Passive heating-induced changes in muscle contractile function are not further augmented by prolonged exposure in young males experiencing moderate thermal stress

Abstract

Background: We investigated the impact of 1) passive heating (PH) induced by single and intermittent/prolonged hot-water immersion (HWI) and 2) the duration of PH, on muscle contractile function under the unfatigued state, and during the development of muscle fatigue. Methods: Twelve young males volunteered for this study consisting of two phases: single phase (SP) followed by intermittent/prolonged phase (IPP), with both phases including two conditions (i.e., four trials in total) performed randomly: control passive sitting (CON) and HWI (44-45°C; water up to the waist level). SP-HWI included one continuous 45-min bath (from 15 to 60 min). IPP-HWI included an initial 45-min bath (from 15 to 60 min) followed by eight additional 15-min baths interspaced with 15-min breaks at room temperature between 75 and 300 min. Intramuscular (Tmu; measured in the vastus lateralis muscle) and rectal (Trec) temperatures were determined. Neuromuscular testing (performed in the knee extensors and flexors) was performed at baseline and 60 min later during SP, and at baseline, 60, 90, 150 and 300 min after baseline during IPP. A fatiguing protocol (100 electrical stimulations of the knee extensors) was performed after the last neuromuscular testing of each trial. Results: HWI increased Tmu and Trec to 38°C-38.5°C (p < 0.05) during both SP and IPP. Under the unfatigued state, HWI did not affect electrically induced torques at 20 Hz (P20) and 100 Hz (P100). However, it induced a shift towards a faster contractile profile during both SP and IPP, as evidenced by a decreased P20/P100 ratio (p < 0.05) and an improved muscle relaxation (i.e., reduced half-relaxation time and increased rate of torque relaxation; p < 0.05). Despite a reduced voluntary activation (i.e., -2.63% ± 4.19% after SP-HWI and -5.73% ± 4.31% after IPP-HWI; condition effect: p < 0.001), HWI did not impair maximal isokinetic and isometric contraction torques. During the fatiguing protocol, fatigue index and the changes in muscle contractile properties were larger after HWI than CON conditions (p < 0.05). Finally, none of these parameters were significantly affected by the heating duration. Conclusion: PH induces changes in muscle contractile function which are not augmented by prolonged exposure when thermal stress is moderate.

Keywords: cardiovascular response; central activation; contractile properties; hot-water immersion; muscle contractility; muscle fatigue; temperature; voluntary muscle activation.

FIGURE 1

Experimental design (A), and neuromuscular testing and fatiguing protocol (B). BP, blood pressure; CON, control condition; HR, heart rate; HWI, hot-water immersion condition; KE, knee extension; KF, knee flexion; MVIC, maximal voluntary isometric contraction; PSI, physiological strain index; P20, electrical stimulation (1-s stimulation) at 20 Hz; P100, electrical stimulation (1-s stimulation) at 100 Hz; Tmu, intramuscular temperature; Trec, rectal temperature; TT, 250-ms test train stimulation at 100 Hz. The fatiguing protocol consisting of 100 trains of electrical stimulation of the knee extensors (100 × 250-ms TT) was only performed after the last neuromuscular testing (i.e., 2nd neuromuscular testing during the single phase, and 5th neuromuscular testing during the intermittent/prolonged phase). *: at the time points 165 and 225 min, Tmu was only measured in the HWI condition.

FIGURE 2

Physiological measurements during the single and intermittent/prolonged protocols. Heart rate (HR), systolic and diastolic blood pressures (BP), rectal temperature (Trec), physiological strain index (PSI) and intramuscular temperature (Tmu) at three depths (1, 2, and 3 cm) during the single (A, C, D, G, I, K) and intermittent/prolonged (B, E, F, H, J, L) protocols. Data are shown as mean ± SD and individual values are presented in panels C–F. SP, single phase; IPP, intermittent/prolonged phase. *, p < 0.05; **, p < 0.01; ***, p < 0.001: significant differences CON vs. HWI. In K and L, significant differences between CON and HWI are at the three depths. In L, Tmu was not assessed in CON at the time points 165 and 225 min. N = 12 for all parameters.

FIGURE 3

Electrically induced isometric torques under the unfatigued state. Peak torques at 20 Hz (P20) and 100 Hz (P100), and P20/P100 during the single (A, C, E) and intermittent/prolonged (B, D, F) phases. Data are shown as mean ± SD and all panels include individual values. SP, single phase; IPP, intermittent/prolonged phase. *, p < 0.05: significant differences CON vs. HWI. N = 12.

FIGURE 4

Peak torques and contractile properties derived from a 250-ms test train stimulation at 100 Hz (TT) under the unfatigued state. TT torques, contraction time/peak torque, half-relaxation time (HRT), peak rate of torque development (RTD) and peak rate of torque relaxation (RTR) during the single (A, C, E, G, I) and intermittent/prolonged (B, D, F, H, J) phases. Data are shown as mean ± SD and all panels include individual values. SP, single phase; IPP, intermittent/prolonged phase. *, p < 0.05; **, p < 0.01; ***, p < 0.001: significant differences CON vs. HWI. N = 12 for all parameters except for RTD (N = 9; (G, H).

FIGURE 5

Maximal voluntary contraction torques and central activation ratio (CAR) under the unfatigued state. Maximal voluntary isometric contraction (MVIC) torques, CAR, knee extension (KE) maximal isokinetic torques, and knee flexion (KF) maximal isokinetic torques during the single (A, C, E, G) and intermittent/prolonged (B, D, F, H) phases. Data are shown as mean ± SD and all panels include individual values. SP, single phase; IPP, intermittent/prolonged phase. *, p < 0.05: significant differences CON vs. HWI. N = 12.

FIGURE 6

Torques and contractile properties during the fatiguing protocol. Torques during 250-ms test train stimulations at 100 Hz (TT torques), contraction time/peak torque, half-relaxation time (HRT), peak rate of torque development (RTD) and peak rate of torque relaxation (RTR) during the single (A, C, E, G, I) and intermittent/prolonged (B, D, F, H, J) phases. Data are shown as mean ± SD and all panels include individual values. *, p < 0.05; **, p < 0.01: significant differences CON vs. HWI N = 12 for all parameters except for RTD (N = 11; (G, H) and RTR (N = 11; (I, J).

FIGURE 7

Peak torques during the fatiguing protocol, expressed in Nm (A) and in percentage (B). Data are shown as the means obtained from the 12 participants.