We evaluated the effects of competitive match play on IPC-F assessed at two joint positions, immediately post-match and at +24 h, +48 h and +72 h later, in elite youth soccer players. We found significant reductions at both positions immediately post-match, at +24 h, recovery at +48 h, while at +72 h values were significantly higher than baseline. We also assessed the association between player’s relative posterior chain strength and the magnitude of force decrement observed at each time point and found that players with higher levels of pre-match posterior chain strength had larger reductions/slower recovery of IPC-F.
4.1. IPC-F Fatigue-Recovery Response and Angle-Specific Differences
Deficits in the immediate post-match period are characterised as acute fatigue and are principally related to ionic and metabolic disturbances [24
]. In agreement with previous literature, evaluating the acute effects of match or simulated match-play on IPC-F in elite youth [17
] and senior soccer players [15
], we found a significant reduction in IPC-F post-match. The present study is the first to assess the IPC-F fatigue-recovery profile in elite youth players using the 90° and 30° tests, but the magnitude of decline post-match is similar to the 17.7% reported by Wollin et al. [17
] in elite youth players evaluated using a fixed dynamometer. We evaluated both angles proposed by Schache et al. [26
] and McCall et al. [15
] as they differ in recruitment pattern, with a higher relative bicep femoris (BF) activation at 30° than 90° [27
]. In the present study, there were notable differences in the IPC-F fatigue-recovery profile; particularly the substantially larger acute IPC-F decrement at 30° than 90° (20.7% vs 10.8%) and at 24 h (12.1% vs 3.1%), and the larger increment at +72 h (17.2% vs 9.2%) that potentially warrant further consideration. Previous research comparing the response to competitive match-play of both IPC-F at 30° and 90° in senior players found a larger decline in IPC-F at 30° than at 90° +24 h post-match (effect size at 30°; 0.91–1.08 at 90°; 0.67–0.77; [11
]) or a similar magnitude of acute IPC-F decline [15
]. Following a simulated match protocol, Matinlauri et al. [25
], observed that semi-professional adult players had a larger decrement in IPC-F assessed in a 90° hip, 20° knee flexion test compared to that observed at 90°. The authors suggested that this was due to the 90°:20° position inducing a higher relative contribution of the BF and a greater knee extension—aligning with previous work showing greater acute fatigue in dynamic knee flexion at greater extension following simulated soccer competition [28
]. The substantially larger response to acute competition induced fatigue in the 30° in the present study may therefore reflect the higher BF activation at that joint angle [27
] in combination with greater relative fatigue in BF than in other posterior chain muscles active in the IPC tests. Interestingly, in the present study and that of Nedelec et al. [11
], the timepoint at which the magnitude of 30° and 90° IPC-F changes (relative to pre-match baseline) were least divergent was at +48 h or “match-day +2” - the time-point at which IPC assessments are typically performed (at least in English Soccer). Deficits at this time point are related to the degree of mechanical damage and the proportion of fibres affected [16
] and driven by exposure to the intense eccentric contractions [11
] implicit in decelerations and change of directions. Overall, these data suggest that the tests have a similar capacity to detect residual deficits, but that the 30° or other tests, which put a greater demand on the BF, have marginally greater sensitivity. However, in deciding which test is “better” to implement, in the time pressured setting typical of weekly monitoring, other practical considerations come into play, such as the ease of setting up the 90° test. Nonetheless, if time/environment permits evaluating in both positions and assessing changes at the two angles within individuals could potentially provide information around the relative fatigue response across the posterior chain muscles differentially involved in these tests.
