This is the first meta-analytic review to assess the role of exercise sequence within the context of the concurrent training interference effect. Pooled estimates revealed that intra-session exercise sequence during a prolonged (≥5 weeks) concurrent training programme significantly affected the improvements in lower-body dynamic strength, with a resistance followed by endurance exercise order superior to the alternative sequence. Meanwhile, the training outcomes of lower-body static strength and muscle hypertrophy were not significantly affected by intra-session sequencing of the exercise mode. Finally, maximal aerobic capacity and body fat percentage, outcomes not associated with concurrent training interference , were unaffected by intra-session exercise sequence.
Evidence exists to support the concurrent interference effect, which consists of decrements in strength-based outcomes when practising this type of training relative to resistance training in isolation . As such, it is of interest to observe whether the manipulation of exercise sequence can play a role in mitigating, or indeed exacerbating, this phenomenon. This was true of lower-body dynamic strength, with resistance-endurance exercise sequence proving superior to the alternative order. Previous research suggests that a resistance followed by endurance exercise sequence is beneficial when prioritising strength-based outcomes [15,16,17], supporting the finding for lower-body dynamic strength. Interestingly, Hickson  failed to report exercise sequence of the concurrent training group. This lack of reporting prevents confirmation as to whether the primary finding of this research would act to mitigate or exacerbate the interference effect associated with concurrent training in this original research.
When contextualising the findings of this research, it is important to understand the factors governing adaptation to contrasting types of maximal efforts. The concept of training specificity is well established, whereby resistance training consisting of dynamic contractions results in greater gains during isotonic vs. isometric contractions , and hence a degree of contraction-type specificity. Further, adaptation is specific to the contraction velocity of the training stimulus; Kanehisa and Miyashita  reported maximal torques at isokinetic speeds, which coincided with the contraction velocity region of the training stimulus. We observed that strength adaptation, following a concurrent training programme, was only susceptible to modification from exercise sequence during dynamic and not static contractions. The greater increase in dynamic vs. static strength, irrespective of exercise sequence, is likely explained by the dynamic training methods of the included studies. However, the effect of training specificity fails to explain why dynamic strength was the only outcome to be modified by intra-session exercise sequence.
There is support from research investigating the order effect that the resistance stimulus should precede endurance exercise, given that residual fatigue from alternate exercise sequence has been suggested to negatively affect the training-induced strength gains [16, 17, 31]. The primary finding of this meta-analysis therefore supports this premise, given that lower-body dynamic strength adaptation was improved following resistance-endurance exercise sequence. What is less clear is why this outcome was modified by exercise order. It is possible that the observed order effect is explained by residual fatigue, with the stress of the preceding endurance stimulus acting to hinder the quality of the resistance session. Indeed, Lepers et al.  reported that 2 h of cycling at 65% maximal aerobic power reduced muscular peak torque by 14% in well-trained cyclists, with these outcomes ascribed to a decline in the neural input to the muscle and peripheral mechanisms. Cadore et al.  postulated that greater adaptation in lower-body dynamic 1-repetition maximum with resistance-endurance exercise sequence might be attributed to improved neuromuscular economy, with improvements in strength and reduced electromyographic activity for a given load. A suggested role for adjustments in the nervous system is also supported by Eklund et al. , with increased maximal force in combination with an increase in muscle activation in the resistance-endurance training group only. However, if residual fatigue or neuromuscular mechanisms were responsible for the observation that exercise sequence modifies the adaptation in lower-body dynamic strength, it remains to be answered why these factors would not facilitate enhanced lower-body static strength also. The finding that hypertrophy and dynamic strength outcomes were not similarly influenced by exercise sequence has been reported previously in the literature, albeit in an older population .
The outcome of power is reported to be most susceptible to interference from concurrent training methods , suggesting that velocity of contraction during maximal efforts may be an important factor. Concurrent training has been reported to attenuate strength adaptation in the high-velocity, low-force region of the force-velocity relationship, relative to resistance training in isolation. Resistance training in isolation improved maximal torque at angular velocities ranging from 0 to 4.19 rad s−1, while improvements from concurrent training were limited to the range of 0–1.68 rad s−1, despite both groups completing resistance training at an angular velocity of 4.19 rad s−1 . This is particularly important in the applied scenario, given that the majority of athletic performances require a limb speed of ≥3.14 rad s−1 .
The susceptibility of higher velocity actions to the interference effect has further support [6, 33]. Häkkinen et al.  reported that concurrent training resulted in attenuated rapid force production, relative to resistance training in isolation, possibly explained by a reduction in rapid voluntary neural activation. If high-velocity contractions against resistance are most affected by the interference effect, it could perhaps be that the order effect would be most apparent during outcomes assessing maximal power, rather than isometric activity. For example, if power is most affected by the addition of endurance stimuli , it would seem logical that prioritising the resistance stimulus (with resistance-endurance exercise sequence) would be of greater importance than for an outcome less affected by the opposing endurance stimuli. Unfortunately, there were insufficient data to include power outcomes in this meta-analysis, but generating sufficient data to analyse the order effect on higher velocity maximal effort outcomes would be a pertinent research question to investigate in the future.
The current study provides an overview of the data available on the effect of manipulating exercise sequence on the interference effect. It should be noted that while a meta-analysis does play a role in causal inference, it is not its primary purpose; rather, it provides an assessment of the consistency of results reported at an individual study level, in addition to offering greater precision of the summary effect outcomes . Some of the outcome measures reported had moderate to substantial heterogeneity, indicating a level of inconsistency in the results of individual studies. This could be a representation of the different methods used between individual studies, or indeed, the breadth of the age and training status in the study treatment groups. Despite symmetry in the funnel plot assessment, a publication bias risk was possible because of the inclusion of published articles only in the meta-analysis, with the risk of published articles showing positive findings and the non-publication of research that shows no effect. Further, the search of English-language sources only might have resulted in missed data. Furthermore, the role of exercise intensity within the context of the interference effect is a topical area of research [35, 36]. It is possible that the relatively untrained cohorts included in the meta-analysis were limited by their ability to perform at higher exercise intensities, and the subsequent effect that this could have had on the interference effect or benefit of a given intra-session exercise order is unknown. These are justifiable avenues for future research. Despite the limitations, this meta-analysis provides an assessment of the potential for intra-session exercise sequence to manipulate strength-based outcomes associated with the concurrent training interference effect.