Understanding and Overcoming the Sticking Point in Resistance Exercise

To summarise our discussion above, many of the popular definitions of the sticking point and the sticking region suffer from some of the following key limitations or inconsistencies:Motivated by these observations, in this work we argue that it is best to adopt the notion of a sticking point rather than a sticking region and propose to define it as the point at which failure occurs when exercise is taken to the point of momentary muscular failure. Different forms of this definition were previously adopted by various authors such as Blackburn and Morrissey [21] and Cotterman et al. [22]. To see how this definition overcomes the difficulties enumerated above, first observe that by its very nature it identifies a single, well-defined point in a lift (thereby addressing issue 7 in the list), which can be anywhere in the range of motion (thereby addressing issues 1 and 2). Considering that failure occurs at this sticking point, issues 3 and 4 are immediately addressed too, as are issues 5 and 6. In addition to being clearly and uniquely defined, and not leading to any of the listed problems, the proposed definition is also ipso facto the performance bottleneck.

Early attempts at explaining the occurrence of sticking points have largely concentrated on biomechanical factors. This includes biomechanical factors specific to a particular exercise as well as, inevitably, to a particular trainee such as limb length ratios [16]. In terms of the primitive elements that affect muscular force production, the focus here is on the structural mechanics that affect muscular torque transfer to the load and the force-length relationship characteristics of muscular force production.

For example, in their study of bench press performance by elite powerlifters, Elliott et al. [4] argue that the observed location of the sticking point is mostly explained by the mechanically disadvantageous position for the exertion of effective force against the load. Similar arguments were put forward by Madsen and McLaughlin [10] and Escamilla et al. [13] amongst others, and this explanation remains a popular one to this day [17, 36‐38]. Nevertheless a more rigorous exploration of this claim readily exposes methodological flaws as well as inconsistencies. With regard to the former, it should be noted that Elliott et al. never actually investigated whether the position in question is indeed the biomechanically weakest one. As correctly noted by Kulig et al. [16] amongst others (also see Németh and Ohlsén [39], Arandjelović [19], and Bryanton et al. [40]), in multi-joint exercises such as the bench press, the effective strength curve is complex and involves a nonlinear combination of strength curves of individual muscles [19]. Indeed even in the simplest case of a single-joint exercise, because of the interplay between the force-length and force-velocity characteristics, and changing levers, the resulting strength curve is often not straightforward to predict as demonstrated by Blackburn and Morrissey in the example of leg extensions [21] and by Arandjelović in the example of arm curls [20] (also see work by Németh and Ohlsén [39]). Therefore the claim that a specific position in a lift is the weakest one requires empirical data, e.g. through a comparison with isometric strength characteristics. Although no data of this nature were provided by Elliott et al. [4], such a comparison in the context of deadlift performance was performed in detail by Beckham et al. [41] who found a poor match between points of isometric weakness and the observed sticking point when the exercise is performed in a conventional, dynamic fashion (though it should be noted that Beckham et al. performed allometric normalisation, which could have introduced artefacts in the data [42]).

Another observation that highlights the shortcomings of the purely biomechanical explanation concerns the changes in the sticking point locus across efforts of different intensities. As we already noted, in general the same individual will experience the sticking point at different stages in a lift taken to failure at different loads [3]. This could not be the case if biomechanics were the sole underlying factor.

Motivated by the inadequacies of the purely biomechanical explanation of the sticking point, in recent years a number of researchers have sought to present an alternative theory that focusses on the changes in relative contributions of the passive and active components of muscular force. Recall that muscle force comprises an active component exerted by the contractive elements of the muscle and a passive component that is exerted by its non-contractile, structural elements [43, 44], both of which are dependent on the elongation of the muscle. Of particular importance here is the observation that the magnitude of the passive component of muscular force increases rapidly after a certain amount of stretch of the muscle has been reached. Considering that in most cases (though not universally) the main muscles involved in a certain lift experience the greatest stretch at the beginning of the exertion phase, the magnitude of passive muscular force decreases as the lift progresses. Hence, van den Tillaar and Ettema [5] argued that the sticking point emerges as a consequence of this decrease—if the force deficit is exhibited over a sufficient amount of time active contractions are insufficient to overcome the experienced external resistance and the lift fails at the sticking point. The finding that the maximal weight an athlete could lift is significantly reduced when the concentric action of prime movers is not preceded by an eccentric stretch [45] supports this hypothesis given that this effect of the so-called stretch-shorten cycle (SSC) [46‐48] is thought to be effected by an elastic recoil of passive components of the muscle [49]. However this explanation too has failed to withstand empirical evidence, as acknowledged by van den Tillaar and Ettema themselves in their subsequent work [38, 50]. In particular there are several findings that speak against the decrease in passive force as the dominant factor in the development of the sticking point. For example, van den Tillaar et al. [38] observed that when it was not preceded by an eccentric portion, the sticking point on the bench press was higher than when the conventional execution of the exercise was adopted. This is contrary to what the proposed theory would predict—the presence of the stretch-shorten cycle would have been expected to result in delayed dissipation of the passive force contribution.

