Data Analysis

EMG signals were processed using MATLAB (Version R2023a, The MathWorks Inc., Natick, MA, USA). Data were trimmed to the first minute of recording, beginning with the onset of muscle activity. Onset was identified via visual inspection, following previously described methods [18], by detecting the first exercise repetition of VL activation from the EMG onset. Signals were then processed using a 4th-order recursive Butterworth band-pass filter (20–500 Hz) to attenuate low-frequency drift and high-frequency noise. Muscle activation magnitudes for the VL, BF, GM, and TA were quantified using two metrics: root mean square (RMS) and integrated EMG (iEMG). EMG RMS values were computed from the filtered EMG signals, while iEMG was calculated by full-wave rectifying the filtered signals and applying numerical integration over the 1 min data collection period using the trapezoidal method (note the discrepancy between the 3 min Eccentron exercise protocol and the 1 min EMG collection period is due to this study being part of a larger study which had originally used a 3 min exercise duration).

2.4. Statistical Analysis

Statistical analyses were conducted using RStudio (Version 1.1.456). All dependent measures were assessed for normality using the Shapiro–Wilk test. Repeated measures ANOVAs were performed to evaluate differences in EMG RMS and iEMG values across muscles, with Bonferroni post hoc comparisons being conducted when appropriate. Cohen’s d effect sizes were calculated based on mean differences and pooled standard deviations, and interpreted according to conventional thresholds: small (d = 0.2), medium (d = 0.5), and large (d = 0.8), where larger effect sizes indicate changes that are more likely to be meaningful and practically important for training or clinical application. A type I error rate (α) of 0.05 was used to determine statistical significance for all hypothesis tests.

3. Results

Normality of all dependent measures was confirmed using the Shapiro–Wilk test (p > 0.05). The ANOVAs revealed a significant difference for both RMS and iEMG parameters (p < 0.001 for both). Follow-up comparisons revealed that both EMG RMS and iEMG values were significantly greater for the VL compared to the BF (p < 0.001 for both), TA (p = 0.003 and < 0.001, for RMS and iEMG, respectively), and GM (p < 0.001 for both), with large Cohen’s d effect sizes ranging from 1.28 to 3.17 (see

Table 2. Central tendency and dispersion data for the EMG RMS and iEMG variables across the four muscle groups.

Skeletal Muscle	EMG RMS (μV)	iEMG (mV·s)
Vastus Lateralis	52.9 (22.1)	377.6 (162.2)
Biceps Femoris	10.8 (4.5) a,b	78.4 (29.9) a,b
Tibialis Anterior	26.6 (19.1) a	166.4 (123.0) a
Medial Gastrocnemius	15.4 (16.4) a	96.8 (97.1) a

^a significantly lower compared to the vastus lateralis (p < 0.05); ^b significantly lower compared to the tibialis anterior (p < 0.05); EMG = surface electromyography; RMS = root-mean-square amplitude; iEMG = integrated surface electromyography. Data are presented as mean (SD).

and

Table 3. Cohen’s d effect sizes.

Comparison	Cohen’s d (EMG RMS)	Cohen’s d (iEMG)
Vastus Lateralis v. Biceps Femoris	3.17	3.11
Vastus Lateralis v. Tibialis Anterior	1.28	1.48
Vastus Lateralis v. Medial Gastrocnemius	1.94	2.17
Tibialis Anterior v. Biceps Femoris	1.34	1.15
Tibialis Anterior v. Medial Gastrocnemius	0.63	0.63
Medial Gastrocnemius v. Biceps Femoris	0.45	0.29

EMG = surface electromyography; RMS = root-mean-square amplitude; iEMG = integrated surface electromyography. Cohen’s d effect sizes were calculated using pooled standard deviations.

). Additionally, EMG RMS and iEMG values for the TA were significantly greater than those for the BF (p = 0.016 and 0.050, d = 1.34 and 1.15, respectively; see Table 2 and Table 3). No significant differences were observed between the GM and either the TA (p = 0.173 and 0.193, respectively) or the BF (p = 1.000 for both) for EMG RMS and iEMG values, with small to medium effects sizes observed (see Table 2 and Table 3). Figure 1 presents the data as box plots for both the RMS and iEMG variables, across the four muscle groups.

