Search results

A total of 207 articles were identified from an initial survey executed by two independent reviewers. 98 of these articles were duplicated, which led to a remaining amount of 109 in the process. The next step involved reading the title and abstract with the purpose of eliminating all those not meeting the inclusion criteria. Finally, twenty-eight articles were fully read, and nineteen of these were eventually selected for the review (Fig 1). The publication date of all selected articles ranged from 2002 to January 2019. Additionally, all studies were categorized as having a good/excellent quality in the methodological process based on the PEDro quality scale.

All selected articles presented a cross-sectional design. In fact, most experimental studies found used an untrained participant sample, so they were excluded. Regarding experience time, all participants had at least six months of previous resistance training experience, although some studies did not report the exact experience time of participants (Table 1).

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Table 1. Data gathered from selected articles regarding intervention, sample size, gender, training experience, age, sEMG collection method, outcomes and main findings.

https://doi.org/10.1371/journal.pone.0229507.t001

No common criteria were followed when referring to the exercise loading at which exercises were evaluated during sEMG recordings. As a matter of fact, only two studies used a similar method, assessing one repetition maximum intensity (1RM) [22, 32]. Some studies measured a number of repetitions of xRM, whereas others measured a number of repetitions of a range between 65–85% of 1RM (Table 1), which could be considered in all cases as a submaximal load intensity [33].

Data regarding the studies’ general description and main findings are presented in Table 1, while Tables 2–5 contain data referring to muscle activation during Deadlift exercise and/or its variants. We found no unified criteria for the sEMG normalization method. Out of all included studies, seven reported data description regarding muscle activation in relation to exercise type and normalized sEMG activity as a percentage of maximal voluntary isometric contraction (% MVIC) (Table 2); three of them as percentage of peak root mean square (% peak RMS) (Table 3); two studies reported data expressed as absolute RMS values in microvolts (mV) (Table 4); and three studies expressed data as a percentage of 1 repetition maximum (% 1RM) (Table 5). In addition, there were four studies which were not included in the tables because they assessed the sEMG only from the upper limbs or showed the muscle activation in a different measurement unit than that used in our analysis [28, 32, 34, 35].

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Table 2. Data description regarding sEMG activity in each study, in relation to exercise type and normalized sEMG activation expressed as mean or peak % MVIC.

https://doi.org/10.1371/journal.pone.0229507.t002

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Table 3. Data description regarding sEMG activity in each study, in relation to exercise type and normalized sEMG activation expressed as % peak RMS.

https://doi.org/10.1371/journal.pone.0229507.t003

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Table 4. Data description regarding sEMG activity in each study, in relation to exercise type and normalized sEMG activation expressed as absolute RMS values in mV.

https://doi.org/10.1371/journal.pone.0229507.t004

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Table 5. Data description regarding muscle activation in mV expressed as a percentage of EMG (mV) during 1RM effort.

https://doi.org/10.1371/journal.pone.0229507.t005

Most researched Deadlift variants include the Conventional Barbell Deadlift (10/19 studies) [8, 13, 22, 27, 28, 32, 36–39] and the Stiff Leg Deadlift (6/19 studies) [16, 35, 40–43], which are followed by Unilateral Stiff Leg Deadlift (2/19 studies) [16, 43], Romanian Deadlift (2/19 studies) [27, 44] and Hexagonal Bar Deadlift (2/19 studies) [22, 38] (Table 1).

It is also important to clarify that exercises such as “Olympic Barbell Deadlift”, “Straight Bar Deadlift”, “Barbell Deadlift”, “No Chains Deadlift” and “Conventional Barbell Deadlift” all refer to the same exercise, so “Deadlift” will be used for all cases.

Concentric and eccentric phases

Generally, studies analyzing electromyographical data assess muscle activation on each repetition, treating it as a single unit. Nonetheless, it has been reported that electromyographical activity could differ significantly between concentric and eccentric phases of the movement. Therefore, some authors have already carried out this division in their research [45–47]. Not all studies included in the current review divided sEMG exercises into concentric and eccentric phases. In fact, only seven studies performed such a subdivision [16, 28, 34, 37, 38, 40, 41], in which the concentric phase showed greater muscle activation than the eccentric phase for every single case.

