Training loadPhysical/physiological load is essential in the training process. Without appropriate (overload) stimulus, fitness of players will deteriorate (or not develop to its full potential in youth players). An example, which all players and coaches have already experienced, is during injury or during off-season/summer-break.

However, an overload that is excessive and inappropriate will most likely result in injury (short term) and/or in an overtraining state (long term).

As a result, monitoring training load seems to be essential in order to: improve interpretation of physical tests used to verify the effectiveness of training programs design periodization strategies; identify athletes who are poor responders (to training) to control the compliance of the training completed to that planned by the coach To modify the training process before the assessment of its outcome, thus optimizing soccer performance (31- see references below).

The following paragraphs will display different solutions how to monitors football players. Particularly we will distinguish the coachs problems in prescribing training (external) loads compared with the actual (and perceived) load (internal load) received by players.

The manuscript is not thought to set guidelines for teams for a specific kind of training, as the load for an individual team and its players needs to be set individually in accordance to the physical state for a given time of the season.Internal vs. external training loadInternal loadInternal load is described as the physiological stress opposed on players and can be measured through physiological variables (21, 36, 52) such as maximum oxygen consumption (VO2max), heart rate (HR), percentage (%) of maximum heart rate (%max HR), percentage (%) of maximum heart rate reserve (%HRR), blood lactate.

Maximum oxygen consumptionVO2max seems to be the gold standard (4) (for more aerobic type activities), however absolute impractical in training (and sometimes even in testing).

Percentage of maximum heart rateAs heart rate (HR) have direct relation with VO2max (28, 51), it seems that HR, more particularly, time spend in different heart rate zones (for example 5 minutes above 70% of heart rate max) indicate how strenuous the particular exercise/game/training is for a player. %max HR was used in football (12, 13, 23, 33, 36, 40, 42, 47, 52, 53). It also needs to be mentioned that HR underestimates the load in more anaerobic type situation (such as 1 vs. 1, 2 vs. 2 and sprints) (34).

Percentage of maximum heart rate reserve% heart rate reserve is thought to be an even more accurate measurement (compared to %of max HR), as a) the heart rate reserve takes the resting HR of players into account and adjust the intended exercise heart rate accordingly and b) the percentage of maximal heart reserve has been proposed to be equivalent to %VO2max (while %HRmax is not) (29). However, to our knowledge, HRR was used only used in one investigation in football (39).

Blood lactateBlood lactate has been used as a physiological measurement of load in football (21). It seems, that blood lactate is a more appropriate measurement for more anaerobic type performances (such as 1 vs. 1.) compared to heart rate in SSG (5, 21), however did not show appropriate sensitivity (compared to HR) during on-field endurance training (17).

Ratio of perceived exertionwhile above variables are (truly) measured, the individual players ratio of perceived exertion (RPE) can be seen as a subjective rating of the training load.

RPE was used frequently in football (1-4, 8, 12-14, 22, 30, 32, 40, 48, 52) as it is a simple and inexpensive method to get information about the individual internal load of players.

From our experience, players need to be educated/trained using RPE; and even despite the training, coaches will need time to familiarize themselves with the interpretation of the results they will receive. For example, individual responses to training might show the same players at the top end (maximal) and or at the bottom end continually. Therefore it is not the best way to interpret the values inter-personally, however longitudinally intra-personally.

RPE was seen a good indicator of the load for SSG as it correlated with heart rate and blood lactate (16), however, taking the session duration into account will furthermore give information about the total load opposed onto players (and not only with regards to a specific part of the session/exercise).

Session-RPEThis method was proposed by Foster et al. (23, 24) and uses (CR10-scale)-RPE and the duration of training (time in minutes) to quantify the internal training load. The session-RPE is a result of the multiplication of the whole training RPE and the total time of training of a particular training session. Athletes are supposed to rate the overall load of the session 30 minutes after the session to ensure that the perceived effort referred to the whole session rather than the most recent exercise intensity (14, 30).The method was validated in soccer players, showing session-RPE as a good indicator of internal load of football training (14, 16, 30). Furthermore, all kinds of football sessions (technical, conditioning, speed) were used to quantify the internal training load with the session-RPE (3). RPE seemed to be the more suitable choice (in comparison to HR) in more anaerobic type situations (such as 1 vs. 1, 2 vs. 2 and sprints) (34).

Scientifically, (and despite the statements above) it seems that: RPE with heart only explained 50% of the perceived load adding blood lactate increased the model to 57.8% (16) The validation process regarding the internal training load does not seemed to be appropriate (enough), as the pure correlation of different monitoring tools (RPE with HR and blood lactate) does not validate one or the other, especially if one or the other (or both) measurement have flaws itself (2).Additionally, session-RPE does not seemed to be a valid factor in prediction of performance (8).

Visual Analogic Scale QuestionnaireThe questionnaire was tested for validity in youth soccer players (43). The players were asked two questions: How do you classify the effort made during the training session or match today? How physically demanding did you perceive the training session or match today?Players had then to mark a distance on a 10 cm scale displaying no effort at all to maximal effort (for question one) and not demanding at all to maximally demanding (for question two). Visual analogic scale (VAS) distances were then converted into scores, with 1 cm between scores equal 1 score point resulting in a score between 0 to 10.