4. Discussion

The primary finding of the present investigation was that a multi-joint isokinetic eccentric lower body leg press movement elicited significantly different muscle activation magnitudes across four distinct muscles of the lower limb.

Unsurprisingly, the VL exhibited the highest magnitude of muscle activation compared to all other muscles during the multi-joint eccentric movement. In fact, VL activity was nearly twice as high as the next most active muscle, the TA, and was approximately five times greater than the least active muscle, the BF. This confirms the VL’s role as one of the primary movers and substantially engaged muscles during the Eccentron exercise.

What is perhaps more surprising, and less intuitive, is that the TA was the second most active muscle during this task, even higher than the GM. Given the nature of the leg’s pushing action against the backward-moving pedals, one might expect the GM to be more engaged than the TA. However, the opposite was observed. This finding partially aligns with Da Silva et al. [9] who reported variable lower leg muscle activation depending on foot placement during multi-joint leg press tasks, but direct comparisons remain limited since their study did not isolate the eccentric phase or quantify dorsiflexor activity.

The reason that the TA was more active is not known, but it may be speculated that the TA is highly engaged for joint stabilization purposes, via coactivation of the ankle joint. Another plausible explanation is that the pushing motion against the pedal may naturally encourage individuals to drive more through the heel or midfoot, subtly lifting the toes and thereby activating the dorsiflexor muscle group. It should be noted that the activation of the TA, in this manner, would seem to be more isometric rather than eccentric in nature. This observation has important functional implications, as the TA may act to stabilize the ankle joint during the backward pedal movement by maintaining dorsiflexion control. Further research is warranted to determine the precise mechanisms underlying this rather unexpected TA engagement during the Eccentron exercise movement. From a training perspective, this finding is noteworthy: the dorsiflexors, particularly the TA, appear to be sufficiently activated during this exercise and as such would seem to indicate that additional exercises specifically targeting the TA may not be necessary in training programs that already include Eccentron-based exercise routines.

Clinically, this finding is particularly relevant in populations affected by dorsiflexor weakness, such as older adults [19] and individuals with neurological conditions who often experience foot drop [20,21]. Given that foot drop results from impaired TA activation and leads to gait dysfunction and fall risk, the incidental yet potentially sufficient activation of the TA during Eccentron exercise suggests its potential utility in rehabilitation protocols. This may reduce the need for supplementary dorsiflexor-specific exercises within comprehensive training programs.

Another notable finding was that the BF, representing the posterior thigh and hamstring group, showed low muscle activation (the least of all muscles). This suggests that the Eccentron exercise is not particularly effective for engaging the hamstrings. A likely explanation for this low activation lies in the biomechanics of the movement such that during the eccentric phase, the knee angle progressively decreases, resulting in the hamstrings shortening rather than lengthening. Since eccentric loading is most effective when a muscle is lengthening under tension, this shortening action likely reduces the mechanical load placed on the hamstrings, thereby limiting their activation.

This pattern of low hamstring activation is consistent with findings by Nishiwaki et al. [22], who observed low hamstring and soleus muscle activity across various squat exercises, regardless of center-of-gravity adjustments. Their work suggests that hamstring under-activation during closed-chain lower limb squat press- or leg press-based tasks may be a broader phenomenon beyond the specific isokinetic eccentric leg press movement studied here. Perhaps the most comparable study relating to the isokinetic aspect is the Luera et al. study. Using a novel squat testing device that performs “isovelocity” movement via a motorized system, Luera et al. [12] demonstrated that during both concentric and eccentric squat movements, the hamstrings exhibited lower EMG amplitudes compared to quadriceps muscles, supporting the notion that multi-joint lower limb exercises, especially during an eccentric movement, may inherently result in differential muscle recruitment patterns, favoring some muscles over others. This aligns with our finding of consistently low BF activation despite the eccentric loading, also providing support for the idea that certain posterior chain muscles may be under-recruited during multi-joint closed-chain eccentric tasks like the Eccentron exercise.

