## 4 Discussion  
This study aimed to examine whether the application of simultaneous TENS and focal joint cooling prior to strengthening exercises in AKP patients produces greater improvements in quadriceps and self-reported functions over ten sessions of rehabilitation and 4 weeks of rest thereafter. We observed that the endpoints (quadriceps and self-reported functions) improved in both treatment conditions (Tables 3 and 4). The levels of improvement were not different between the conditions, indicating that there was no therapeutic advantage in receiving an additional 30 min of disinhibitory modality. Improved training effects on quadriceps (RTD and PT) and self-reported functions (pain during ADL, and at least- and worst-pain perception) persisted for 4 weeks of detraining. Interestingly, however, the first increase in quadriceps MVIC and the second improvement in LEFS were seen 4 weeks after rehabilitation (Tables 3 and 4). An immediate increase in quadriceps function by the use of TENS [4,9] or knee joint cooling [10,28] is well-documented. Sensory inputs from either TENS or knee joint cooling are thought to inhibit Ib inhibitory interneuron and facilitate Ia excitatory interneurons [28]. Additionally, TENS also activates descending mechanism, mediated at supraspinal level, which assists in the inhibition of Ib interneurons [4]. Knee joint cooling is thought to reduce the recruitment threshold of the motor unit and nerve conduction velocity [31]. Therefore, we expected that patients who received electro-cyotherapy would be able to exert greater external load when performing resistance exercises. Variations in treatment parameters, knee pathologies, and muscle contraction mode may explain why we did not observe such disinhibitory effects. First, a large volume of TENS treatments (at least 8 hours per day for 4 weeks) was applied for the study reported beneficial effect on the use of TENS in patients with knee osteoarthritis [14]. A shorter TENS application (60 min for 20 sessions) did not show additional improvements in the same pathological population while our treatment (30 min for ten sessions) was insufficient to elicit what we expected. Second, a therapeutic advantage (greater quadriceps improvement) was also observed in patients with ACL reconstruction [12]. Osteoarthritis and ACL reconstructions are more severe knee pathologies where the potential for improvement could be larger, relative to in patients with chronic AKP. Lastly, a recent study observed that an application of 20 min electro-cyotherapy did not restore altered running mechanics due to experimentally induced knee pain [32]. Since previous quadriceps disinhibition was observed in a static supine position [4] or with isometric contraction [9,10,28], our isotonic exercise (also requiring co-contractions with other muscles) may have suppressed a disinhibitory effect. Pain perception (4.2 to 1.3 cm in VAS, ES = 2.28) and lower-extremity functional outcomes (53 to 60 points in LEFS, ES = 0.60) were also improved regardless of modality usage. Even if there was little or no disinhibitory effect, greater improvements in self-reported functions were expected in patients receiving electro-cyotherapy due to the known analgesic effect of TENS and focal joint cooling. Additionally, numbing the painful area by electro-cyotherapy application could have possibly allowed for pain-free exercises or increased the exercise tolerance. These results are, in fact, consistent with previously conducted similar training studies [11,12,14,33] where functional outcome measures were not different among the treatment groups. While TENS and/or knee joint cooling resulted in an immediate reduction in pain perception by experimentally induced AKP [9,32,34], the limitations on the cumulative analgesic effect of repetitive use of TENS [35] and cryotherapy [36] have also been suggested. Hence, long-term benefits on the use of therapeutic modalities, as the addition to strengthening exercises, in patients with knee joint pathologies remain unclear.