Banisters TRIMPBanister (6) assumed that exercise elicits a training impulse (TRIMP) and hismethod uses the exponential relationship between fractional elevation in heart rate and blood lactate concentration. It serves as a function to weight exercise at a particular intensity (2).

The effective training session duration (in minutes) is multiplied withHR Ratio (based on heart rate recovery)HRratio = (HRex HRrest) / (HRmax HRrest)

Where HRex = the average heart rate during exercise, HRrest = the average heart rate during restAnd further multiplied with e and its exponent b (1.92) x HRratio

To our knowledge, this method was used in football players (2, 3, 30).

Edwardss methodThe Edwards method to quantify the internal load uses the total volume of the training session (in minutes) and the total intensity of the training session. The total intensity is based on a categorical system in which different heart rate zones receive a multiplier (20). 50-60%HRmax = 1 60-70%HRmax = 2 70-80%HRmax = 3 80-90%HRmax = 4 90-100%HRmax = 5

The total (internal training load) score is then the sum of the calculated load (duration in each zone times the multiplier) in each zone.

To our knowledge, there is no scientific information investigating the validity of this methods and we argue that the chosen categories (10%HRmax equals one factor point) might not be the appropriate relationship. The given categories implies linearity (training at 80-90%HRmax is two times harder then training at 60-70%HRmax), which is false above the anaerobic threshold (50).

To our knowledge, this method was used in football players (2, 3, 30).

Lucias TRIMPThis TRIMP model is similar in approach to Edwards methods, however used ventilator thresholds (35) and therefore might be seen as a step into individualization of internal training load.

Zone 1 was defined as below the ventilatory threshold ( 4 mmol/l). The duration spent in each zone is then multiplied by a coefficient (k) relative to each zone (k=1 for zone 1, k=2 for zone 2, and k = 3 for zone 3) (7).

Impelizzeri et al. (30) used this methods in football players.

Individual TRIMPIndividual TRIMP (iTRMP) is a product from heart rates values (based on Banisters TRIMP), the total training time and a nonlinear coefficient for an individual player. The nonlinear coefficient derives from individual blood lactate (in a prior performance test) measurements and therefore display an individual approach to monitor internal load.

To our knowledge there are three individual (i) TRIMPs published so far (2, 37, 46) with one in football (2).

The iTRIMP seems a valid monitoring tool as it correlates with velocity at given lactate thresholds (2, 37, 46), changes in VO2max (46) and running performance (37).

Team TRIMPTeam TRIMP is an equation that is based on individual data from players iTRIMP however used to calculate the internal load for each player. The following formula was obtained from Akubat et al. (2).

Training duration HRratio 0.2053e3.5179x

Where HRratio is the same in Banisters TRIMP, e = the base of the Napierian logarithms, 3.5179 is the exponent of e, and x = HRratioGlobal positioning systemsIn recent years, global positioning systems (GPS) have been used to (better) quantify time motion analysis (10-12, 18, 19, 26, 38, 41, 45, 49) and total body load, such as accelerations and decelerations (15, 27). While the devices seemed to be reliable, it is impossible to interchange models and data (9) and furthermore, 1 Hz-units seemed not be appropriate (15) as football is to quick for 1 Hz (meaning one measurement per second).

Furthermore, monitoring accelerations/decelerations only (without taking other aspects such as dribbling a ball, running backwards or change of direction into account) might mislead whilst quantifying load (25), as it was shown that these modes of motion accentuate metabolic loading (44).External training loadAs the name suggests, the external load is defined by variables such as time, distance, weight etc. that is opposed on players. In a football language, it is the exercise prescription by the coach (unless he describes his load in internal load variables).

Problems with the external load (such as 5 km run in 20 minutes) is that the load is not tailored to the individual physical ability of a player meaning the same external load will be experienced differently (too much vs. not enough load) by players with unequal physical capacity.

However, this seems to be the approach by most of the coaches world-wide just because a) measuring (and furthermore interpreting) the internal load appropriately seems to be somewhat hard and b) prescribing load derived from internally measured variables is very impractical whilst training in a team set-up.ConclusionWe believe that monitoring is essential in football and both the internal AND the external training load should be measured. While measuring heart rate is very time consuming, it provides useful information regarding training intensity despites its limitation. As a consequence RPE (session-RPE) as an additional measurement should be used to get a feeling from the athlete. Well-educated athletes who seek help from (physical) coaches will give a true indication. To us, GPS units are still very expensive, their data are even more complicated to handle (compared to HR) and therefore require even more attention, whilst having questionable benefits (compared to other TMA measurements). Using an individual approach (iTRIMP) based on blood lactate seems the most appropriate and valid option, however, not all football teams are able to accommodate the initial blood testing.References1. Aguiar, M.V., Botelho, G.M., Goncalves, B.S., and Sampaio, J.E. Physiological responses and activityprofiles of football small-sided games. J. Strength. Cond. Res. 27: 1287-1294, 2013.

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