Muscle activation

The biceps femoris has been the most investigated muscle in terms of sEMG for the Deadlift exercise and its variants (13/19 studies). Gluteus maximus is the next muscle most evaluated (10/19) followed by vastus lateralis and erector spinae muscles (9/19). The semitendinosus and rectus femoris are positioned in fourth position (5/19) followed by vastus medialis, external oblique and medial gastrocnemius (3/19) (Table 1).

Due to the diversity regarding methodology, it was considered appropriate to report the results by grouping the studies according to the sEMG normalization process carried out in each study (mean or peak % MVIC, % peak RMS, RMS mV or % 1RM).

Studies in which muscle activation was expressed as a mean or peak % MVIC are shown in Table 2. Erector spinae showed the greatest muscle activation during the Stiff Leg Deadlift exercise [41], and also showed a similar muscle activation than the gluteus maximus or biceps femoris during Deadlift and Hexagonal Bar Deadlift exercises [22]. Except for the Deadlift exercise [22], the gluteus maximus showed greater muscle activation than biceps femoris [13, 22, 40, 41]. When comparing muscle activation within the hamstrings, there was a greater activation for the semitendinosus muscle than the biceps femoris during Stiff Leg Deadlift [16, 41, 42], which is even more pronounced when performing Unilateral Stiff Leg Deadlift [16]. The concentric phase showed a greater activation in the gluteus maximus and hamstring muscles than the eccentric phase for all exercises evaluated [16, 40, 41] (Table 2).

Data regarding muscle activation expressed as percentage peak RMS (% peak RMS) are shown in Table 3. The erector spinae and lumbar multifidus showed greater muscle activation than the gluteus maximus and biceps femoris [26, 36]. However, conflicting results have been reported for the Deadlift exercise. Lee et al. [27] reported more activation in the biceps femoris than the gluteus maximus, while Snyder et al. [36] reported more activation in the gluteus maximus than the biceps femoris (Table 3). Whereas the vastus lateralis showed greater muscle activation than the biceps femoris [26, 36], and the rectus femoris showed greater muscle activation than the biceps femoris and gluteus maximus during Deadlift exercise [27] (Table 3).

Data regarding muscle activation expressed as RMS in mV are shown in Table 4. Erector spinae and semitendinosus are the most activated muscle in the Deadlift exercise [22]. When comparing muscle activation within the hamstrings, there was a greater activation recorded for the semitendinosus muscle in comparison to that for the biceps femoris [22, 44] (Table 4). The concentric phase showed greater activation than the eccentric phase in all muscles and exercises evaluated [44].

Data regarding muscle activation in mV expressed as a percentage of sEMG (mV) during a 1RM effort are shown in Table 5. The Erector spinae presented higher muscle activation than the gluteus maximus [37] and biceps femoris [38]. The vastus lateralis and vastus medialis showed greater muscle activation than the biceps femoris and gluteus maximus during Deadlift exercises and its variants [37–39]. The concentric phase showed greater activation in the biceps femoris, vastus lateralis and erector spinae than the eccentric phase during the Deadlift exercise as well as during the hexagonal bar Deadlift exercise [38].

Methodological issues

One concern about the findings of the review was the lack of unification of collecting data methodological process amongst studies. This includes the kind of muscle contraction evaluated, the number of participants, the participants’ resistance training experience, exercise intensity during evaluation, sEMG collection method, electrode location, and number of evaluation days. All studies following a specific methodology process had diverse aims and different outcomes, which made difficult to deliver consistent results. Only one study evaluated just an isometric position of the movement, the preparatory position [35], whereas the rest evaluated exercises from a dynamic perspective.