However, this pattern may not apply uniformly across all posterior chain muscles. For example, the gluteus muscle group, which lengthens during hip flexion in the eccentric phase, would be expected to exhibit heightened activation. This preferential engagement of the quadriceps (and possibly gluteus) muscles during this form of eccentric exercise may have important clinical implications given these two muscle groups play a primary role in eccentric control when attempting to prevent/counteract a fall.

One limitation of the present study is that it did not include EMG assessment of the gluteus muscle group, leaving this interpretation speculative. Future research should include gluteal EMG data to confirm whether these muscles are more engaged than the hamstrings during the Eccentron-based exercise.

From a practical standpoint, these findings have important implications for exercise programming. When the Eccentron is used as a primary lower body exercise, it may not sufficiently target the hamstrings. Therefore, it would be advisable to include additional exercises that specifically recruit the hamstrings (such as Nordic hamstring curls, leg curls, etc.) to ensure a balanced posterior chain development and to reduce injury risk. Additionally, the relatively high standard deviations observed in some EMG parameters highlight notable inter-individual variability in muscle activation patterns, which may have practical significance for customizing eccentric training or rehabilitation programs to individual needs and movement strategies. This is especially relevant given that exercises such as the Nordic hamstring curl have been shown to elicit significantly higher EMG activity in the hamstrings compared to other modalities [23], and that eccentric hamstring exercises are indicated as important for reducing hamstring-related injuries [24].

The GM muscle exhibited relatively low muscle activity during the Eccentron exercise, which is somewhat unexpected. Given the pushing motion against the pedal, a degree of plantarflexion involvement would seem to be expected, which would elicit significant activation of the GM. However, several biomechanical and neuromuscular factors may contribute to the observed modest activation of the GM. For example, the backward-moving pedal in a seated position likely limits hip extension range and alters pelvis positioning, which in turn would reduce the amount of GM recruitment. Coactivation patterns between the ankle and knee muscles may also indirectly modulate GM activity by altering neuromuscular coordination during the eccentric phase. Additionally, participants may naturally push more through the heel or midfoot than the forefoot, reducing the need for plantarflexion and thus diminishing GM involvement. This complements Luera et al.’s observations of relatively lower activation of the GM in isovelocity squat conditions, suggesting that limited hip extension and altered force transfer may similarly constrain GM engagement during Eccentron-based movements.

Although the GM showed moderately greater activation than the BF, with an effect size of 0.45 for EMG RMS versus the BF, the multi-joint eccentric movement performed on the Eccentron did not elicit substantial muscle activation in this muscle group. While the GM could plausibly receive a moderate amount of workload during a prolonged Eccentron-based exercise routine, these findings suggest that supplementary exercises may be warranted to adequately target and enhance the function of the GM within a comprehensive lower body resistance training regimen.

In conclusion, the present study demonstrated that different lower limb muscle groups exhibited varying levels of muscle activation during a multi-joint isokinetic eccentric exercise performed on the Eccentron dynamometer. Muscle engagement, from highest to lowest, was observed in the following order: VL, TA, GM, and BF. This information may be valuable for both practitioners and researchers utilizing this form of exercise, as it provides insight into which muscles are being most actively engaged and which may require additional or alternative exercises to ensure comprehensive lower body training is achieved. Multi-joint resistance training movements offer efficiency by recruiting several muscle groups simultaneously, thereby reducing the total time needed for a complete training session that involves most muscle groups. While this appears to hold true in the current study, with the VL and TA, and to a lesser extent the GM, demonstrating moderate to substantial activation, the BF likely did not reach activation levels that would typically be associated with effective hamstring training. As such, supplementary exercises targeting the hamstrings may be necessary to ensure balanced muscular development when performing an Eccentron-based lower body exercise routine. Additionally, given the incidental activation of the dorsiflexors, these findings may have clinical relevance for populations with dorsiflexor weakness, such as individuals recovering from a stroke or living with neurological conditions (e.g., foot drop), where improving functional muscle engagement during a coordinated eccentric task could support gait rehabilitation and fall prevention.