As we sought to illustrate, a number of challenges in the understanding of the phenomenon of the sticking point remain. This is not for lack of empirical data. The topic has received a remarkable amount of research attention and numerous well-designed studies in a variety of settings have been conducted; many of them are referenced in this article, and many others exist. Upon an examination of the corpus of relevant literature a disinterested researcher, we would argue, is led to the conclusion that much of the difficulty in trying to explain the sticking point is the result of the apparent desire to formulate an overly reductive yet universal model. It should come as no surprise that this may not be a realistic goal—different exercises in which sticking points are of interest are characterised by vastly different biomechanics (relative lever lengths, their changes over time, numbers of major contributing muscles, etc.), and different lifters exhibit different abilities (maximal force production, ability to sustain force, rate of force development, relative development of different muscle groups, etc.). These differences can greatly change the relative contributions of different elementary factors that contribute to the development of the sticking point: the dependence of the maximal voluntary force on muscle elongation and the speed of contraction, the elastic energy dissipated in the stretch-shortening cycle, the changes in internal and external levers, and fatigue. While in some circumstances one of these may indeed dominate, evidence suggests that this is not universally the case. The full understanding of the sticking point therefore requires a consideration of all of these factors and an explanation of a particular sticking point demands a thorough analysis of the particular lifter in the context in which the sticking point is observed.

Lastly it is worth nothing that research to date has very much focussed on what may be described as the impact of zeroth-order force dependence on the development of sticking points—both the force-length relationship and the torque effected by muscular force are factors that depend on the position in the lift only. In contrast the impact of the force-velocity (first-order force dependence) relationship on the sticking point has received little attention [20]. This is particularly surprising given that the development of momentum has been recognised as an especially important aspect in training and competition performance in practice [51], as we describe in further detail in the next section. The interplay of the aforementioned factors and fatigue, although noted by several authors [15, 52, 53], also demands more extensive study before its role in the context of sticking points is understood with some clarity and practical insight; notable research in this direction includes the work by Drinkwater et al. [2]. Finally, to the best of the authors’ knowledge the effect of muscular fibre type composition (both across individuals as well as across different exercises and muscle groups) on the occurrence and the location of sticking points has not been investigated at all.

Many studies on the sticking point examined the stage in a lift at which the sticking point was observed for different exercises [6, 9, 13, 54, 55]. These findings can offer valuable insight into different strategies that can be employed to improve performance. In particular, by considering the biomechanical context (lever arms, elongation, etc.) in which different muscles contribute to the lift in the vicinity of the sticking point as well as the corpus of collected electromyography (EMG) data, in many cases it is possible to identify the muscle (or more broadly a functional muscle group) that can be considered the ‘weakest link’ [8, 20]. A straightforward application of this observation involves the strengthening of these muscles and especially so at the elongation at which failure occurs. Indeed power- and weightlifters have a long tradition of so-called assistance work, which accomplishes precisely this [8, 56, 57]. Common examples include the inclusion of chest isolation exercises by athletes who exhibit the sticking point at the onset of the concentric phase in the bench press [58] or the use of various isolation exercises for elbow extensors by athletes who encounter difficulties in the terminal stages of the lift [59].