We observed a broadly similar pattern of IPC-F fatigue recovery profile in the +24 h to +72 h post-match period as previously reported, but with some aspects worthy of note. Firstly, while the acute (post-match) IPC-F deficit we observed at 90° was slightly lower than that previously reported in senior players at 90°, at +48 h we found no mean deficit and most players IPC-F returned to pre-match values, while players in those studies still showed significant deficits at +48 h (of~8%–10%) [11
]. Our findings align more closely to that of Wollin et al. [17
], who reported non-significant (~3%) deficits at +48 h in elite youth players of a similar mean age as players in the present study but used a fixed dynamometer. Intuitively, recovery from intense exercise is slower in older individuals, but contrasting our data and that of Wollin et al. [17
] with previous findings in adults [11
] raises the possibility that even in their early 20s, players may already experience a slower rate of recovery and larger/more persistent neuromuscular deficits following match-play, a possibility which warrants further investigation.
4.2. 72 h Post-Match—“Positive Adaptation”
A notable finding of the present study was that at 72 h, mean IPC-F was significantly higher than pre-match values, in contrast to a return to pre-match values [25
] or persistent deficits [11
] previously reported at this timepoint. It is important to note that unique to the present study, players were only included in the analysis if they did not perform any training or match-play in the +72 h post-match period. While this may be considered an artificial situation in elite football, it did allow us to describe the “pure” fatigue-recovery response of IPC-F in response to competitive match i.e. without the potentially confounding influence of additional training delaying recovery. Our data suggests that when combined with adequate recovery, match-play may provide a stimulus for posterior chain muscle strength development, measurable with these isometric tests. However, we acknowledge that this positive adaptation may have been inflated by factors such as; increased practice and familiarisation with tests, pre-match “baseline” values that did not represent full recovery from previous days training or priming/potentiation of IPC-F by match-play [31
]. Nonetheless, while a novel finding with respect to IPC-F, the observation of a positive adaptation to competition does concur with Morgans et al. [32
] who reported increased CMJ height and peak power three days post-competition in elite professional soccer players. They suggested that match-play and in particular HSR volume, with which positive adaptation was most strongly associated, represented the highest physiological load a player is exposed to, and is an important stimulus for muscle power adaptations. These findings however, might lead one to ask; if match-play represents a strong stimulus for neuromuscular adaptations, is the player adequately conditioned for it? Future research in this area is warranted.
4.3. Variation in IPC-F and VariaVariability in IPC-F Response
While our sample was too small to allow a robust statistical comparison of positional differences in IPC-F, there appeared to be positional trends in IPC-F; for example a full-back within the sample; Pre-match IPC-F at 30° = 6.7 N/kg and at 90° = 6.6 N/kg, compared with a central midfielder; at 30° = 4.7 N/kg and at 90° = 5.0 N/kg. These differences in IPC-F force could relate to the specific adaptations of imposed demands of training and match-play [33
], influenced by positional differences in external load [34
]. In elite soccer academies players develop a playing position from a young age based on technical, tactical and physical capabilities, which are further developed during their academy years [35
]. For example, a full-backs match demands involve larger volumes of high-speed, sprint and high-acceleration distances [36
] and would favour greater stimulus for adaptations in type 2 fibres and for force and power development. In contrast, central midfielders who cover greater total distance comprising of less high-speed and sprint distance [37
], match stimulus would favour adaptations in type I fibres and improvements in oxidative capacity [38
]. As in previous studies, we noted a large inter-individual variability in acute and residual IPC-F force deficits following real and simulated match-play [15
]. The relationship between IPC-F deficits and external load have not been described, but it is likely that the large inter-individual variability in running loads in matches [5
] and in particular, the volume of HSR is one of the determinants of the variability in the IPC-F fatigue-recovery profile. Our study was limited by the lack of micro-sensor technology such as global positioning systems (GPS) within the match to quantify the running load demands of the players. Our correlation analysis did however show that players with higher IPC-F, those who also tend to play in positions known to perform higher volumes of high-speed, sprint and high-acceleration distances in youth soccer [36
], showed greater declines in IPC-F 30° at all time-points, and IPC-F 90° at all timepoints except post-match, suggesting that stronger players may require extended recovery before return to loading. Although, our study was limited by sample size and the lack of technology to monitor player-specific running loads, these observations represent interesting patterns which warrant further research.
