Foundational work relating to overtraining utilized "rating of perceived exertion" times activity duration to estimate training load magnitude. In recent years, wearable inertial measurement units (IMUs) have been used to precisely quantify change in acceleration every 1/100 second over the duration of a practice session or competitive event. Most of the available research on training load relates to elevated risk for muscle strains when accumulated load exceeds a critical threshold (i.e., insufficient recovery time between training sessions). Very recent work has related variability in IMU load data to risk for acute musculoskeletal injuries among college football players.

can be measured and detected by controls of heart frequency (intensity, training volume, recovery, HR variability), you can also control with subjective indicators, such as the questionnaire Brunel (scale BRUMS), Visual Analog Scale of Pain (EVA), and direct measures of grip strength manual pinch, ankle range of goniometer, horizontal jump; It can be used blood physiological longitudinal enzymatic markers (CK, LDH, SGOT, SGPT), cortisol, testosterone ratio; Biomechanical tests as surface electromyography subsidized by thermography; can be reached until the evaluation by isokinetic testing.

Moreover the tools that were mentioned up, I would suggest other subjective scale, Recovery-stress Questionnaire for Athletes (RESTQ-Sport), which evaluate the perceived recovery-stress balance both physical and psychosocial, because we regularly use to monitor only the physical stress, but another source of overtraining syndrome is the psychosocial stress. The use of the RESTQ-Sport with the Rating Perceived Exertion and Heart Rate Variability can give you a complete analysis of the psychophysiological Recovery-Stress balance of the Athletes with non-expensive tools.

Hi, Christian. In addition to what has been mentioned above, the following articles may be of assistance. Of particular interest is a recent systematic review which demonstrated that keeping it simple is best in terms of monitoring athletes' subjective reports in regards to loading/training response. 

I've also attached a link to a recent systematic review detailing the changes in different heart rate measures in response to training/loading. Hopefully these are of help!  

http://bjsm.bmj.com/content/early/2015/09/30/bjsports-2015-094758.abstract

http://www.ncbi.nlm.nih.gov.ezproxy.otago.ac.nz/pubmed/?term=Monitoring+Athletic+Training+Status+Through+Autonomic+Heart+Rate+Regulation%3A+A+Systematic+Review+and+Meta-Analysis

Hi Christian,

I would suggest to use the session rating of perceived exertion (sRPE) method in combination with some measure of external load (such as the total covered distance in football or number of thrown balls in cricket). 

Based on these training loads you can then calculate the acute (average workload over the last 7 days) and chronic (average training load past 28 days) workload and the acute to chronic workload ratio. Spikes in this ratio (>1.5) have been found to be associated pretty well with non-contact soft tissue injuries in several recent studies. 

Suggested reads:

Bahr R (2016). Why screening tests to predict injury do not work-and probably never will...: a critical review. Br J Sports Med, Epub ahead of print.

Whiteley R (2016). Screening and likelihood ratio infographic. Br J Sports .            Med, Epub ahead of print.

Gabbett TJ (2010). The development and application of an injury prediction model for noncontact, soft-tissue injuries in elite collision sport athletes. The Journal of Strength and Conditioning Research, 24 (10), 2593-2603.

Gabbett TJ et al. (2016). High training workloads alone do not cause sports injuries: how you get there is the real issue. Br J Sports Med, 50 (8), 444-445.

Hope this helps.

Bas