At the point in the ROM of a lift at which failure occurs, it can be readily seen that increasing the effective force that a trainee can exert against the load at this point will improve performance (note that this does not imply that the sticking point is the only or even the optimal such point, as discussed at length by Arandjelović [20]). Owing to the principle of specificity of strength adaptations [19, 60]—that is, the observation that the inducted adaptational stimulus to resistance exercise is the greatest for lifting conditions similar to those experienced during exercise—the most direct manner of addressing a sticking point is by employing partial repetitions [18] or isometric training [61, 62]. In particular, numerous studies have demonstrated that partial repetitions, whereby the load is lifted only through a limited part of the ROM in an exercise, is effective at increasing strength at approximately ±10°–20° from the trained joint angle [63, 64]. Similarly, functional isometrics that involve the application of force by the trainee against a load against a practically immovable obstacle (e.g. the pushing of a barbell against pins in a power rack) [65] have been shown to be successful at increasing strength at the specifically trained ROM [61, 62, 66, 67].

Considering the general consensus of empirical findings that suggest that partial and isometric training has limited potential for providing a sustained stimulus for muscular hypertrophy [62], these training modalities are of most direct interest to performance-oriented athletes. For athletes seeking increases in muscle mass the potential benefit may be indirect in that overcoming a specific sticking point may facilitate the use of greater loads in conventional training (which involves a combination of eccentric, concentric, and isometric contractions). However this potential value has to be carefully considered in the context of the invested time and effort, the associated neural fatigue, and psychological factors [68].

In Sect. 2 we noted that the sticking point in a lift may not necessarily occur at the point of greatest biomechanical disadvantage. For example, even if at a certain point in the ROM there is a net force deficit (i.e. the effective force an athlete is able to exert against the load is lower than the experienced external resistance), if the load has significant momentum the deficit may not effect a difficulty in overcoming this part of the motion. This observation leads to a popular training strategy employed by strength and power athletes that focusses on increasing force and its rate of development in the phases of a lift that precede a sticking point [69‐71]. In particular so-called speed work involves the use of repeated low-intensity (\(\approx 50\)–60 % of one repetition maximum) sets, typically with short rest periods (45–60 s), with repetitions performed in a maximally accelerated fashion. This modality has been widely used by powerlifters [51, 69, 72, 73] in training for all three of the competition lifts (bench press, squat, and deadlift), and recent models described in the academic literature have started to elucidate the mechanisms underlying its effectiveness [20].

A different use of momentum for overcoming a sticking point involves the application of external momentum. In contrast to speed work training whereby the load is supplied momentum via the action of the muscles inherently involved in a particular exercise, external momentum is developed through the use of muscles otherwise not involved in a lift [14]. Though widely used by both recreational trainees and elite athletes [74, 75], this practice is often, if not usually, dismissed (as suggested by the morally loaded colloquial term ‘cheating’ used to describe it [76‐79]) on the grounds that the use of excessive resistance increases the risk of injury and reduces the load experienced by the target muscles [76]. However recent models suggest that when used in moderation, external momentum can be safely used to apply greater force on target muscles as well as increase their time under tension (TUT) [14]. Considering safety and practical constraints (e.g. external momentum is easier to impart on isolation exercises, which generally involve the use of lighter loads), external momentum is of most use to athletes seeking increases in muscle size such as bodybuilders.

The motion against resistance can be thought of as being effected by the sum of forces of muscles dynamically contributing to the lift, nonlinearly modulated by the given mechanical context [16]. Even when a single functional muscle group and its effects on motion around a single hinge joint are considered, the isolated characteristics of the effective force are greatly different from those of the muscle in isolation [20]. For complex multi-joint lifts, which involve a greater number of functional groups of muscles, the characteristics are far more multifaceted. This observation provides a powerful means of modifying a lift in a manner that eliminates or reduces the impact of a sticking point—by changing the style of exercise execution the biomechanical context can be changed. Distally speaking, this means that the points in the ROM at which a particular muscle are particularly strong (or weak) can be altered [4, 54], the time under tension (and with it fatigue) preceding the sticking point can be affected [8, 20] as well as the speed of contraction of contributing muscles at different points in the lift [19, 72]. In proximal terms the aim is to “flatten out” the difficulty of the lift [80]. Specific examples of how this may be achieved include alterations to the grip [54] or the stance [81, 82] width of the lifter, changes in the orientations of joint flexion/extension (or adduction/abduction) planes [3, 4, 82], adjustments in the synchronisation of movements across different joints [7], as well as numerous others [83].