The included studies were variate in number of participants (8–34) but similar in their sample population ages (18–34), who had a minimum of 6 months resistance training experience. It is important to highlight the impact that training status have upon muscle activation pattern, since familiarization with the movement could substantially modify muscle activation elicited during each exercise [49–51]. Furthermore, twelve of the studies had a male sample, while the rest combined both genders [34–36, 41, 43], and only two studies included exclusively females [39, 40]. This raises the necessity to invest more research into females in this field.

In line with previous reviews, exercise loading for sEMG recordings has been one of the biggest concerns [52–54]. Only two studies performed same 1RM intensity [14, 25], whereas others performed exercises at a predetermined repetition maximum load, and the rest measured a number of repetitions within a range of 65–85% 1RM (Table 1). Differences in the applied methodology should be reduced for future studies, providing an enhanced outcomes reliability [42].

No unified criterion has been followed in categories of time management during exercise phase among study methodologies, which could also be treated as a potential bias risk. For future studies focused on sEMG, it would be of significant interest to report divided electromyographical data into concentric and eccentric phases, as well as exercise timing. Such information would help coaches and trainers when choosing one or another exercise for a concrete target when prescribing an optimized training [7].

In relation to the electrode location, reports on surface recording of sEMG should include electrode shape and size, interelectrode distance, electrode location and orientation over muscle with respect to tendons and fiber direction among others (Merletti & Di Torino, 1999). It is vital to report in detail the placement of electrodes over the muscle belly when we aim to compare outcomes with other similar studies.

Different protocols for surface electromyography electrode placement have been described in the literature. One of the most popular protocols is the SENIAM Guidelines (Surface ElectroMyoGraphy for the Non-Invasive Assessment of Muscles). Eight studies following the SENIAM Guidelines have been included in our review [8, 16, 22, 35–37, 41, 42]. The rest followed some other Guidelines or a previous reference, and only four studies did not report any protocol for electrode location [26, 28, 32, 34].

In regard to interelectrode distance, five studies reported using the recommended 2 cm center-to-center distance between electrodes according to SENIAM Guidelines [8, 22, 27, 35, 43]. In addition to not following these Guidelines, some other studies also did not report the inter-electrode distance [26, 32, 34, 38–40, 44]. Furthermore, four studies reported to have placed the electrodes with a center-to-center distance ranging between 15–35mm but different from 20 mm [13, 16, 28, 37, 41]. The higher the interelectrode distance, the wider the detection volume and consequently the detected amplitude [55]. Future research should attempt to follow established Guidelines, so they can reach optimum research quality and diminish the risk of data collection bias.

On the other hand, most of the reviewed studies included between 2–4 days/sessions (visits to the laboratory) for the measurement process, normally leaving 2–7 days’ rest between each visit. Tasks performed during those days cover anthropometric data gathering, familiarization with exercises, RM testing and sEMG data collection. To ensure reliable sEMG data outcomes, sEMG data must be collected at the same session [8]. Otherwise, some studies collected sEMG data on two different days, which might have entailed electrode location mistakes [32, 40, 41, 44].

In order to avoid fatigue bias risks, a randomized counterbalanced order for exercise testing was followed in all studies but one, which followed a preset exercise order [41]. In addition with the same aim, a minimum break of 2–5 min was considered between exercise testing trials [22, 27, 28, 34, 36, 37, 40–42, 44].

Most studies did not report hand grip and stance position in any depth of detail. Some studies allowed a preferred stance position for each participant but maintained the same for all exercises tested [8, 22], whereas others also indicated a hand grip slightly wider than shoulder width [26, 27, 32, 40–43].

Likewise, information about MVIC should be strictly reported. Our reviewed studies reported a range between 2–3 trials, 3–5 seconds holding and 15–60 seconds rest between trials [13, 16, 22, 32, 40, 43].