While the greater HSR loads performed by the stronger players would be expected to play an important part in the negative correlation between IPC-F and IPC-F recovery following match-play observed, other factors such as greater damage to type 2 than type 1 fibres [39
] may also contribute to the greater fatigue/slower recovery in players with greater IPC-F. Nonetheless, this finding does appear to run counter to the notion that higher strength is associated with greater resilience or robustness; i.e. the ability to cope with and recover from the demands of competition. To our knowledge, the association between hamstring/posterior chain strength and the recovery profile in that muscle group has not been previously examined. However, there is evidence that “physical qualities” including strength, in knee/hip extension may mediate the neuromuscular fatigue response to competition as measured in a triple extension activity—the countermovement jump (CMJ) [41
]. In two studies by Johnston et al. [19
] examining elite youth rugby league players in which changes in CMJ peak power were used to quantify fatigue, it was shown that higher levels of aerobic fitness/intermittent running performance (measured with the Yo-Yo Intermittent Recovery test) and muscle strength (defined by 3RM back squat) were associated with reduced fatigue/more rapid recovery of neuromuscular function. The authors noted that stronger players covered significantly higher (GPS assessed) total and high-speed distances than weaker players during competition, yet post-match, and +24 h and +48 h they displayed the same deficits in CMJ peak power as the weaker players, suggesting that higher strength conserves at least one aspect of neuromuscular function under fatigue allows more work to be done for the same level of fatigue [41
]. Similarly, in elite senior soccer players, Owen et al. [42
] found moderate to large negative correlations between lower limb power (in a fixed load squat) and serum creatine kinase, a marker of muscle damage temporally associated with the residual fatigue response to high-intensity competition. On face value, our observations conflict with the findings of these studies, as we found that higher relative IPC-F was associated with larger acute and residual IPC-F decrements. Potentially, maximum isometric strength in the posterior chain is not protective but higher maximum eccentric strength [2
], already known to be protective against hamstring injury, might be. Furthermore, Johnston et al. [19
], reported that higher dynamic strength was associated with lower CMJ peak power changes, while lower limb strength was not measured, limiting direct comparisons. Lastly, it is well established from repeated sprint/soccer-specific activity results in significantly greater acute fatigue in hamstring than quadriceps peak force (torque) [26
]. This greater susceptibility to fatigue in the hamstrings muscle group may mean that other physical qualities, such as repeated sprint ability, aerobic capacity, or posterior chain local strength-endurance, are more important in determining fatigue-recovery profile in this muscle group. This may also suggest that in players with the highest HSR and match demands, overall aerobic and buffering capacity [43
] and metabolic adaptations in the relevant muscle fibres may not only confer greater fatigue resistance during competition [45
] but may also influence residual neuromuscular deficit/enhance recovery.
One of the limitations of our study is the lack of day-to-day reliability of the IPC-F tests within the present cohort, therefore we cannot determine the smallest worthwhile change or other metrics for meaningful change. While good reliability of both IPC positions tested has been previously reported in professional soccer players [15
]; dominant-leg 90° (CV = 4.3%, ICC = 0.95), non-dominant leg at 90° (CV = 5.4%, ICC = 0.95), and non-dominant leg at 30° (CV = 4.8%, ICC = 0.93) and for dominant leg at 30° (CV = 6.3%, ICC = 0.86), it is recommended that within-population reliability is assessed and signal-to-noise determined using internal data [46
]. Furthermore, increased familiarisation and practice on a daily basis through the testing cycle could have contributed to the ‘positive adaptation’ relative to pre-match values at +72 h post-match rather than true change in neuromuscular function. Although the present study was conducted in an elite population, results may not be generalizable as testing was conducted with a small sample (n = 14) from one team and with only one match per player included in the analysis, although this is the case with majority of similar studies [11
]. However, this was partly due to the strict inclusion criteria of match-recovery cycles with no training in the subsequent 72 h, which is also a strength of the study, in that it allowed us to characterize the response to the match alone—without the influence of varying superimposed training loads.