It is important to stress that safety should always be an important consideration when attempting a modification of lifting technique. An unfamiliar biomechanical context itself can lead to injury so any changes should be done in a gradual fashion and using conservative loads until the lifter is familiarised with the newly adopted technique. In addition certain lifting styles may inherently carry certain risks, e.g. a wide grip on the bench press may increase the risk of shoulder injury and pectoralis major rupture [84], rounding of the back in the deadlift (which minimises the moment arm of the load around the hip) the risk of spinal injuries [85], and buckling of the knees (valgus collapse—poorly synchronised or excessive tibial internal rotation and adduction relative to the knee flexion angle in a given stance) in the squat the risk of knee injuries [86].

Exercise technique alterations are of most obvious utility to strength athletes whose primary aim is to complete a lift with the greatest amount of load, providing that the alterations are within the range permitted by their sport (e.g. see The International Powerlifting Federation [87] and The International Weightlifting Federation [88]). However employed in a targeted manner they can benefit a wide range of trainees. Bodybuilders for example may use them to place a greater emphasis on a certain muscle group (thereby possibly increasing the resistance experienced by the target muscles while reducing the total load lifted) while athletes may benefit from a style that is more suitable to their individual strengths and weaknesses and more effective at mimicking the manner in which they would perform a certain mechanical action.

The term accommodating resistance refers to purposeful modifications of the effective load experienced in an exercise throughout a repetition [19, 89‐92]. This technique is most often used in training by powerlifters [69] but also by other types of athletes in general strength and conditioning work [89, 91, 93]. One popular method of introducing accommodating resistance involves the fixing of an elastic band between the load (such as a barbell) and floor (or other fixed object, e.g. the power cage or the frame of a resistance machine). Typically, as the weight is lifted, the band is stretched and the resistance felt by the trainee increased [94‐96]. Another commonly used alternative involves the use of heavy chains [97, 98], which are uncoiled and lifted off the floor during the lift thereby effecting an increase in resistance.

Both types of accommodating resistance are commonly recommended in powerlifting training for “overloading the top of the range of motion” [99‐101] or increasing the rate of force development [93, 102, 103]. As such, when it comes to performance-oriented athletes (such as powerlifters), they are of most use in cases when the sticking point occurs in the terminal stages of a lift. For bodybuilders, or indeed other athletes looking to increase their muscle size, for whom the immediate aim is not the increase in performance in a particular exercise per se, the opposite prescription seems reasonable, i.e. an overload of the part of the ROM that is overcome easily. In this manner the entire ROM of an exercise can be made approximately uniformly challenging and closer to maximal resistance experienced throughout a set [80].

The mechanics of training aids such as elastic bands and chains limit the functional form of resistance alterations that can be achieved [104] (for a detailed review see Arandjelović [19]). Nevertheless other means of applying variable resistance are readily available in many training facilities [48, 104]. The most common ones include machines that achieve more complex loading patterns through the use of cams [80, 105], counterweights [106‐108], and viscous resistance [104, 109]. The resistance modification achieved by each of these is quite different in nature: cams offer resistance variability as a function of the position in a lift, counterweights as a function of the acceleration of the load (i.e. the second derivative of position), and viscous resistance as a function of the speed of the load (i.e. the first derivative of position) [109]. By choosing an appropriate modification, which may include a combination of two or more of the aforementioned modalities, sophisticated effects can be achieved that best suit a particular athlete’s goals [109‐111].

By varying the length of the moment arm of the force transmitted by the machine, cams allow a fixed force (the weight of the load) to produce a changing effective force experienced by the athlete. The force envelope is determined by the design, i.e. the shape of the cam [111]. One of the key ideas motivating the use of cams is that of attempting to match the resistive force of the machine with the force-length characteristics of human skeletal muscles [80, 104, 112, 113]. This would make them more suitable for hypertrophy-oriented athletes such as bodybuilders. The alteration of resistance characteristics through the use of counterweights is rather different in nature and may be described as reactive in the sense that the resistance is not dependent on the part of the exercise ROM per se but rather the instantaneous ease or difficulty of lifting exhibited by the trainee. As the detailed analysis presented by Arandjelović [109] demonstrated, at times when the load is moving with ease, i.e. with an increased acceleration of the load, the acceleration deficit between the load and the counterweights acts in a manner that increases resistance. The converse is true as well: when the acceleration of the load reduces, the effect of the counterweight is increased and the resistance felt by the trainee lessened, the least resistance being felt when acceleration reaches zero or becomes negative (as is the case in the exercise ROM preceding the sticking point) [109]. This is of most use to hypertrophy-oriented athletes for whom high tension sustained over time is crucial [114, 115].