Other Deadlift variations

Apart from the above-mentioned Deadlift exercises, there are some other studies which focused on less conventional variants of this movement. The Good Morning exercise appears to be an appropriate substitute to Romanian Deadlift when it is preferable to place the load on the back instead of lifting it from the floor. Good Morning provokes a similar muscular pattern activation as Romanian Deadlift, but it showed more muscle activation for the semitendinosus and less muscle activation for the biceps femoris than Romanian Deadlift [44].

In addition, some authors proposed interesting alternatives for the Deadlift exercise with the goal of overcoming the sticking region. This involves a phase during the lift in which there is a mechanical disadvantage that elevates injury risk and leads to a deceleration on the speed lift [56]. In relation to this issue, Nijem et al. (2016) compared Deadlift versus Deadlift with chains and reported the existence of a lightest load at the sticking point which would allow one to maintain a neutral spine during Deadlift with chains. Regarding muscle activation, there were significant differences for the gluteus maximus muscle, which present greater activity during Deadlift than Deadlift with chains. Furthermore, Andersen et al. (2019) reported another resource by using the addition of elastic bands attached to the ceiling to displace the sticking point. This method would reduce the load from lower phases of the lift and increase the resistance as the bar goes up.

Elastic bands have also been used as a tool in Deadlift learning processes, when the athlete is not ready to lift high loads with a proper technique or in those cases when some injury prevents the athlete from using conventional resistance equipment. Muscular activation presented during elastic bands Stiff Leg Deadlift was lower than that elicited during free weights Stiff Leg Deadlift, with significant differences when referring to the gluteus maximus, biceps femoris and semitendinosus muscles [41].

Furthermore, it should be noted that if your aim is to increase muscle activation from forearm musculature during the Deadlift exercise, it is recommended to use a Fat Gripz device, a wider grip implement that sticks to the bar. Worth mentioning that a significant reduction in 1RM strength would appear when using this kind of implement [32].

Comparing Deadlift to other exercises

Some studies included in this review also compared muscle activation elicited during Deadlift exercises versus other typical weight bearing exercises performed in weight rooms. McCurdy et al. (2018) reported significantly greater muscle activation for the gluteus maximus and hamstring muscles during Modified Single Leg Squat in comparison to Back Squat and Stiff Leg Deadlift. Whereas, Korak et al. (2018) reported the highest muscle activation for the gluteus maximus during Front Squat comparing to Deadlift exercise, with no differences for this muscle between Front and Back Squat.

Moreover, the Hip Thrust exercise has also been found to elicit greater muscle activation for the gluteus maximus than Deadlift and Hexagonal Bar Deadlift. Also, lower muscle activation for the biceps femoris muscle was shown during Hip Thrust compared to Deadlift. No muscle activation differences were presented among those three exercises for the erector spinae muscle. Hence, a greater torque and greater stress in the hip joint during Deadlift compared to both other exercises was also reported [22].

Additionally, several authors have compared Deadlift exercises to single joint and machine-based exercises in their research. For example, Bourne et al. (2017) reported significantly greater muscle activation during 45º Hip Extension and Nordic Hamstring Exercise than Stiff Leg Deadlift and Unilateral Stiff Leg Deadlift for biceps femoris and semitendinosus muscles. Similar results support these findings, showing a greater muscle activation during the Nordic Hamstring Exercise and during Seated Leg Curl for hamstring muscles in comparison to the muscle activation elicited for hamstring muscles during Stiff Leg Deadlift and Unilateral Stiff Leg Deadlift [43].

On the other hand, the Prone Leg Curl in machine was found to elicit higher muscle activation for both upper and lower sections of the biceps femoris muscle than during Stiff Leg Deadlift but showed no significant differences for the semitendinosus muscle (Schoenfeld et al., 2015). On the contrary, McAllister et al. (2014) reported greater biceps femoris muscle activation during Romanian Deadlift than during the Prone Leg Curl. It would be necessary to unify the muscle activation normalization method and protocol carried out. Likewise, researchers should ascertain a proportional exercise load when comparing bilateral multi joint exercises to single leg and machine-based exercises, in order to obtain consistent outcomes.